Observation optical system and optical apparatus having the same

Abstract: An observation optical system whose eye relief is below 25 mm includes an eyepiece lens (7) having a water absorptive antifogging coating (21) applied on one surface thereof, and a member holding the eyepiece lens (7). Further, an eyeball-characteristic detecting system includes a light source for illuminating the eyeball of an observer, a light receiving element arranged on detection of the reflected light from the eyeball (201) through the eyepiece to produce a signal, an optical member positioned in the path of the light from the light source to the light receiving element and having an antifogging coating (21) on one surface thereof, and a circuit receptive of the signal for detecting the characteristic of the eyeball (201).

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an observation optical system, and, more particularly, to such systems which are suited to be used in the viewfinder optical systems of cameras or the like. Description of Related Art Fig. 1 is a longitudinal section view of the conventional single-lens reflex camera comprising an observation optical system having the features summarized in the preamble of claim 1. In Fig. 1, a camera body 101 with a photographic lens 102 contains a return mirror 103, a viewfinder system 104 composed of a focusing screen 105, a pentagonal prism 106 and an eyepiece lens 107, and a back cover 108. The photographer, while keeping hold of his or her eye 109 in axial alignment with the eyepiece lens 107, observes an image formed on the focusing screen 105 by the photographic lens 102. In such a holding position of the camera, as the photographer breathes through the mouth or nose, a warm humid air ascends along the surface of the back cover 108, reaching the space between the'eyepiece lens 107 and the eye 109 of the photographer. When the ambient temperature is low to make the temperature of the surface of the camera body 101 or the eyepiece lens 107 low, the cold surface of the eyepiece lens 107 partly cools the breath. So, in some events, dewdrops are deposited on the eyepiece lens 107 to make the eyepiece lens 107 fogged, so that it becomes difficult to observe the viewfinder image. Particularly, with a mask put on the nose and mouth for the purpose of protection against the cold, the eyepiece lens 107 tends to get fogged more easily. To avoid such a state from occurring, the photographer is obliged to stop breathing until a shot is completed. In another situation that, when the camera is moved from one place to another where the environment greatly changes in temperature or humidity, it also happens that condensation of moisture, not owing to the breath, takes place on the optical parts constituting the viewfinder. To solve such problems, a method is known which uses solutions of surface active agents or the like having the antifogging effect. The solution is thinly applied to the surface of the eyepiece lens 107. Besides this, as shown in Fig. 2, it can be also considered to use a hood 110 surrounding the periphery of the eyepiece lens 107 so that the breath hardly touches the eyepiece lens 107. For the breath fogging problem that becomes serious when the camera is used at a low temperature, different methods are available as described above. However, any of these methods has the following drawbacks. The method using the surface active agent does not insure that, as the outer surface of the eyepiece lens is somewhat deep from the frame thereof, the solution is uniformly applied to the entire area of that surface. The antifogging effect is ensured only temporarily. Further, the surface active agent when applied to the surface makes it hydrophile. In other words, the wettability is enhanced, thus producing the antifogging effect. Therefore, a thin film of water is formed on the surface. As the condensed amount of moisture increases, the image to be observed is possible to eventually distort. In the other method which uses the hood shown in Fig. 2, the length L of the hood 110 must be made longer than a certain value to obtain a sufficient antifogging effect. It varies depending on the conditions of the environment. For example, at a temperature of -10°C, the desired length L is found to be 20 mm or longer. Otherwise, no antifogging effect can be obtained. In this case, the eye 109 of the observer is necessarily spaced away from the eyepiece lens 107 by a distance of about 25 mm. To measure how long the eye may be put away from the eyepiece lens to observe the entire area of the field of the viewfinder, a numerical variable called "eye relief" is usually adopted. For a viewfinder system whose eye relief exceeds 25 mm, its physical size becomes large. Therefore, a problem arises in that its price becomes high. In the meantime, it has been known to provide the observation optical system such as the viewfinder of the camera with a visual line detecting device for detecting that location on the viewfinder image which the observer looks at, or for detecting the direction of a visual line of the observeobserver (document EP-A-0 657 767). With such a visual line detecting device, in detecting the current visual line of the observer, it becomes possible either to determine a specific focusing position on which the photographic lens is to be adjusted for focusing by the automatic focusing device built in the camera, or to selectively choose the menu displayed in the viewfinder. Thus, the photographer can operate the camera with higher mobility. It is also known to provide a technique of detecting the refractive power of the eyeball of the observer, i.e., the diopter, to automatically drive a diopter correcting optical system, thus adjusting the diopter. To make determination of the above-described eyeball characteristic such as the visual line and diopter of the observer, many principles have been proposed. Of these, there is a one that a light-emitting diode or like light source illuminates the eyeball of the observer and an optical system adjacent to the eye receives the reflected light from the eyeball and forms an image of the eyeball on a light receiving element (sensor). Using the image information as the signal from the sensor, the current eyeball characteristic is detected. This method is employed in many ordinary optical apparatus. In the case of, for example, the camera, depending on the environment in which the camera works, it is possible that, as described before, fogging due to the temperature or humidity affects some optical parts of the above-described optical systems for illumination and for receiving the reflected light. If the fogging occurs, then not only the visual line and diopter become impossible to detect but also a faulty operation will take place to effect an unintentional result. Document EP-A-0 716 051 discloses an antifogging coating which has the property of absorbing water and which can be formed on substrates like optical lenses, spectacles, window glasses for vehicles and films for ink-jet printers. The thickness of the antifogging coating is preferably 0.01 to 1 µm. Document JP-A-57 130 022 discloses a viewfinder attachment which can be attached to a viewfinder of a camera and which, when attached to the camera, provides an eye relief of not less than 50 mm. BRIEF SUMMARY OF THE INVENTION The present invention has been made with the above-described problem in mind, and it is a general object of the invention to provide an observation optical system which has a permanent antifogging effect while still not sacrificing the comfortableness of viewing. This object is achieved by the observation optical system according to claim 1. According to the invention, on that eyepiece surface of said eyepiece optical system which is closest to the observer an antifogging coating is formed which comprises a first antifogging layer having a property of absorbing water formed on said eyepiece surface, a thin film of high refractive index made from at least one of metal alkoxide and composites of metal alkoxide and inorganic materials formed outside of said first antifogging layer, and a second antifogging layer having a property of absorbing water formed outside of said thin film layer, wherein said first antifogging layer has a thickness between 0.5 µm and 10 µm inclusive In accordance with another aspect of the invention, there is provided an optical apparatus comprising the observation optical system of the invention. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING Fig. 1 is a schematic view of the construction of a conventional single-lens reflex camera. Fig. 2 is a schematic view of the construction of the conventional single-lens reflex camera provided with a hood. Fig. 3 is a schematic view of the construction of a single-lens reflex camera comprising an observation optical system according to a first embodiment of the invention. Fig. 4 is a schematic view of the construction of a single-lens reflex camera comprising an observation optical system according to a second embodiment of the invention. Figs. 5A and 5B are schematic views of the construction of an eyepiece portion in the second embodiment. Fig. 6 is a sectional view of the structure of layers in an antifogging coating of the observation optical system according to the invention. Fig. 7 is a schematic view of the construction of a single-lens reflex cammera comprising an observation optical system according to a third embodiment of the invention. Fig. 8 is a schematic view of an ideal image of the eyeball. Fig. 9 is a schematic view of the construction of a single-lens reflex camera comprising an observation optical system according to a fourth embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION Hereinafter, preferred embodiments of the invention will be described in detail with reference to the drawings. Fig. 3 is a longitudinal section view of the construction of a single-lens reflex camera having an observation optical system according to an embodiment of the invention. In Fig. 3, a camera body 1 with a photographic lens 2 contains a return mirror 3, a viewfinder system 4 composed of a focusing screen 5, a pentagonal prism 6 and an eyepiece lens 7, and a back cover 8. The photographer, while keeping hold of his or her eye 9 in axial alignment with the eyepiece lens 7, observes an image formed on the focusing screen 5 by the photographic lens 2. While holding the camera in such a position, the observer looks through the viewfinder system. Then, the breath from the mouth or nose of the photographer ascends along the surface of the back cover 8, passing through the space between the eyepiece lens 7 and the eye 9. When the ambient temperature is low, it is in the conventional arrangement that part of the breath touches the cold surface of the eyepiece lens 7, where the condensation of moisture results in fogging. In the present embodiment, on the other hand, of the optical members constituting the eyepiece lens 7, at least one surface which is exposed to the open air, i.e., the surface 10 that is closest to the observer, is provided with an antifogging coating having a property of absorbing water. The eyepiece lens 7 is thus prevented from being fogged by the breath of the observer. In a case where a long eye relief can be secured, as described before, the hood may be attached to the eyepiece lens 7, reducing the proportion of that part of the breath which directly hits the eyepiece lens 7. Fogging is thus made possible to avoid. However, in a case where the eye relief is shorter than 25 mm., such a method can hardly prevent the fogging from occurring completely. Therefore, the observation optical system of the invention, in which the eye relief is shorter than 25 mm, is provided with the antifogging coating, produces the greatest effect when. For an observation optical system of smaller size, especially the viewfinder with the eye relief made shorter than 15 mm, the invention is more advantageous. The antifogging coating has a multi-layer construction the details of which will be explained with reference to Fig.6 and comprises a first antifogging layer having a property of absorbing water. To form the antifogging layer having a property of absorbing water, many antifoggy agents are known. In the first embodiment, use is made of what is disclosed in document JP-A-8 231 944, equivalent to document EP-A-071605. This antifoggy agent is a composition of (1) inorganic alkoxide and at least one polymer formed from this alkoxide by hydrolysis and poly-condensation and having OH radicals, (2) polyalkylene oxide, (3) catalyst, and (4) water-contained organic solvent. As the alkoxide cited above, at least one of the compounds expressed by the following formula is preferably usable. M(OR) n (X) a-n where M is an inorganic atom selected from the group consisting of Si, Al, Ti, Zr, Ca, Fe, V, Sn, Li, Be, B and P, R is alkyl radicals, X is alkyl radicals with or without functional groups, or halogen, "a" is the valence of M, and "n" is an integer of from 1 to "a". As the polyalkylene oxide cited above, polyethylene oxide is preferably used. As the basic catalyst cited above, N,N-dimethylbenzylamine is preferably used. In addition to the ingredients described above, at least one of polyacrylic acids may be included. More specifically, polyacrylic acid, poly-methacrylic acid and their salts may be mentioned. At least one selected out of these is preferably used. A specific example of the above-described antifoggy agent in respect to the ingredients with weight parts is shown in Table-1 below. 10% polyethylene oxide in methanol 81.52 5% polyacrylic acid in methanol 0.26 N,N-dimethylbenzylamine 0.01 γ-glycid propyltrimethoxysilane 13.59 Aluminum isopropoxide in ethanol 4.62 According to the weight proportion of the Table-1, the polyacrylic acid, N,N-dimethylbenzylamine and γ-glycidoxy propyltrimethoxysilane are added to the methanol solution of 10% polyethylene oxide (mean molecular weight: 300,000). The mixture is stirred for 10 minutes. After that, as aluminum isoproxide was previously hydrolyzed, its methanol solution (containing 5% Al 2 O 3 ) is added. The mixture is stirred for 30 minutes. The thus-obtained solution is applied either to both surfaces of the eyepiece lens, or to the one of the surfaces which is exposed to the outside and is dried (annealed) by heating to, and keeping at, 150°C for 10 minutes. The obtained eyepiece lens was incorporated in the camera and left lying for 2 hours in an atmosphere of -10°C. After that, in the same atmosphere, the observer looked through the viewfinder, while breathing through the mouth or nose. However, the eyepiece lens did not get fogged in every test. Since the ability the above-described antifoggy agent has to prevent fogging from occurring relies on its property of absorbing water, so that it produces such an antifogging effect, the first antifogging layer coated on the lens surface must have a certain thickness. In the case of the antifoggy agent or composition described above, to get satisfactory results in the usual photographic situations, the first antifogging layer is thickened to not less than at least 0.5 µm. A more desirable coating is 1 µm or thicker. If too much thick a layer is formed on the lens, on the other hand, the dimensional accuracy of the surface becomes worse and, therefore, the image to be observed deteriorates. To assure observation without extreme deterioration of the image, the first antifogging layer on the lens surface is 10 µm or less. If thinner than 5 µm, it is more desirable. In the first embodiment, the application of the antifoggy agent is carried out by any technique known in the art using the dipping method, the spray method or the spin method. In a case where one cycle of coating operation hardly assures formation of a layer thick enough, the cycle of coating and annealing operations may be repeated twice or more. Also, although the first embodiment has been described in connection with the first antifogging layer applied directly to an optical member having a refractive power, that is, the eyepiece lens, a variation may be made by using a transparent plate of glass as the protection member having no refractive power at a closest position of the eyepiece optical system to the observer. The antifogging layer is applied to the one of the parallel flat surfaces of the plate which faces the observer, or both of them. The flat plane makes more uniform the layer than the surface of curvature and the plane accuracy is easier to hold than the surface of curvature accuracy. Further, if it is a plane surface, a plate of large size can be used as the substrate for applying the layer thereon. In the later process, it can be cut to a necessary size. So, it is advantageous even in the point of view of the mass production. Fig. 4 is a longitudinal section view of a single-lens reflex camera according to a second embodiment of the invention, where the same parts as those of Fig. 3 are denoted by the same reference numerals. In the second embodiment, an eyepiece lens 7' is different from the eyepiece lens 7 of the first embodiment in that its surfaces are not coated with the antifoggy agent. Instead of this, the second embodiment employs an antifogging unit 12 (optical attachment) including a transparent member 11 on the observer side of the eyepiece lens 7'. The transparent member 11 is a flat plate of glass whose both surfaces are parallel to each other, and at least the one of them which confronts the eye of the observer, namely, the surface 13, is coated with an antifoggy agent similar to that of the first embodiment. The antifogging unit 12 of the second embodiment is made attachable to and detachable from the camera body 1 by a mechanism described below. Figs. 5A and 5B in an enlarged scale show only the eyepiece lens of Fig. 4 and its neighborhood, with Fig. 5A in an axial sectional view and Fig. 5B in an elevation back view as looked from the observer. In Figs. 5A and 5B, the antifogging unit 12 has an axial opening 14 of rectangular shape in which the transparent member 11 is fixedly mounted, and is movably fitted on a coupling frame 15 on the camera body 1. A claw member 16 fixedly secures the antifogging unit 12 to the coupling frame 15 of the camera. Referring to Fig. 5B, the left and right sides of the coupling frame 15 have grooves 17 formed therein. The claw member 16, which is made of a resilient material, has a letter "U" shape with the open side down, so that the space between its free ends 18 is horizontally expandable. When attaching the antifogging unit 12, the user puts the free ends 18 of the claw member 16 above the coupling frame 15 in alignment with the grooves 17 and slides the antifogging unit 12 downward until the free ends 18 of the claw member 16 drop over the corners of the coupling frame 15. The antifogging unit 12 is thus locked. To release the antifogging unit 12 from the locking connection, the user presses the antifogging unit 12 from both sides at the upper half 19 thereof, thereby opening the free ends 18 of the claw member 16. While keeping the pressure, the user slides the antifogging unit 12 upward. The antifogging unit 12 is thus detached from the camera body 1. In the state that the antifogging unit 12 is attached to the camera, the space between the eyepiece lens 7' and the transparent member 11 is desired to be air-tightly shielded, especially at the bottom, so that the breath of the observer cannot enter that space. However, to make easy the attaching or detaching operation, a gap of necessary minimum may be provided in between the coupling members. Even if so, almost no decrease of the antifogging effect has been found because the flow of breath is sufficiently blocked from entering. Although the second embodiment has been described as using the transparent member 11 of the antifogging unit 12 in the form of a parallel flat plate of glass, it is also possible that the transparent member 11 takes the form of a lens. With the antifogging layer on both surfaces or one surface of that lens, the antifogging unit 12 then gets not only the antifogging function but also a function of correcting the diopter. As a surface in the optical parts of the viewfinder of the camera is selected for application of the antifogging layer, that surface after being coated is desired to improve the light transmission by suppressing the reflection as far as possible. With this regard, the antifogging coating is constructed to have the antireflection effect. To this purpose, many schemes are already available. Of these, the one that allows the antifogging aspect and the antireflection aspect to be compatible and is excellent in durability is suitably used, as disclosed in document EP-A-0 896 229, published on February 10, 1999. This scheme is that a thin film of high refractive index is made from metal alkoxide or at least one of the composites of metal alkoxide and inorganic materials and that this film is sandwiched between antifogging layers. Fig. 6 is a sectional view of the construction of the multi-layer antifogging coating which is given the antireflection effect by the scheme described above. In Fig. 6, a substrate glass 20 on which to apply such an antifogging coating corresponds to the eyepiece lens 7 of Fig. 3 or the transparent plate 11 of Fig. 4. The coating comprises the first antifogging layer 21, an intermediate layer 22 of titan tetraisopropoxide and a second antifogging layer 23 made of a similar material to that of the first antifogging layer 21. For the purpose of convenience, the substrate glass 20 is depicted as a flat surface, but may have a curved surface, of course. The thicknesses of these layers are shown in Table-2 below. First Antifog Layer 3 µm Titan Tetraisopropoxide Layer 30 nm Second Antifog Layer 135 nm In Table-2, the first antifogging layer 21 and the titan tetraisopropoxide layer 22 are in actual thicknesses, while the second antifogging layer 23 is in optical thickness (= refractive index × thickness.) The first antifogging layer 21 is formed in the away and by using the antifoggy agent already described in connection with the first embodiment. Its thickness is concerned with the antifogging performance and the surface accuracy and is determined to a selected value out of the range of 0.5 µm to 10 µm, or desirably of 1 µm to 5 µm, depending on the aims, as mentioned before. For the titan tetraisopropoxide layer 22, a solution of titan tetraisopropoxide in isobutyl acetate is applied by the dipping method and then dried. This layer has a refractive index of 1.7 or higher and plays a role of increasing the antireflection effect. The second antifogging layer 23 is very thin as compared with the first antifogging layer 21. Therefore, the antifoggy agent has to be diluted to a low viscosity by a solvent or the like, before it is applied. The thickness of the second antifogging layer 23 is determined to be 1/4 of a wavelength at which to reduce the reflectance to a lowest value (or the design wavelength). In the case of the second embodiment, the design wavelength is 540 nm (= 135 nm × 4). The above-described coating was formed on the transparent member 11 of Fig. 4 and, when tested, proved a sufficient antireflection effect without sacrificing the antifogginess. For the antireflection capability to impart, the construction of the coating and the method of forming the layers are not limited to those described in connection with Fig. 6. Likewise as that disclosed in Japanese Patent Application No. Hei 9-212088, corresponding to JP-A-11052104, a porous inorganic layer may be formed on the antifog layer. By the way, in the atmospheric condition that the ordinary optical system without the antifogging provision gets fogged, moisture condenses not only on the optical system but also on the exposed-to-air surfaces of the members that hold it. As the condensed amount increases, dewdrops grow, eventually running down to the surfaces of the optical system. With such situations in mind, the invention is desired to apply its antifogging coating to that optical system which is mounted in a mechanism constructed with parts of materials which exhibit somewhat high a hydrophilicity. The above-described running of dewdrops is thus prevented or lessened. For the mounting members, the hydrophilicity in terms of the angle of contact with water is necessarily less than 90° and more desirably 70° or less. In actual practice, resin materials that meet such a condition is chosen to use, or the surface is finished to a minute roughness. It will be appreciated from the foregoing that, according to the first and second embodiments, it becomes possible to realize an observation optical system which has a permanent antifogging effect while still not sacrificing the comfortability of observation. A third embodiment of the invention is next described as applied to the viewfinder of camera which is provided with a visual line detecting means for detecting the visual line of the eyeball of an observer. Fig. 7 is a sectional view of the main parts of the third embodiment. In Fig. 7, an image of an object to be photographed is formed on a focusing screen 31 by a photographic lens (not shown). A pentagonal prism 32 laterally reverses and erects the image, which is observed through an eyepiece lens 33. A dichroic prism 34 has a cemented surface 35 that transmits the visual light but reflects an infrared light which cannot be perceived by the observer, constituting an optical unit of the eyeball-characteristic (visual line) detecting system together with an image forming lens 36, a light receiving element 37 such as area sensor and a light-emitting diode 38. The light-emitting diode 38 emits an infrared light of that nature which undergoes reflection from the cemented surface 35 of the dichroic prism 34. To the observer looking through the viewfinder, this light is invisible when illuminating his or her eyeball 201 by light projection means (not shown). The eyeball 201 has a cornea 39, an iris 40 and a pupil 41. The third embodiment, although using the dichroic prism 34, insures that, because its cemented surface 35 has a spectral characteristic that reflects the infrared light but completely transmits the visible light, the object image on the focusing screen 31 the observer views as the laterally reverse, erected image through the pentagonal prism 32 can retain the same brightness as in the conventional camera. In the meantime, the light emitted from the light-emitting diode 38 is projected by a projection lens or like means (not shown) onto the eyeball 201. The reflected light from the cornea 39 and iris 40 passes through the eyepiece lens 33 in the reverse direction to that with the light coming from the focusing screen 31 and going to be observed. It then reflects from the cemented surface 35 in the dichroic prism 34 upward, and then passes through the image forming lens 36, by which information concerning the eyeball, for example, Purkinje image, is formed on the light receiving element 37. Therefore, the eyepiece lens 33 and the dichroic prism 34 are used as the common members in both of the viewfinder optical system and the optical system for detecting the visual line of the eyeball 201. Fig. 8 is a top view of the image receiving surface of the light receiving element 37, schematically showing what images about the eyeball 201 are cast thereon at this time. In Fig. 8, images 42 and 43 are of the iris 40 and pupil 41, respectively. An image of the light-emitting diode 38 is formed with the light reflecting from the cornea 39 of Fig. 7, constituting a first Purkinje image 44. Using the signal outputted from the light receiving element 37, the visual line of the observer is detected by a method known in the art. As this detecting method, use may be made of that disclosed in, for example, document JP-A-61 172 552. In this document, the image signal from the light receiving element 37 is processed to find the center of the pupil image 43 and the location of the first Purkinje image 44. From the relationship between their current positions, which direction the observer looks to is detected. To assure detection of the visual line with high accuracy and reliability even in a wide variety of atmospheric conditions described before, it is in the third embodiment that at least the closer surface 45 of the eyepiece lens 33 of Fig. 7 to the observer is provided with an antifogging coating. Another coating of such property may be applied to the inner surface 46 of the eyepiece lens 33. If so, the inside fogging resulting from the change of the ambient temperature also can be prevented well. The antifogging coating may be applied to not only the eyepiece lens 33 but also other members such as dichroic prism 34 and image forming lens 36. According to this, the accuracy of detection can be prevented from lowering due to the fogging. Further, the light receiving element 37 and the optical parts through which the light from the light-emitting diode 38 travels may be given the antifogging coating likewise. As the antifogging coating in the third embodiment, the coating is employed which, as described in the first and second embodiments, has the property of absorbing water in producing the antifogging effect. This is because a hydrophilic coating lets a thin film of water be left on the surface and a water repellent coating is difficult to remove the dewdrops completely. In either case, the optical performance is caused to deteriorate. Unlike these, the water absorptive coating diminishes dewdrops as soon as they appear, so that it is most excellent in the point of view of the good stability of optical performance. For the antifogging coating of water absorptiveness to make up, a specific example of the antifoggy agent is similar to those shown in the first and second embodiments. So, the description thereof is omitted here. Also in the third embodiment, the antifogging coating is given the antireflection property like that in the first and second embodiments. However, when putting the third embodiment to practice, it is preferred that the antireflection layer has such characteristics that not only the light transmission through the viewfinder of the camera is improved but also the electrical signal representing the detected visual line is deprived of noises resulting from the ghost and flare. That is, even for the infrared light from the light-emitting diode 38, a sufficient antireflection effect has to be produced. Based on the various experiments, it has been found that the visual line is detected in good results when the reflectance for the wavelength of the light-emitting diode of the antifogging coating is limited to not more than 2%. On consideration of the situation that, as the observer wears glasses, strong unnecessary light rays are liable to arise, the reflectance must be further reduced to 1% or lower. In such a situation, it is preferred to employ the scheme like that disclosed in Japanese Patent Application No. Hei 9-212088, corresponding to JP-A-11052104, where a porous inorganic layer is formed on the top of the antifog layers. Fig. 9 is a sectional view of the main parts of a fourth embodiment of the invention, where the same reference numerals as those of Fig. 7 are used to denote the same elements. The fourth embodiment is different from the third embodiment of Fig. 7 in a point that a protection member 47 in the form of a parallel flat plate is provided on the observer side of the eyepiece lens 33 and at least one of its surfaces, namely, the closer surface 48 to the observer is formed with antifogging coating thereon. The other constructional features are the same. In general, uniform coatings are more easily applied to flat surfaces than to curved surfaces. In the fourth embodiment, such a plate is selected to form the antifogging coating thereon, thus retaining the good surface accuracy. Further, if the substrate is a plate whose both surfaces are flat and parallel to each other, the size of the plate can be increased greatly. In the later process, it can be cut to necessary sizes. So, it is advantageous even in the point of view of the mass production. Further, the protection member 47 may be built in an appropriate carrier, which is made releasably attachable to the coupling member for the diopter correcting lens or the like provided on the housing of the eyepiece lens of the camera. If so, the invention becomes applicable to the existing cameras having no antifogging coatings. In the situation where no antifogging effect is necessary, the carrier can be removed to elongate the eye relief. The viewfinder becomes more comfortable to look through. Although the foregoing embodiments have been described as applied to part of the viewfinder of the camera, the invention is not confined thereto and applicable to binoculars, telescopes, microscopes, endoscopes, etc., generally speaking, to a wide variety of commonly available observation optical systems. Also, the eyeball characteristic to be detected includes not only the visual line of the observer mentioned in the foregoing embodiments, but also the degree of sight, the state of the eyeground and other factors that can be detected by the reflected light from the eyeball. It will be appreciated from the foregoing that, according to the third and fourth embodiments, at least one of the optical members constituting the optical system for detecting the characteristic of the eyeball of the observer, is provided with an antifogging coating, thereby making it possible to achieve an eyeball-characteristic detecting system which can detect well the characteristic of the eyeball no matter how the ambient atmosphere and the environmental conditions may change, and to achieve an optical apparatus having the eyeball-characteristic detecting system. In particular, despite of any changes of the ambient atmosphere and the environmental condition, the accuracy of detection is prevented from lowering. An eyeball-characteristic detecting system which has such characteristic features, or an observation optical system having the eyeball-characteristic detecting system, is possible to realize easily.

Claims: An observation optical system having an eye relief of not more than 25 mm, said observation optical system comprising an eyepiece optical system (6, 7; 6, 7', 11; 32, 33, 34; 32, 33, 34, 47) and a holding member arranged to hold the eyepiece optical system, wherein light from an object being observed is led through said eyepiece optical system to an observer, characterized in that on that eyepiece surface (10, 13, 45, 48) of said eyepiece optical system (6, 7; 6, 7', 11; 32, 33, 34; 32, 33, 34, 47) which is closest to the eye of the observer, an antifogging coating is formed which comprises a first antifogging layer (21) having a property of absorbing water formed on said eyepiece surface (10, 13, 45, 48), a thin film layer (22) of high refractive index made from at least one of metal alkoxide and composites of metal alkoxides and inorganic materials formed between said first antifogging layer (21) and a second antifogging layer (23), the second antifogging layer (23) having a property of absorbing water formed burside of said thin film layer (22), wherein said first antifogging layer (21) has a thickness between 0.5 µm and 10 µm inclusive. An observation optical system according to claim 1, wherein said eyepiece optical system (6, 7; 32, 33, 34) comprises an eyepiece lens (7, 33), and wherein said antifogging coating is formed on said eyepiece lens (7, 33). An observation optical system according to claim 1, wherein said eyepiece optical system (6, 7', 11; 32, 33, 34, 47) comprises an eyepiece lens (7', 33) and a transparent protection member (11, 47) for protecting the eyepiece lens (7', 33), and wherein said antifogging coating is formed on said protection member (11, 47). An observation optical system according to claim 3, wherein said protection member (11) is attachable to and detachable from the remainder of said eyepiece optical system (6, 7', 11). An observation optical system according to one of claims 1 to 4, wherein said first antifogging layer (21) is made of a composition containing inorganic alkoxide and at least one polymer having OH radicals produced by hydrolysis and poly-condensation of the inorganic alkoxide. An observation optical system according to one of claims 1 to 5, wherein said holding member has an angle of contact with water of not more than 90°. An observation optical system according to one of claims 1 to 6, further comprising; a light source (38), which emits light for illuminating an eyeball of the observer, a light receiving element (37) which receives the light reflected by the eyeball, through said eyepiece optical system (32, 33, 34; 32, 33, 34, 47); and a detecting system (33 to 38) which detects a characteristic of the eyeball by utilizing a signal outputted from said light receiving element (37). An optical apparatus comprising: an observation optical system according to one of claims 1 to 7.

Inventors: IKEMORI KEIJI, OHTAKA KEIJI, IKEMORI, KEIJI, OHTAKA, KEIJI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1998
Main classification: G03B13
IPC codes: G03B13, G02B1