Finder optical device

A secondary image forming type finder opitcal device having an objective lens solely used therefor, comprising, from front to rear, an objective lens unit including at least one positive lens, a relay lens unit formed by arranging a lens of positive refractive power and a lens of negative refractive power in spaced relation, a field lens unit consisting of a positive lens whose front surface is of strong curvature, and an eyepiece lens unit consisting of two positive lenses whose confronting surfaces are of strong curvature, satisfying the following conditions: ##EQU1## where f.sub.P is the focal length of the lens of positive refractive power of the relay lens unit, .nu..sub.P is the Abbe number of its material, R.sub.P is the radius of curvature of a lens surface of the lens of positive refractive power of the relay lens unit which is of strong curvature and faces the lens of negative refractive power of the relays lens unit, f.sub.N is the focal length of the lens of negative refractive power of the relay lens unit, .nu..sub.N is the Abbe number of its material, and R.sub.N is the radius of curvature of a lens surfaces of the lens of negative refractive power of the relay lens unit which is of strong curvature and faces the lens of positive refractive power of the relay lens unit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to secondary image forming type finder optical 
devices and, more particularly, to secondary image forming type finder 
optical devices provided with an objective lens solely used therefor 
separately from the photographic lens and having a predetermined optical 
total length, which devices are suited to, for example, electronic still 
cameras or video cameras. 
2. Description of the Related Art 
Recently, a variety of kinds of photographic systems on the electronic 
still camera which magnetically records video information in the 
small-sized floppy disc have been proposed. Of these proposals, 
particularly for the finder optical device, various types are adopted. 
The electronic still camera differs largely from the conventional camera 
for silver halide photosensitive material in the shape of the entirety of 
the camera depending on how to arrange the floppy disc in the camera body. 
For example, in the case of containing the floppy disc in a chamber whose 
plane is parallel to the optical axis of the photographic lens, the shape 
becomes an axially elongated one as in the motion video camera of the 
unified type of recorder and reproducer, or the like. 
The so-called reverse Galilean finder optical device which has so far been 
suited well to the external finder optical device for the silver halide 
camera, and the real image finder optical device of the primary image 
forming type using the prism for non-reverse erecting image, when applied 
to the electronic still camera, etc., because of their optical total 
length being too short, have given rise to, for example, the following 
problems. 
That is, to secure a sufficiently long eye point by arranging the eyepiece 
lens of the finder optical device at or near the rear plane of the camera, 
the shortage of the optical total length of the finder optical device 
causes the front vertex of the objective lens to be arranged in a 
considerably secluded position from the front plane of the camera. For 
this reason, to secure the finder light beam without eclipse, the size of 
the opening portion for the finder optical device in the front panel of 
the camera housing must be increased, which calls for an increase of the 
distance from the optical axis of the photographic lens to that of the 
finder optical device. Thus, a problem of intensifying the finder parallax 
and others arose. 
The conventional secondary image forming type finder optical devices for 
use in the video cameras or the like, on the other hand, generally become 
too long in the axial direction. Hence, they are not very suited to be 
used in, for example, electronic still cameras. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a secondary image forming 
type finder optical device wherein light from a finder image formed by an 
objective lens unit is further focused by a relay lens unit or the like to 
form a non-reverse erecting finder image to be observed through an 
eyepiece lens unit, and wherein the construction and arrangement and the 
refractive powers of the constituent lenses of each lens unit are so 
properly designed that the optical total length takes a desired value, 
while still permitting the possibility of observing a finder image of high 
quality. 
Another object is to provide a secondary image forming type finder optical 
device suited to the electronic still camera or video camera.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 to FIG. 3 and FIG. 7 to FIG. 9 schematically show numerical examples 
1 to 6 of embodiments of finder optical devices according to the 
invention, respectively. It should be noted that the finder optical device 
of the invention is arranged separately from the photographic lens (not 
shown). In FIG. 1 to FIG. 3, T is an objective lens unit comprising two 
positive lenses Ta and Tb arranged so that their lens surfaces of strong 
curvature face each other. Incidentally, the lens Tb plays chiefly the 
role of a field lens. A relay lens unit R comprises a negative lens Ra and 
a positive lens Rb. A field lens unit F is arranged in the neighborhood of 
a secondary image plane Q, and comprises a positive lens convex toward the 
front. An eyepiece lens unit E comprises two positive lenses Ea and Eb 
arranged so that their lens surfaces of strong curvature face each other. 
In the secondary image forming type finder optical device in this 
embodiment, at first, the objective lens unit T forms a first finder image 
on a primary image plane P, and the relay lens unit R and the field lens 
unit F then focus the light from the first finder image to form a 
non-reverse erecting second finder image on a secondary image plane Q. 
And, the non-reverse erecting second finder image formed on the secondary 
image plane Q is made to be observed by the eyepiece lens unit E. 
The finder optical device of the invention satisfies the following 
conditions: 
EQU 1.0&lt;f.sub.R /f.sub.T &lt;1.8 . . . (1) 
EQU 1.0&lt;f.sub.F /f.sub.E &lt;1.7 . . . (2) 
where f.sub.T, f.sub.R, f.sub.F and f.sub.E are respectively the focal 
lengths of the objective lens unit T, the relay lens unit R, the field 
lens unit F and the eyepiece lens unit E. 
The inequalities of condition (1) concern with the refractive power 
arrangement of the relay lens unit R and the objective lens unit T, which 
is most important in the present embodiment. Now, on the assumption that 
the focal length of the photographic lens and the finder field rate are 
constant, it is possible that, as the composite focal length f.sub.T of 
the objective lens unit T increases, the finder magnification increases. 
But, because the magnification at the primary image plane P becomes large, 
the secondary image forming system comprised of the relay lens unit R and 
the field lens unit F gets harder to correct for aberrations. Meanwhile, 
the secondary image forming system, when the image magnification is unity, 
has a shortest optical total length, taking a value of about 4f.sub.R. 
Thus, the shorter the focal length of the relay lens unit R, the more 
advantageously the optical total length is shortened, but the more 
difficult the aberration correction becomes. 
On account of such a reason as described above, in the present embodiment, 
the focal lengths of the objective lens unit T and the relay lens unit R 
are made so determined that their ratio or f.sub.R /f.sub.T satisfies the 
condition (1). 
When the lower limit of the inequalities of condition (1) is exceeded, it 
is advantageous for the finder magnification and the shortening of the 
optical total length, but the Petzval image surface gets harder to correct 
well. When the upper limit is exceeded, on the other hand, the optical 
total length is increased objectionably, although the aberrations can 
advantageously be corrected. 
The inequalities of condition (2) have a main aim to minimize the diameter 
of the relay lens unit R. In this embodiment, the focal lengths of the 
field lens unit F and the eyepiece lens unit E are made so determined that 
the principal ray of the off-axis pupil which is to pass through the 
center of the observation pupil passes through almost the center of the 
relay lens unit R. Therefore, despite the strengthening of the refractive 
power of the relay lens unit R, the light beam which would otherwise be 
refracted from the marginal zone of the lens can be avoided. Hence, the 
good quality can be secured over the entire area of the observation pupil. 
When the upper limit of the inequalities of condition (2) is exceeded, the 
diameter of the relay lens unit R increases largely, and the diameter of 
the eyepiece lens unit E also becomes larger. Conversely when the lower 
limit of the inequalities of condition (2) is exceeded, the diameter of 
the relay lens unit R becomes larger, the curvature of field produced in 
the field lens unit F becomes impossible to correct. 
Next, when the relay lens unit R is constructed in the cemented form as 
shown in FIGS. 1 to 3, conditions for preserving good optical performance 
are given below. 
They are for the refractive indices N.sub.N and N.sub.P of the materials of 
the negative lens Ra and the positive lens Rb of the relay lens unit R 
respectively, the Abbe numbers .nu..sub.N and .nu..sub.P of the materials 
of the negative lens Ra and the positive lens Rb of the relay lens unit R 
respectively and the radius of curvature RA of the cemented lens surface 
of the relay lens unit R to satisfy the following conditions: 
##EQU2## 
Particularly, the relay lens unit R is constructed so as to satisfy the 
conditions (3) to (5), and the objective lens unit T and the eyepiece lens 
unit E each are constructed with the two lenses whose confronting surfaces 
are of strong curvature, so that the various aberrations are well canceled 
in each lens unit itself, thus achieving good balance of aberration 
correction. 
The inequalities of condition (3) concern with the radius of curvature of 
the cemented lens surface of the relay lens unit R. When the upper limit 
is exceeded, the curvature of field becomes difficult to correct. 
Conversely when the lower limit is exceeded, the spherical aberration on 
the secondary image plane Q becomes over-corrected. 
The inequalities of conditions (4) and (5) concern with the refractive 
indices and Abbe numbers of the materials of the negative lens Ra and the 
positive lens Rb constituting the relay lens unit R. Mainly the condition 
(4) concerns with the refractive index difference for enabling the 
curvature of field to be corrected well, and the condition (5) concerns 
with the Abbe number difference for enabling, among others, the 
longitudinal chromatic aberration to be corrected. 
When the condition (4) is violated, the curvature of field toward the 
marginal zone of the image frame becomes larger. Also, when the condition 
(5) is violated, the chromatic aberration increases. In any case, it 
becomes difficult to obtain the good finder image. 
It should be noted that of the singlet lenses constituting the objective 
lens unit, the field lens unit and eyepiece lens unit, arbitrary one or 
ones may otherwise be constructed in cemented form, comprising a positive 
lens and a negative lens cemented together. According to this, a finder 
optical device better corrected for chromatic aberrations and other 
aberration and having a higher grade of optical performance can be 
achieved. 
Next, desirable conditions in another embodiment which is different from 
the embodiment of FIGS. 1 to 3 in that the relay lens unit of the finder 
optical device is divided as shown in FIGS. 7 to 9 are shown. 
It should be noted that this embodiment, too, satisfies the above-described 
conditions (1) and (2). 
The objective lens unit T comprises two positive lenses Ta and Tb arranged 
so that their lens surfaces of strong curvature face each other. 
Incidentally, the lens Ta may be made up by a plurality of lenses for the 
purpose of improving aberration correction. Also, the lens Tb plays the 
role of a field lens. Hence the primary image is formed in the 
neighborhood of the lens Tb. 
The relay lens unit R comprises a lens Ra of positive refractive power and 
a lens Rb of negative refractive power. An air lens is formed between the 
lenses Ra and Rb. The curvature of one of lens surfaces of the lens Ra of 
positive refractive power which faces the lens Rb of negative refractive 
power is stronger than that of the other surface. The field lens unit F 
comprises one positive lens turning its strong convexity to the object 
side. A secondary image is formed in the neighborhood of the field lens 
unit F. The eyepiece lens unit E comprises two positive lenses Ea and Eb, 
the surfaces of strong curvature of the lenses Ea and Eb facing each 
other. 
What is important in this embodiment is the refractive power arrangement of 
the relay lens unit R. In a case where the focal length of the 
photographic lens and the finder field rate are constant, a longer 
composite focal length of the objective lens unit T enables the finder 
magnification to be greater, but causes the size at the primary image 
plane to get larger. Thus, the difficult point is in the aberration 
correction of the secondary image forming system. Meanwhile, the secondary 
image forming system, when the image magnification is unity, becomes 
shortest in the total length. The shorter the focal length of the relay 
lens unit R, the more advantageously the total length can be shortened, 
but the more difficult the aberrations become to correct. 
Next, conditions for maintaining the desired optical performance are set 
forth as follows: 
##EQU3## 
where f.sub.P is the focal length of the lens Ra of positive refractive 
power of the relay lens unit R, .nu..sub.P is the Abbe number of its 
material, R.sub.P is the radius of curvature of its lens surface of strong 
curvature, f.sub.N is the focal length of the lens Rb of negative 
refractive power of the relay lens unit R, .nu..sub.N is the Abbe number 
of its material, and R.sub.N is the radius of curvature of its lens 
surface of strong curvature. 
The inequalities of condition (6) represent a preferable range on 
aberration correction for the focal lengths of the positive lens Ra and 
the negative lens Rb constituting the relay lens unit R when a shortening 
of the total length by strengthening the refractive power of the relay 
lens unit R is achieved. 
Since the composite focal length of the relay lens unit R has a positive 
refractive power, when the refractive power of the positive lens Ra 
becomes strong as exceeding the lower limit, although it is advantageous 
to shortening the total length, because the diverging action in the relay 
lens unit R weakens, under-corrected spherical aberration is produced. 
Meanwhile, when the refractive power of the negative lens Rb strengthens 
as exceeding the upper limit, it gets harder to achieve a shortening of 
the total length while well correcting the spherical aberration. 
The inequality of condition (7) concerns with the difference between the 
Abbe numbers of the materials of the positive lens Ra and the negative 
lens Rb constituting the relay lens unit R. When the difference between 
the Abbe numbers becomes smaller than the limit, correction of 
longitudinal chromatic aberration gets harder. 
The inequalities of condition (8) are to determine the shape of an air lens 
between the positive lens Ra and the negative lens Rb constituting the 
relay lens unit R. Incidentally, this air lens has a negative refractive 
power. When the upper limit is exceeded, spherical aberration and 
curvature of field both get under-corrected. Conversely when the lower 
limit is exceeded, both of the spherical aberration and the curvature of 
field get over-corrected objectionably. 
It will be appreciated from the foregoing discussion and is even apparent 
from the aberration curves of FIG. 10 to FIG. 12 that according to this 
embodiment, the relay lens unit R of the secondary image forming system is 
divided into the positive lens Ra and the negative lens Rb, and their 
refractive powers are properly arranged, whereby an increase of the degree 
of freedom on aberration correction and a shortening of the optical total 
length can be achieved. 
Another advantage arising from the use of the divided form of the relay 
lens R into the positive lens Ra and the negative lens Rb is that it 
becomes even possible to choose synthetic resin or the like as the optical 
material. 
Next, numerical examples 1 to 6 of the invention are shown. In the 
numerical examples 1 to 6, Ri is the radius of curvature of the i-th lens 
surface counting from front, Di is the i-th lens thickness or air 
separation counting from front, and Ni and .nu.i are respectively the 
refractive index and Abbe number of the glass of the i-th lens element 
counting from front. 
Also, the relations of each of the before-described conditions (1) to (5) 
with the various numerical values in the numerical examples 1 to 3 are 
shown in Table-1. 
______________________________________ 
Numerical Example 1 (FIGS. 1 and 4): 
Exit Pupil Diameter .phi.3; Max. Emergence Angle tan .THETA. = 0.17 
______________________________________ 
R1 = 33.14 D1 = 2.00 N1 = 1.49171 
.nu.1 = 57.4 
R2 = -9.63 D2 = 9.43 
R3 = 7.79 D3 = 4.28 N2 = 1.49171 
.nu.2 = 57.4 
R4 = .infin. D4 = 31.40 
R5 = 25.85 D5 = 0.72 N3 = 1.84666 
.nu.3 = 23.9 
R6 = 7.24 D6 = 2.43 N4 = 1.77250 
.nu.4 = 49.6 
R7 = -22.44 D7 = 29.00 
R8 = 10.62 D8 = 3.20 N5 = 1.49171 
.nu.5 = 57.4 
R9 = .infin. D9 = 24.86 
R10 = .infin. D10 = 1.50 N6 = 1.49171 
.nu.6 = 57.4 
R11 = -20.00 D11 = 0.15 
R12 = 20.00 D12 = 1.50 N7 = 1.49171 
.nu.7 = 57.4 
R13 = .infin. 
______________________________________ 
Note: 
The eye point lies 16 mm behind the vertex of the lens surface R13. 
f.sub.T = 11.35, f.sub.R = 18.0, f.sub.F = 21.61, f.sub.E = 20.37 
______________________________________ 
Numerical Example 2 (FIGS. 2 and 5): 
Exit Pupil Diameter .phi.3; Max. Emergence Angle tan .THETA. = 0.17 
______________________________________ 
R1 = .infin. D1 = 1.80 N1 = 1.77250 
.nu.1 = 49.6 
R2 = -12.44 D2 = 9.60 
R3 = 7.85 D3 = 2.80 N2 = 1.49171 
.nu.2 = 57.4 
R4 = -148.41 D4 = 28.89 
R5 = 17.85 D5 = 0.80 N3 = 1.84666 
.nu.3 = 23.9 
R6 = 7.27 D6 = 2.60 N4 = 1.71300 
.nu.4 = 53.8 
R7 = -18.74 D7 = 25.07 
R8 = 11.09 D8 = 2.40 N5 = 1.49171 
.nu.5 = 57.4 
R9 = .infin. D9 = 22.01 
R10 = 144.93 D10 = 1.80 N6 = 1.49171 
.nu.6 = 57.4 
R11 = -20.70 D11 = 0.15 
R12 = 20.70 D12 = 1.80 N7 = 1.49171 
.nu.7 = 57.4 
R13 = - 144.93 
______________________________________ 
f.sub.T = 11.34, f.sub.R = 15.39, f.sub.F = 22.56, f.sub.E = 18.61 
______________________________________ 
Numerical Example 3 (FIGS. 3 and 6): 
Exit Pupil Diameter .phi.3; Max. Emergence Angle tan .THETA. = 0.17 
______________________________________ 
R1 = 5458.52 D1 = 2.60 N1 = 1.49171 
.nu.1 = 57.4 
R2 = -8.01 D2 = 9.09 
R3 = 8.01 D3 = 2.60 N2 = 1.49171 
.nu.2 = 57.4 
R4 = -5458.52 D4 = 32.92 
R5 = 17.72 D5 = 0.80 N3 = 1.84666 
.nu.3 = 23.9 
R6 = 7.24 D6 = 2.60 N4 = 1.69680 
.nu.4 = 55.5 
R7 = -20.84 D7 = 27.93 
R8 = 12.13 D8 = 2.40 N5 = 1.49171 
.nu.5 = 57.4 
R9 = .infin. D9 = 22.07 
R10 = 389.49 D10 = 1.80 N6 = 1.49171 
.nu.6 = 57.4 
R11 = -19.15 D11 = 0.15 
R12 = 19.15 D12 = 1.80 N7 = 1.49171 
.nu.7 = 57.4 
R13 = -389.49 
______________________________________ 
f.sub.T = 11.30, f.sub.R = 17.0, f.sub.F = 24.68, f.sub.E = 18.66 
TABLE 1 
______________________________________ 
Numerical Example 
Condition 1 2 3 
______________________________________ 
(1) f.sub.R /f.sub.T 
1.59 1.36 1.50 
(2) f.sub.F /f.sub.E 
1.06 1.21 1.32 
(3) .vertline.RA.vertline./f.sub.R 
0.40 0.47 0.43 
(4) N.sub.N -N.sub.P 
0.074 0.134 0.150 
(5) .nu..sub.P -.nu..sub.N 
25.7 29.9 31.6 
______________________________________ 
______________________________________ 
Numerical Example 4 (FIGS. 7 and 10): 
Exit Pupil Diameter .phi.3; Max. Emergence Angle tan .THETA. = 0.17 
______________________________________ 
R1 = 33.142 D1 = 2.00 N1 = 1.49171 
.nu.1 = 57.4 
R2 = -9.632 D2 = 9.43 
R3 = 7.799 D3 = 4.28 N2 = 1.49171 
.nu.2 = 57.4 
R4 = 0.000 D4 = 30.49 
R5 = 20.766 D5 = 2.43 N3 = 1.69680 
.nu.3 = 55.5 
R6 = -7.369 D6 = 0.15 
R7 = -6.828 D7 = 0.72 N4 = 1.58347 
.nu.4 = 30.2 
R8 = -45.349 D8 = 31.19 
R9 = 10.627 D9 = 3.20 N5 = 1.49171 
.nu.5 = 57.4 
R10 = 0.000 D10 = 24.86 
R11 = 0.000 D11 = 1.50 N6 = 1.49171 
.nu.6 = 57.4 
R12 = -20.000 D12 = 0.15 
R13 = 20.000 D13 = 1.50 N7 = 1.49171 
.nu.7 = 57.4 
R14 = 0.000 
______________________________________ 
Note: 
The eye point lies 16 mm behind the vertex of the lens surface R14. 
f.sub.p = 8.09 f.sub.N = -13.87 
______________________________________ 
Numerical Example 5 (FIGS. 8 and 11): 
Exit Pupil Diameter .phi.3; Max. Emergence Angle tan .THETA. = 0.17 
______________________________________ 
R1 = 5032.000 D1 = 2.60 N1 = 1.49171 
.nu.1 = 57.4 
R2 = -8.086 D2 = 9.05 
R3 = 8.086 D3 = 2.60 N2 = 1.49171 
.nu.2 = 57.4 
R4 = -5032.000 D4 = 29.78 
R5 = 10.078 D5 = 0.80 N3 = 1.84666 
.nu.3 = 23.9 
R6 = 6.343 D6 = 0.15 
R7 = 6.986 D7 = 2.45 N4 = 1.49171 
.nu.4 = 57.4 
R8 = -11.800 D8 = 25.07 
R9 = 11.091 D9 = 2.40 N5 = 1.49171 
.nu.5 = 57.4 
R10 = -5032.000 D10 = 23.13 
R11 = 144.930 D11 = 1.80 N6 = 1.49171 
.nu.6 = 57.4 
R12 = -20.709 D12 = 0.15 
R13 = 20.709 D13 = 1.80 N7 = 1.49171 
.nu.7 = 57.4 
R14 = -144.930 
______________________________________ 
f.sub.P = 9.33 f.sub.N = -22.41 
______________________________________ 
Numerical Example 6 (FIGS. 9 and 12): 
Exit Pupil Diameter .phi.3; Max. Emergence Angle tan .THETA. = 0.17 
______________________________________ 
R1 = 5458.520 D1 = 2.60 N1 = 1.49171 
.nu.1 = 57.4 
R2 = -8.017 D2 = 9.09 
R3 = 8.017 D3 = 2.60 N2 = 1.49171 
.nu.2 = 57.4 
R4 = -5458.520 D4 = 32.72 
R5 = 15.012 D5 = 0.72 N3 = 1.58347 
.nu.3 = 30.2 
R6 = 4.807 D6 = 0.15 
R7 = 4.936 D7 = 2.50 N4 = 1.49171 
.nu.4 = 57.4 
R8 = -11.802 D8 = 27.67 
R9 = 12.135 D9 = 2.40 N5 = 1.49171 
.nu.5 = 57.4 
R10 = 0.000 D10 = 22.07 
R11 = 389.490 D11 = 1.80 N6 = 1.49171 
.nu.6 = 57.4 
R12 = -19.154 D12 = 0.15 
R13 = 19.154 D13 = 1.80 N7 = 1.49171 
.nu.7 = 57.4 
R14 = -389.490 
______________________________________ 
f.sub.P = 7.44 f.sub.N = -12.44