Optical instrument and image photographic method

A lens-fitted photo film unit for photographing an object includes a flash charger button as mode selector, which designates a selected one of a flashless mode and a flashing mode. An exposure aperture forms a color image by object light from the object. The image, if the flashless mode is designated, is recorded with first color balance in the exposure aperture, and if the flashing mode is designated, is recorded with second color balance. An electronic flash unit applies flash light to the object. A light balancing filter as color balance adjustor subjects the object light to correction if the flashless mode is designated, so as to set the first and second color balance substantially equal to one another.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to an optical instrument and an image
 photographing method. More particularly, the present invention relates to
 an optical instrument and an image photographing method for photographing
 an object image with appropriate color balance in indoor photography in
 particular.
 2. Description Related to the Prior Art
 Optical instruments, such as a photographic camera and a digital still
 camera, are widely used for photographing an object image. The
 photographic camera is used with silver halide photo film. The digital
 still camera incorporates a CCD image sensor. Any of such optical
 instruments is generally provided with an electronic flash device, which
 makes it possible to photograph an object indoors with insufficient
 illumination.
 A photographic camera is used with a daylight type of color negative photo
 film which is the most suitable for outdoor photography under daylight.
 The flash device incorporated in the camera has color balance (color
 temperature) of flash light corresponding to color balance of daylight.
 Recently, there have been color negative photo films having high speed
 without lowering the image quality. As cameras of types with the flash
 device have been widely used, users takes indoor photographs the more
 frequently.
 It is likely indoors that the use of flash light creates a high contrast
 frame in which an image of a principal object is over-exposed and recorded
 in a whitened manner, and/or an image of a background scene is
 under-exposed and darkened. To avoid such unwanted exposures, various
 suggestions have been made, including heightening speed of the photo film,
 lowering a shutter speed, and lowering the f-number of a photographic
 optical system. Also the principal object is illuminated with flash light
 being weakened, and the background scene is illuminated suitably by indoor
 illumination of an indoor light source.
 However it is impossible to obtain a photographic print with sufficiently
 high color reproduction or color balance. When the principal object is
 effectively illuminated by flash light, the principal object is
 photographed on the photo film with coloration or tint near to that of its
 image which would be photographed while illuminated by daylight. The
 background scene behind the principal object is not illuminated by flash
 light, and thus photographed with the coloration of the indoor
 illumination. In general, the indoor illumination has color different from
 the daylight color, to cause a problem in the color reproduction. When the
 principal object is reproduced from the photo film on the photographic
 print neutrally or with the coloration or chromaticity very near to color
 of the photographic object, the color of the indoor illumination strongly
 remains in the background scene. In contrast, when the background scene is
 reproduced on the photographic print with neutral coloration, the color
 complementary to that of the indoor illumination strongly remains in the
 principal object. In any case, color failure occurs.
 Let the indoor illumination be light of a tungsten lamp, the photo film of
 a daylight type be used under the indoor illumination, and the principal
 object be photographed while illuminated with flash light of the daylight
 color. When the photographic print is produced, the background scene has
 deep orange color while the principal object has its neutral coloration on
 the photographic print. If a printer of a type capable of correcting the
 colors effectively is used to produce the photographic print, the whole of
 the frame is finished with the coloration near to the neutral coloration.
 In such a frame, the background scene only has light orange color. But the
 cyan color is conspicuously added to the portion of the principal object,
 the cyan color being complementary to the orange color. An image of a
 person's face will be finished with very unnatural color reproduction.
 If the indoor illumination is light of fluorescent lamps, pale green is
 added to an image of the background scene of the photographic print. Or
 the magenta color is conspicuously added to the image of the principal
 object for example a flesh-color portion of a person, the magenta color
 being complementary to the green color. A portrait of a person results in
 very unnatural color reproduction.
 In order to prevent such problems, professional photographers selectively
 use photo films of a daylight type and a tungsten light source type. Also
 they use selectively lens filters suitable for the types of light sources
 for the purpose of obtaining optimized color balance. Those methods of
 indoor photography are well-known in the field of photography, but very
 difficult for amateur photographers to use. In addition, similar problems
 occurs with the digital still camera as optical instrument which uses the
 CCD image sensor instead of the silver halide photo film.
 SUMMARY OF THE INVENTION
 In view of the foregoing problems, an object of the present invention is to
 provide an optical instrument and an image photographing method for
 photographing an object image with acceptable color balance in indoor
 photography in particular.
 In order to achieve the above and other objects and advantages of this
 invention, an optical instrument for photographing an object comprises an
 electronic flash unit for applying flash light to the object. An exposure
 station forms a color image by object light from the object. The optical
 instrument satisfies a condition of:
EQU 0.ltoreq..DELTA.E.ltoreq.0.08
 where .DELTA.E is a light source color difference amount between the flash
 unit and a white fluorescent lamp of F6 type, and obtained from conditions
 of:
EQU .DELTA.E= [(X.sub.1 -X.sub.2).sup.2 +(Y.sub.1 -Y.sub.2).sup.2 ]
EQU X.sub.1 =[(RF-R0)-(GF-G0)]-[(BF-B0)-(GF-G0)]/2
EQU Y.sub.1 =[(BF-B0)-(GF-G0)].times.sin (.pi./3)
EQU X.sub.2 =[(RS-R0)-(GS-G0)]-[(BS-B0)-(GS-G0)]/2
EQU Y.sub.2 =[(BS-B0)-(GS-G0)].times.sin (.pi./3)
 where RF, GF and BF are Status-M converted density of red, green and blue
 colors of a gray reference reflector plate having a reflectance factor of
 18%, the Status-M converted density being obtained by illuminating the
 gray reflector plate with the white fluorescent lamp of the F6type, by
 photographing an image of the gray reflector plate in the exposure
 station, and by detection of the image.
 RS, GS and BS are Status-M converted density of the red, green and blue
 colors of the gray reflector plate, the Status-M converted density being
 obtained by illuminating the gray reflector plate with the flash unit, by
 photographing an image of the gray reflector plate in the exposure
 station, and by detection of the image.
 R0, G0 and B0 are Status-M converted density of the red, green and blue
 colors of the gray reflector plate, the Status-M converted density being
 obtained by illuminating the gray reflector plate with a standard light
 source C, by photographing an image of the gray reflector plate in the
 exposure station, and by detection of the image.
 Preferably, the optical instrument satisfies a condition of:
EQU 0.ltoreq..DELTA.E.ltoreq.0.06.
 More preferably, the optical instrument satisfies a condition of:
EQU 0.ltoreq..DELTA.E.ltoreq.0.04.
 Furthermore, a mode selector designates a selected one of first and second
 predetermined photographing modes, wherein if the first and second
 photographing modes are designated, the image is recorded with
 respectively first and second color balance in the exposure station. A
 color balance adjustor subjects the flash light or the object light to
 correction if the first photographing mode is designated, to satisfy the
 condition of:
EQU 0.ltoreq..DELTA.E.ltoreq.0.08
 so as to set the first and second color balance substantially equal to one
 another.
 By the construction of the present invention, an object image can be
 photographed with acceptable color balance in indoor photography in
 particular, because flash light or object light is subjected to correction
 when the particular one of the two photographing modes is designated.
 In a preferred embodiment, the optical instrument satisfies a condition of:
EQU 40.degree..ltoreq..theta..ltoreq.90.degree.
 where .theta. is a phase angle depending upon RF, GF, BF, R0, G0 and B0,
 and defined in an X-Y orthogonal coordinate system between a vector
 (X.sub.1, Y.sub.1) and a portion of X-axis having a positive sign in a
 predetermined rotational direction according to a point (X.sub.1, Y.sub.1)
 plotted in the X-Y orthogonal coordinate system.
 The phase angle .theta. is a value satisfying conditions of:
 if X.sub.1 =0 and Y.sub.1 =0, then .theta.=0.degree.,
 if X.sub.1.noteq.0, then .theta. tan.sup.-1 (Y.sub.1 /X.sub.1),
 if X.sub.1.gtoreq.0, Y.sub.1.gtoreq.0, then 0.degree..ltoreq.tan.sup.-1
 (Y.sub.1 /X.sub.1).ltoreq.90.degree.,
 if X.sub.1.ltoreq.0, Y.sub.1.gtoreq.0, then 90.degree..ltoreq.tan.sup.-1
 (Y.sub.1 /X.sub.1).ltoreq.180.degree.,
 if X.sub.1.ltoreq.0, Y.sub.1.ltoreq.0, then 180.degree..ltoreq.tan.sup.-1
 (Y.sub.1 I/X.sub.1).ltoreq.270.degree.,
 if X.sub.1.gtoreq.0, Y.sub.1.ltoreq.0, then 270.degree..ltoreq.tan.sup.-1
 (Y.sub.1 /X.sub.1).ltoreq.360.degree..
 In a preferred embodiment, the optical instrument is loadable with an
 unexposed photo film of a predetermined type, adapted to recording of the
 image of the object, and having spectral sensitivity satisfying the
 condition of:
EQU 40.degree..ltoreq..theta..ltoreq.90.degree.
 so as to record a light source color of the white fluorescent lamp of the
 F6 type in a manner tinted with a yellow, reddish yellow or greenish
 yellow color in comparison with a light source color of the standard light
 source C.
 In another preferred embodiment, the first and second photographing modes
 are a flashless mode and a flashing mode, and the mode selector allows the
 flash unit to operate if the flashing mode is designated.
 If the first and second photographing modes are designated, respectively
 first and second object light advances from the object through a
 photographing light path toward the exposure station. The color balance
 adjustor converts the first object light into color-balance-revised object
 light of which color balance is substantially equal to color balance of
 the second object light.
 The color balance adjustor comprises a light balancing filter, having a
 predetermined color, disposed movably, and if the first photographing mode
 is designated, positioned inside the photographing light path, and if the
 second photographing mode is designated, positioned outside the
 photographing light path.
 The optical instrument is a lens-fitted photo film unit pre-loaded with
 unexposed photo film, and photo film is set in the exposure station.
 The first photographing mode is the flashless mode.
 In still another preferred embodiment, the optical instrument is a digital
 still camera loadable with a recording medium. Furthermore, an image
 sensor is disposed in the exposure station for outputting image
 information by detecting object light. The color balance adjustor is an
 image processor for processing the image information, for correcting the
 image information if the first photographing mode is designated, and for
 writing the image information to the recording medium after the processing
 and correction, wherein the image information after the correction
 represents object light according to the object light but with a change in
 color balance.
 The first photographing mode is the flashing mode.
 The mode selector includes a photometric circuit for measuring object
 brightness of the object light. A controller compares the object
 brightness with reference brightness, to designate the flashing mode if
 the object brightness is equal to or less than the reference brightness,
 and to designate the flashless mode if the object brightness is more than
 the reference brightness.
 In another preferred embodiment, the optical instrument is a camera, and is
 loadable with photo film of first and second predetermined types, and the
 photo film is set in the exposure station and adapted to recording of the
 image of the object. The mode selector is operated according to which the
 photo film is of the first and second types, and designates the first and
 second photographing modes for respectively the first and second types.
 The color balance adjustor converts the flash light directly emitted by
 the flash unit into flash light with a change in coloration in accordance
 with spectral sensitivity of the first and second types and coloration of
 the flash light, the first and second color balance being set
 substantially equal irrespective of a difference between the first and
 second types in the spectral sensitivity.
 The color balance adjustor comprises a flash filter, having a predetermined
 color, disposed movably, and if the first photographing mode is
 designated, set away from a front of a flash emitter of the flash unit,
 and if the second photographing mode is designated, set in front of the
 flash emitter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) OF THE PRESENT
 INVENTION
 There are various types of indoor light sources in the field of
 illumination. The most widely used types of them are a white flourescent
 lamp and a tungsten lamp such as incandescent lamp. Recently numerous
 types of flourescent lamps have been known, for example a daylight type
 and a three-wavelength type which is provided with improved bulb color and
 color rendering properties. In consideration of this situation,
 investigation and experiments were effected. As a result, it has been
 confirmed that, only if the white flourescent lamp and the tungsten light
 source are considered, 90% or more of the indoor light sources can be
 safely used for photography.
 In FIG. 1, the white flourescent lamp of the F6 type has energy
 distribution with very low uniformity. When light from the white
 flourescent lamp is reflected by a gray reference reflector plate with
 reflectance of 18% (hereinafter referred to as a gray reflector plate),
 the light becomes reproduced in various colors in an optical instrument or
 camera according to spectral sensitivity of the optical instrument. If the
 optical instrument has ideal spectral sensitivity of R-G-B color matching
 functions according to CIE 1931, the gray reflector plate photographed by
 the optical instrument is reproduced in a pale yellow color, which is
 illustrated in FIG. 2 in the coordinate system of the L*a*b* space
 according to CIE 1976. The reproduced color of the gray reflector plate is
 the light source color of the white flourescent lamp of the F6 type as
 viewed through the optical instrument having the spectral sensitivity of
 the R-G-B color matching functions. According to the degree at which an
 actual spectral sensitivity of the optical instrument is less near to the
 ideal spectral sensitivity, the reproduced color is less near to the pale
 yellow color. Note that the black dots in FIG. 2 are determined by
 plotting twelve colors in the center of the Macbeth color checker.
 There is a color negative photo film of a daylight type known with the
 trade name of Super G Ace 800 (manufactured by Fuji Photo Film Co., Ltd.).
 When the gray reference reflector plate is photographed with this photo
 film, the reproduced color becomes greenish in comparison with the photo
 film of the spectral sensitivity of the R-G-B color matching functions.
 Let a photograph be taken on such a daylight type of photo film while a
 principal object is illuminated with flash light. When a photographic
 print is finished from this frame by reproducing the principal object in a
 neutral tone of color, a background scene behind the principal object
 becomes reproduced in a greenish tone in an unwanted manner, as the
 background scene is illuminated by the white flourescent lamp.
 To avoid reproducing the background scene in the greenish coloration due to
 the white flourescent lamp, it may be possible to provide the optical
 instrument with improved spectral sensitivity. However the changes in the
 optical instrument cause problems which will be described later. Another
 solution is to use a different type of photo film, for example a daylight
 type known with the trade name of Reala Ace (manufactured by Fuji Photo
 Film Co., Ltd.). A camera is loaded with this photo film, on which the
 gray reflector plate is photographed while illuminated by the white
 flourescent lamp of the F6 type. In FIG. 3, reproduced coloration or tint
 of the gray reflector plate is plotted in a hexagonal coordinate system,
 namely the light source color of the white flourescent lamp of the F6 type
 in view of this particular photo film.
 This hexagonal coordinate system has six coordinates of Red (R), Yellow
 (Y), Green (G), Cyan (C), Blue (B) and Magenta (M), and adapted to
 indicate a deviation of coloration to be reproduced with reference to an
 origin as reference coloration. To be precise, the hexagonal coordinate
 system is defined by the following.
 1. Let (R0, G0, B0) be Status-M converted density of Red (R), Green (G) and
 Blue (B) determined by the reference coloration at the origin. Let (RP,
 GP, BP) be Status-M converted density of Red (R), Green (G) and Blue (B)
 determined by coloration to be plotted. Differences (R', G', B') are
 defined by
 R'=RP-R0,
 G'=GP-G0,
 B'=BP-B0.
 The Status-M converted density will be described later in detail.
 2. Let an X-Y orthogonal coordinate system be defined by taking its origin
 at the origin of the hexagonal coordinate system, and by taking a positive
 (+) direction of its X-axis along the R-axis of the hexagonal coordinate
 system. Coordinates (XP, YP) are plotted in the X-Y orthogonal coordinate
 system, and are defined by:
EQU XP=(R'-G')-(B'-G)/2
EQU YP=(B'-G').times.sin (n/3)
 The coordinates of the coloration to be plotted in the hexagonal coordinate
 system are defined by the point in the X-Y orthogonal coordinate system
 overlapped with the hexagonal coordinate system and according to the
 coordinates (XP, YP).
 The hexagonal coordinate system according to the present specification has
 the axes of Red (R), Green (G), Blue (B), Cyan (C), Magenta (M) and Yellow
 (Y) for the coloration of an image on a photographic print.
 In FIG. 3, coloration of daylight color is taken as the origin of the
 hexagonal coordinate system. The graph illustrates deviations of the
 coloration from the daylight color. The coloration of daylight color is
 defined equal to the coloration of a gray reference reflector plate
 illuminated by the CIE standard light source C. In relation to the
 above-described type of photo film, the gray reflector plate illuminated
 by the white fluorescent lamp of the F6 type has the coloration being
 palely yellowish white in comparison with the coloration of daylight
 color, as is read from FIG. 3. The coloration can be obtained in a manner
 similar to the color reproduction according to the R-G-B color matching
 functions.
 In FIG. 4, deviations of the coloration from the daylight color are
 illustrated in using a digital still camera DS-300 (trade name,
 manufactured by Fuji Photo Film Co., Ltd.) as an optical instrument
 incorporating a CCD image sensor. The gray reference reflector plate is
 illuminated by the white fluorescent lamp and picked up by the digital
 still camera DS-300, and reproduced with the coloration, which is plotted
 in the graph as light source color of the white fluorescent lamp as viewed
 through the digital still camera. As a result, a palely yellowish white is
 created in a manner similar to that of the photo film.
 Problems still remain in any of known optical instruments. Although the
 above-described use of the optical instrument pre-loaded with the photo
 film or the optical instrument incorporating a CCD image sensor can avoid
 greenish reproduction of the background scene illuminated by the white
 fluorescent lamp, there remains yellowish tone in the reproduced image. It
 is certain that improvement of spectral sensitivity is effective in the
 lens-fitted photo film unit in which only the predetermined type of the
 photo film is used, as this optical instrument is pre-loaded with the
 photo film. However improvement of spectral sensitivity may be ineffective
 in a compact camera or single-lens reflex camera, as any type of photo
 film can be selected and inserted in it by a user.
 In view of this situation, researches were made to discover possibility of
 reproducing the coloration effectively and agreeably by suitably
 determining light source color (color temperature) of flash light. It was
 found that neutral color reproducibility could be obtained hen the flash
 light had the light source color near to that of the white fluorescent
 lamp as background light source in view of the photo film, the CCD image
 sensor or the like.
 Experiments were conducted by use of the flash light with various colors of
 light in combination with the photo films Super G Ace 800 and Reala Ace.
 FIG. 5 is a graph with the hexagonal coordinate system illustrating the
 coloration of an image of the gray reference reflector plate illuminated
 with each of light sources and photographed on Reala Ace.
 As a result, it was found that the color of the flash light could be
 determined to satisfy the condition of:
EQU 0.ltoreq..DELTA.E.ltoreq.0.08
 for the purpose of reproduction of both the principal object illuminated by
 the flash light and the background scene illuminated by the white
 fluorescent lamp with fidelity in color, where .DELTA.E is a light source
 color difference amount between the flash unit and a white fluorescent
 lamp of the F6 type. Namely, .DELTA.E is a distance defined in the
 hexagonal coordinate system having the origin at the coloration reproduced
 by photographing the gray reference reflector plate illuminated by the
 standard light source C and at a proper exposure, and which is determined
 between the coloration reproduced by photographing the gray reflector
 plate illuminated by the white fluorescent lamp of the F6 type and at a
 proper exposure, and the coloration reproduced by photographing the gray
 reflector plate illuminated by the flash light and at a proper exposure.
 Furthermore, the color of the flash light, more preferably, should satisfy
 the condition of:
EQU 0.ltoreq..DELTA.E.ltoreq.0.06
 and desirably, should satisfy the condition of:
EQU 0.ltoreq..DELTA.E.ltoreq.0.04.
 The distance .DELTA.E is defined by the following equations:
EQU .DELTA.E= [(X.sub.1 -X.sub.2).sup.2 +(Y.sub.1 -Y.sub.2).sup.2 ]
EQU X.sub.1 =[(RF-R0)-(GF-G0)]-[(BF-B0)-(GF-G0)]/2
EQU Y.sub.1 =[(BF-B0)-(GF-G0)].times.sin (.pi./3)
EQU X.sub.2 =[(RS-R0)-(GS-G0)]-[(BS-B0)-(GS-G0)]/2
EQU Y.sub.2 =[(BS-B0)-(GS-G0)].times.sin (.pi./3)
 In the equations:
 RF, GF and BF are Status-M converted density of Red (R),
 Green (G) and Blue (B) of the gray reference reflector plate which is
 illuminated by the F6 white fluorescent lamp and reproduced at a proper
 exposure;
 RS, GS and BS are Status-M converted density of Red (R), Green (G) and Blue
 (B) of the gray reflector plate which is illuminated by an electronic
 flash device and reproduced at a proper exposure;
 R0, G0 and B0 are Status-M converted density of Red (R), Green (G) and Blue
 (B) of the gray reflector plate which is illuminated by the standard light
 source C and reproduced at a proper exposure.
 The definition of the Status-M converted density is now described. The
 Status-M converted density can be used to obtain .DELTA.E and the like by
 plotting points in the hexagonal coordinate system.
 Silver halide camera as optical instrument for use with color photo film as
 recording medium. The Status-M converted density of the color photo film
 is used. If the photo film is color reversal photo film, the sign of the
 value of the density is reversed. .gamma. of the recording medium is
 linearly converted to 0.65, which is a standard value of .gamma. for the
 color negative photo film. Thus the Status-M converted density is
 obtained.
 Digital still camera as optical instrument. Object brightness of a
 photographic object is measured by the CCD unit, digitized, processed
 suitably according to halftone processing, and stored as image data. The
 operation of the halftone processing can be recognized by picking up the
 object such as a gray chart with known reflectance. The image data, when
 subjected to reverse conversion, can be converted into the object
 brightness. The object brightness being obtained is logarithmically
 converted. .gamma. of the recording medium is converted to 0.65, which is
 a standard value of .gamma. for the color negative photo film. Thus the
 Status-M converted density is obtained.
 Further experiments were conducted. A background scene was illuminated by a
 tungsten light source or the CIE standard light source A. The gray
 reference reflector plate was illuminated by flash light of various
 colors, and was photographed at a proper exposure, so as to be observed
 for color reproducibility. As a result with the photo film Super G Ace
 800, when the flash light satisfying .DELTA.E.ltoreq.0.05 was used,
 reddish coloration of the background scene due to the flash light was
 remarkable to cause unwanted color reproducibility. In using the photo
 film Reala Ace, when the flash light satisfying .DELTA.E.ltoreq.0.06 was
 used, very agreeable color reproducibility was obtained, as the
 colorfulness was decreased without changes in coloration or chromaticity
 of the background scene despite the use of the tungsten light source. When
 photographic prints were produced by use of a photographic printer
 operable for effective correction of colors, very agreeable results were
 obtained, as color failure of coloring a principal object in cyan was
 decreased. Also similar experiments were conducted with the still camera
 DS-300 which had characteristics related to fluorescent lamps similar to
 those of Reala Ace. Finally, a result similar to that of Reala Ace was
 obtained.
 After a research on those results, it was found that a difference in the
 color reproducibility between the photo films Super G Ace 800 and Reala
 Ace was derived from a phase angle or hue angle .theta. defined in the
 hexagonal coordinate system from the gray reference reflector plate
 reproduced while illuminated by the F6 white fluorescent lamp. The hue
 angle .theta. according to Super G Ace 800 was 103.degree.. The hue angle
 .theta. according to Reala Ace was 55.degree.. Note that the hue angle
 .theta. in the hexagonal coordinate system is defined as an angle at which
 a vector defined from the origin O to a point at the coordinates to be
 represented by the hue angle .theta. is open rotationally with reference
 to a vector O-R or a direction along the R-axis. The hue angle .theta. is
 measured in a counterclockwise direction about the origin O from the
 vector O-R, which itself has the hue angle .theta. of zero (0) degree.
 Results from various experiments were observed and investigated, so that a
 preferable construction of an optical instrument was discovered. In
 consideration of a tungsten light source, the above-described preferable
 condition of the distance .DELTA.E should be satisfied. Also, the phase
 angle or hue angle .theta., which was obtained from the gray reference
 reflector plate and the F6 white fluorescent lamp and defined in the
 hexagonal coordinate system, should satisfy a condition of:
EQU 40.degree..ltoreq..theta..ltoreq.90.degree..
 More preferably, the optical instrument should satisfy a condition of:
EQU 55.degree..ltoreq..theta..ltoreq.80.degree..
 To be precise, the phase angle or hue angle .theta. is defined as a value
 satisfying the conditions of:
 if X.sub.1 =0 and Y.sub.1 =0, then .theta.=0.degree.,
 if X.sub.1.noteq.0, then .theta.=tan.sup.-1 (Y.sub.1 /X.sub.1),
 if X.sub.1 &gt;0, Y.sub.1.gtoreq.0, then 0.degree..ltoreq.tan.sup.-1
 (Y.sub.1 /X.sub.1).ltoreq.90.degree.,
 if X.sub.1 &lt;0, Y.sub.1.gtoreq.0, then 90.degree..ltoreq.tan.sup.-1
 (Y.sub.1 /X.sub.1).ltoreq.180.degree.,
 if X.sub.1 &lt;0, Y.sub.1.ltoreq.0, then 180.degree..ltoreq.tan.sup.-1
 (Y.sub.1 /X.sub.1).ltoreq.270.degree.,
 if X.sub.1 &gt;0, Y.sub.1.ltoreq.0, then 270.degree..ltoreq.tan.sup.-1
 (Y.sub.1 /X.sub.1).ltoreq.360.degree..
 It is to be noted that
 if X.sub.1 =0, Y.sub.1 &gt;0, then .theta.=90.degree.,
 if X.sub.1 =0, Y.sub.1 &lt;0, then .theta.=270.degree..
 Those conditions being satisfied, the optical instrument can provide good
 color reproducibility particularly for indoor photography. Note that,
 after conducting experiments, it was observed that desirable color
 reproducibility was obtained after the use of a white fluorescent lamp, a
 tungsten light source, and most of all the other indoor light sources.
 It is to be noted that problems are likely to occur when the optical
 instrument satisfying the above-described preferable condition of the
 distance .DELTA.E is used outdoors under daylight, as follows:
 Problem A. A background scene to be photographed is illuminated by
 daylight, while a principal object is illuminated by mixture of daylight
 and flash light.
 Problem B. A single recording medium is provided with frames of both
 photographic scene photographed outdoors under daylight and the
 photographic scene photographed with the mixed light described in
 connection with problem A. If a certain type of automatic printer is used
 to produce prints from such frames, it is impossible to specify the
 characteristic of the recording medium, the type of light source or the
 like. Even though a first one of the scenes is normally printed, the
 second one of the scenes is involved with color failure due to influence
 of printing of the first scene. Inevitably at least part of all the frames
 are unsuitably printed in view of the color reproducibility.
 As a result of observation, it has been found that problem A is not
 serious, because there occurs no great difference in the color
 reproducibility when the principal object receives mixture of the flash
 light and daylight. Problem B should be resolved by ideas which will be
 described heretofore. Such ideas are different between a silver halide
 camera as the optical instrument for use with photo film and a digital
 still camera as the optical instrument incorporating the CCD image sensor.
 In a silver halide camera, it is necessary to obtain acceptable color
 balance in a state recorded in the photo film. The coloration or color
 balance (chromaticity) of object light from a photographic object to be
 incident on an emulsion surface of the photo film can be changed for this
 purpose. There are various structures for changing the coloration, such as
 an insertable color filter or light balancing (LB) filter, and a color
 liquid crystal display panel (LCD). Among them, the light balancing filter
 is the simplest. At the time of photographing an object under daylight,
 the light balancing filter is inserted in such a manner that the
 coloration obtained by properly photographing the gray reference reflector
 plate illuminated by daylight is substantially equal to the coloration
 obtained by properly photographing the gray reflector plate illuminated by
 the flash light. Thus it is possible that the color balance obtained by
 use of an indoor light source can be near to that obtained by use of
 daylight as light source. Photographic prints can be obtained with
 suitable color reproducibility even when the above-described type of
 automatic printer is used.
 The light balancing filter may be inserted either manually or
 automatically, but desirably should be automatically inserted. For the
 automatic structure, the insertion of the light balancing filter can be
 associated with flash emission. Most of the flashed scenes are indoor
 scenes. An externally operable flash charger button for the flash emission
 is connected with a mechanism for insertion of the light balancing filter.
 Alternatively a camera may have a unit for measuring object brightness, a
 unit for determining the flash emission when the object brightness is
 sufficiently low, and a mechanism actuated according to a result of
 evaluating the object brightness for insertion of the light balancing
 filter. Also, the insertion of the light balancing filter can be
 associated with measurement of object brightness without relation to flash
 emission. If object brightness is equal to or more than reference
 brightness, the light balancing filter is inserted. Note that a position
 where the light balancing filter is inserted may be determined at any
 plane surface crosswise to the light path from a face of a photographic
 optical system to a focal plane which is a photo film surface of a photo
 film.
 Furthermore, it is possible to record information to the photo film for
 representing the use or lack of the use of the flash emission, and color
 of the flash light, either the daylight color or the color changed to such
 near to the light source color of white fluorescent lamp. A photographic
 printer for use with this type of the photo film is caused to read the
 information, and designate a printing condition suitable for the state of
 the photo film related to the flash emission.
 In a digital still camera as optical instrument, the color balance of an
 image being picked up is electrically treated, because the image picked up
 by CCD image sensor is converted electrically to an image signal. Japanese
 Patent Application No. 10-33643 suggests a setup unit incorporated a
 digital still camera. The setup unit determines color correction amounts
 and a density correction amount according to the halftone condition of the
 image and the pick-up condition relating to, for example, the environment
 of picking up the image, for the purpose of correction processing to
 optimize the color reproduction of the image on a recording medium.
 To simplify the construction, the optical instrument such as a digital
 still camera is caused previously to store parameters for correction of
 color balance in association with indoor scenes and outdoor scenes under
 daylight. After the image is picked up, the color balance of the image is
 corrected according to the environment at the time of picking up.
 Corrected information of the image is stored, so that a principal object
 can be reproduced with neutral coloration in the image. Also, the
 correction of the color balance according to the environment can be
 preferably determined to be selectable according to the use or lack of the
 use of the flash emission, which is the same manner as the photo film.
 Lens-fitted Photo Film Unit as Embodiment with Photo Film
 In FIG. 6, a lens-fitted photo film unit as an optical instrument is
 illustrated, and is constituted by a housing 2 and an outer belt 3, which
 partially covers the housing 2. The housing 2 incorporates mechanisms for
 taking an exposure, and is pre-loaded with a photo film cassette of the
 IX240 type available in the market.
 The front of the housing 2 is provided with a taking lens 4, a viewfinder
 5, an electronic flash unit 7 and a flash charger button 8 as mode
 selector. The top of the housing 2 is provided with a release button 10, a
 frame counter window 11 and an opening 13. The frame counter window 11
 indicates the number of remaining available frames. The opening 13 is
 formed for emerging of a light guide member 12, which indicates completion
 of flash charging. A winder wheel 14 is disposed to be operable through
 the rear wall of the housing 2, and rotated at each time of taking one
 exposure. An inner surface of the outer belt 3 is coated with adhesive
 agent to attach the outer belt 3 to the center of the housing 2. There are
 openings formed in the outer belt 3 for emerging of the taking lens 4, the
 viewfinder 5, the frame counter window 11 and the like externally.
 A flash discharge tube (not shown) is incorporated in the flash unit 7. A
 protector/diffuser plate 7a is disposed in front of the flash discharge
 tube in the flash unit 7, protects the same, and diffuses flash light in a
 predetermined distribution pattern. Flash light emittable by the flash
 unit 7 is provided with light source color (color temperature)
 predetermined by such a structure as a dye filter secured to a front of
 the protector/diffuser plate 7a or a colored coating formed on the
 protector/diffuser plate 7a. In other words, flash light emitted by the
 flash unit 7 is set at a predetermined spectral energy distribution.
 A hexagonal coordinate system is defined by setting an origin at coloration
 or tint which is reproduced by photographing a gray reference reflector
 plate at a proper exposure with illumination from the standard light
 source C, the gray reflector plate having reflectance of 18%. First
 coloration of the gray reflector plate is plotted in the hexagonal
 coordinate system as photographed at a proper exposure with illumination
 from the F6 type of white fluorescent lamp. Second coloration of the gray
 reflector plate is plotted in the hexagonal coordinate system as
 photographed at a proper exposure with flash light. A distance or light
 source color difference amount .DELTA.E is defined between the first and
 second colorations. The color of the flash light with reference to
 spectral sensitivity of the photo film to be used is determined to satisfy
 the condition of:
EQU .DELTA.E.ltoreq.0.06.
 The flash charger button 8 is connected with a flash switch for turning on
 and off the flash unit. When the flash charger button 8 is slid up to its
 ON position, the flash switch is turned on to start charging. Also the
 flash emission is allowed. Upon completion of the charging, the release
 button 10 is depressed for taking an exposure with the flash charger
 button 8 kept in the ON position. In synchronism with the exposure, the
 flash unit 7 emits flash light to a photographic object 18. When the flash
 charger button 8 is slid down to its OFF position, the flash switch is
 turned off to stop charging and to inhibit flash emission.
 In FIG. 7, the lens-fitted photo film unit is depicted. A photo film
 cassette 20 is inserted in the housing 2 at the time of manufacturing in a
 factory. The photo film cassette 20 is a combination of a cassette shell
 21 and photo film 22 as recording medium. As is similar to the known
 lens-fitted photo film unit, the photo film 22 is drawn from the cassette
 shell 21, wound in a roll form and contained in a roll holder chamber in
 the housing 2. The cassette shell 21 is contained in a cassette holder
 chamber. Again the above-described hexagonal coordinate system is referred
 to. A phase angle or hue angle .theta. of the first coloration of the gray
 reference reflector plate is plotted in the hexagonal coordinate system as
 photographed at a proper exposure with illumination from the F6 type of
 white fluorescent lamp. The spectral sensitivity of the photo film 22 is
 determined for the hue angle .theta. to satisfy the condition of:
EQU 40.degree..ltoreq..theta..ltoreq.90.degree..
 An example of the photo film 22 meeting this condition is Reala Ace
 (manufactured by Fuji Photo Film Co., Ltd.).
 The photo film 22 of the IX240 type includes a transparent magnetic
 recording layer, which is formed on a back surface of a support opposite
 to an emulsion surface. The photo film 22 has magnetic data tracks for
 storing various kinds of data in the magnetic recording layer. Among the
 magnetic data tracks, there are common magnetic data tracks associated
 with the photo film 22 and common between frames of the photo film 22, and
 specified magnetic data tracks associated with respectively frames of the
 photo film 22. The magnetic data tracks are used to store various kinds of
 information including the photo film 22, a photo film type, an ID number,
 the use or lack of the use of flash emission, which are written by photo
 film manufacturer, a photo laboratory, a camera or the like. In an
 ineffective region defined in a leader portion of the photo film 22,
 information including the photo film information and the ID number is
 printed by side printing in forms of numbers and bar codes. When the photo
 film 22 is developed, the information becomes visible images. In the
 course of development and printing, the photo film information and the
 like are read from the magnetic data tracks or the bar code of the photo
 film 22. Developing and printing operation can be effected in a condition
 determined according to the obtained information.
 A spool 23 is contained in the cassette shell 21 in a rotatable manner. A
 trailer end of the photo film 22 is retained to the spool 23. The winder
 wheel 14 has a drive shaft portion, which is engaged with an axial end of
 the spool 23. When the winder wheel 14 is rotated, an exposed portion of
 the photo film 22 is wound into the cassette shell 21. An unexposed frame
 portion of the photo film 22 is set behind the taking lens 4.
 A light-shielding tunnel (not shown) is disposed between the taking lens 4
 and the photo film 22 to cover a photographing light path in a state
 shielded from ambient light. A shutter opening 25 is formed in the front
 of the light-shielding tunnel for introducing object light from the
 photographic object 18 into the light-shielding tunnel. An exposure
 station or exposure aperture 26 is formed in the rear of the
 light-shielding tunnel for determining a size of each frame on the photo
 film 22. An aperture stop unit 27 and a shutter blade 28 are disposed
 between the shutter opening 25 and the taking lens 4. A shutter drive
 mechanism includes a spring, which biases the shutter blade 28 in a
 direction to close the shutter opening 25.
 When the release button 10 is depressed, the shutter blade 28 is caused by
 the shutter drive mechanism 30 to rotate against the spring and in a
 direction to open the shutter opening 25. After the shutter opening 25
 becomes fully open, the spring causes the shutter blade 28 to rotate in
 the direction to close the shutter opening 25. The shutter opening 25 is
 opened and closed for taking an exposure by this back and forth movement
 of the shutter blade 28. Object light entered through the taking lens 4
 passes the shutter opening 25 into the light-shielding tunnel, and exposes
 the photo film 22 in a region inside the exposure aperture 26.
 When the flash charger button 8 is slid to the ON position by a user
 desiring flash emission, the spring in the shutter drive mechanism 30
 associated with the shutter blade 28 is changed over in the biasing force.
 The shutter blade 28 is driven to swing at a shutter speed higher than
 that for photography without flash emission.
 The aperture stop unit 27 includes a larger opening 27a and a smaller
 opening 27b. A selector mechanism 31 in a color balance adjustor is
 connected with the aperture stop unit 27, and sets one of the larger
 opening 27a and the smaller opening 27b to a position in a photographing
 light path 4a along an optical axis of the taking lens 4. When the flash
 charger button 8 is slid to the ON position, the selector mechanism 31
 sets the larger opening 27a in the photographing light path 4a. When the
 flash charger button 8 is slid to the OFF position to disable the flash
 emission, the selector mechanism 31 sets the smaller opening 27b in the
 photographing light path 4a.
 This being so, the shutter speed and the aperture stop are set suitably
 both in the indoor photography with flash light and in the outdoor
 photography with daylight without flash light, because of changing over of
 the shutter drive mechanism 30 and the selector mechanism 31.
 A color filter or light balancing (LB) filter 32 included in the color
 balance adjustor is mounted in the smaller opening 27b. If no flash is
 emitted in taking an exposure, the light balancing filter 32 is kept
 inserted in the light path. The light balancing filter 32 has a color to
 change first coloration to second coloration, the first coloration being
 obtained by properly photographing the gray reference reflector plate with
 illumination of daylight, and the second coloration being obtained by
 properly photographing the gray reflector plate with illumination of flash
 light.
 An electronic flash unit 33 consists of a combination including the flash
 unit 7, the flash switch, a trigger switch, a main capacitor, a dry
 battery, circuit elements for charging and discharging and the like. The
 flash switch is turned on and off in connection with the positions of the
 flash charger button 8. The trigger switch is turned on when the shutter
 blade 28 moves to its fully open position. Both the flash switch and the
 trigger switch being turned on, the flash unit 7 applies flash light to
 the photographic object 18.
 Also an optical indicia imprinting unit 35 is incorporated in the
 lens-fitted photo film unit. When flash light is emitted in taking an
 exposure, an LED in the optical indicia imprinting unit 35 is turned on to
 imprint optical indicia on the photo film 22 in a portion outside a frame
 region as flash emission information.
 The operation of the above construction is described now. To take a
 photograph, a user rotates the winder wheel 14 to advance the photo film
 22 by one frame. The shutter is charged by operation of the one-frame
 advance of the photo film 22. He or she operates the flash charger button
 8 selectively as desired, to set use or lack of use of the flash emission.
 If flash emission is desired, the flash charger button 8 is slid up and set
 in the ON position. The flash switch is turned on to start charging the
 flash unit. The flash unit becomes ready to emit flash light. Also the
 selector mechanism 31 is caused by the flash charger button 8 to position
 the larger opening 27a in the photographing light path 4a of the taking
 lens 4. The shutter drive mechanism 30 is changed over by operation of the
 flash charger button 8 in such a manner that the spring in the shutter
 drive mechanism 30 is changed over to provide lower shutter speed.
 After the user finds the completion of charging by checking a
 light-emitting state of the light guide member 12, he or she pushes the
 release button 10. In response to this the shutter unit is actuated to
 open and close the shutter blade 28. Upon the movement of the shutter
 blade 28 to the fully open position, the synchro switch is turned on to
 emit flash light. While the shutter blade 28 is open, object light having
 entered the taking lens 4 is passed through the larger opening 27a, the
 shutter opening 25 and the exposure aperture 26 and reaches the photo film
 22 to take an exposure thereon. In synchronism with the flash emission,
 the optical indicia imprinting unit 35 is driven to imprint an optical
 indicia to the photo film 22 outside the exposed frame.
 If an exposure is desired outdoors under daylight without using flash
 light, the flash charger button 8 is slid down and set in the OFF
 position. Then the flash switch is turned off to disable the flash
 emission. Also the smaller opening 27b is caused by the selector mechanism
 31 to move into the photographing light path 4a behind the taking lens 4.
 The shutter drive mechanism 30 is changed over in such a manner that the
 spring in the shutter drive mechanism 30 is changed over to provide higher
 shutter speed.
 When the release button 10 is depressed, the shutter unit is actuated to
 open and close the shutter blade 28. The shutter speed is higher than in
 photography with flash emission. Object light passed through the taking
 lens 4 is introduced in the smaller opening 27b at the light balancing
 filter 32, the shutter opening 25 and the exposure aperture 26, becomes
 color-balance-revised object light, and reaches the photo film 22 which is
 exposed.
 Similarly an exposure is taken in each frame. After exposing all the
 frames, a user rotates the winder wheel 14 continuously to wind the photo
 film 22 entirely into the cassette shell 21. He or she forwards the
 lens-fitted photo film unit to a photo laboratory. An operator in the
 photo laboratory removes the photo film cassette 20 from the housing 2,
 and processes the photo film 22. Photographic prints are produced from the
 photo film 22.
 Samples 1 and 2 are hereinafter described in relation to the
 above-constructed lens-fitted photo film unit.
 Sample 1
 A lens-fitted photo film unit according to Sample 1 was produced and
 incorporated the color negative photo film 22 of a daylight type. A fourth
 photosensitive emulsion layer was formed according to Japanese Patent
 Application No. 10-111196 between green-sensitive and red-sensitive
 emulsion layers. The fourth photosensitive emulsion layer had sensitivity
 to light with a wavelength of approximately 520 nm. Although the photo
 film according to Japanese Patent Application No. 10-111196 has photo film
 speed of ISO 400, the photo film 22 according to Sample 1 had photo film
 speed of ISO 800 by use of a great grain size of silver halide.
 In the flash unit 7, a filter was mounted on the front of the
 protector/diffuser plate 7a to set spectral energy distribution in an
 adjusted manner. In FIG. 8, the spectral energy distribution is
 illustrated. As the light balancing filter 32 was mounted on the smaller
 opening 27b, object light passed through the light balancing filter 32 was
 focused on the photo film 22 as color-balance-revised object light when
 the smaller opening 27b was set in the photographing light path 4a of the
 taking lens 4. In FIG. 9, spectral transmittance of the light balancing
 filter 32 is depicted.
 In FIG. 10, values of colorations of the gray reference reflector plate
 with reflectance of 18% are plotted in the hexagonal coordinate system in
 the manner of FIG. 3, the values being obtained according to Sample 1
 including the photo film 22. In FIG. 10, a sign F.sub.1 represents
 reproduced coloration of the gray reflector plate illuminated by the F6
 white fluorescent lamp without using the light balancing filter 32. A sign
 A.sub.1 represents reproduced coloration of the gray reflector plate
 illuminated by the tungsten lamp or standard light source A without using
 the light balancing filter 32. A sign P.sub.1 represents reproduced
 coloration of the gray reflector plate illuminated by the standard light
 source C and reproduced by using the light balancing filter 32. A sign
 P.sub.2 represents reproduced coloration of the gray reflector plate
 illuminated by flash light and reproduced without using the light
 balancing filter 32.
 The distance .DELTA.E is defined between the coloration of the gray
 reference reflector plate with light from the F6 white fluorescent lamp
 and the coloration P.sub.2 of the same with flash light without light
 balancing filter 32. The distance .DELTA.E according to Sample 1 was 0.03,
 and satisfied the condition of .DELTA.E.ltoreq.0.04. The phase angle or
 hue angle .theta. of the coloration of the gray reflector plate reproduced
 while illuminated by the F6 white fluorescent lamp was 55.degree.
 according to Sample 1 with the photo film, and satisfied the condition of
 55.degree..ltoreq..theta..ltoreq.80.degree..
 Relationships of the states of the flash switch being turned on and off in
 relation to the aperture stop, the shutter speed and the filter insertion
 of the light balancing filter 32 into the light path are as indicated in
 the following table.

Aperture Shutter Filter
 Stop Speed Insertion
 Flash Switched On f/5.6 1/60 sec No
 Flash Switched Off f/16 1/125 sec Yes
 This being so, exposures were taken in an appropriate manner both in the
 outdoor photography under daylight and in the indoor photography. As the
 light balancing filter 32 was inserted in the photographing light path 4a,
 it was possible to prevent existence of frames of different color balances
 on the single photo film 22.
 In the photo film 22 to be loaded in Sample 1, information particular to
 Sample 1 was written to the magnetic data tracks of the photo film 22. At
 the time of an exposure with flash light, the optical indicia imprinting
 unit 35 was driven to imprint optical indicia in the position of the
 corresponding frame.
 To produce color prints from the photo film 22, the following operations
 were effected in a color printer for the purpose of obtaining high quality
 of prints.
 1. The taking lens being used was recognized by referring to information
 recorded in the magnetic data track. Distortions and lateral chromatic
 aberration of the optical system were compensated for.
 2. Time information of the photo film was read by referring to the
 information recorded in the magnetic data track. The halftone of the image
 was suitably corrected. Also the image was subjected to the grain-shape
 suppressing process.
 3. The use or lack of the use of the flash emission was recognized from the
 optical indicia. Correction of the color balance and the density at the
 printing time was determined according to the information of the use or
 lack of the use of the flash emission.
 Comparative Example 1
 In contrast to Sample 1, a lens-fitted photo film unit of Comparative
 example 1 incorporated a flash unit for emitting flash light of color near
 to the color of the standard light source C, and did not have the light
 balancing filter 32 to be inserted. Frames were experimentally exposed,
 and then printed. In Comparative example 1, the distance .DELTA.E between
 the colorations was 1.0.
 Comparative Example 2
 A lens-fitted photo film unit was pre-loaded with a conventionally known
 daylight type of color negative photo film without the fourth
 photosensitive emulsion layer. The phase angle or hue angle .theta. of the
 coloration of the gray reference reflector plate reproduced while
 illuminated by the F6 white fluorescent lamp was measured according to
 Comparative example 2 with the photo film, and was 103.degree.. Also, the
 flash light of Comparative example 2 was changed to have the light source
 color of the F6 white fluorescent lamp with reference to coloration of
 reflected gray reflector plate. In Comparative example 2, the distance
 .DELTA.E between the colorations was 0.04.
 Specifics of Sample 1 and Comparative examples 1 and 2 are indicated in the
 following table.