Digital cameras abstract
A finder for single-lens reflex type digital cameras includes a
reflecting plane for splitting a light beam from an imaging optical
system of a camera into two beams directed toward an image sensor
and a finder system, a prism system for erecting an image, and an
ocular optical system. The prism system has an entrance surface,
a reflecting surface consisting of one plane, a reflecting surface
of roof shape, and an exit surface. An optical path between the
entrance surface and the reflecting surface consisting of one plane
is practically perpendicular to an optical path between the reflecting
surface of roof shape and the exit surface in the prism block.
Digital cameras claims
What is claimed is:
1. A finder for single-lens reflex type digital cameras, comprising:
a reflecting plane constructed and arranged to split a light beam
from an imaging optical system of a camera into two beams directed
toward an image sensor and a finder system;
an image-erecting prism system; and
an ocular optical system,
wherein said image-erecting prism system and said ocular optical
system are arranged along a traveling direction of light in said
finder system placed after a position of the image formed by said
imaging optical system,
wherein said prism system comprises, in order along the traveling
direction of the light in one prism block, an entrance surface,
a first reflecting surface consisting of one plane face, a second
reflecting surface of roof shape, and an exit surface,
wherein said first reflecting surface folds an optical path toward
an opposite side of said ocular optical system in reference to said
exit surface, and
wherein the optical path between said entrance surface and said
first reflecting surface intersects, inside the prism block, an
optical path between said second reflecting surface and said exit
surface.
2. A finder according to claim 1, wherein said finder satisfies
the following conditions:
where L is an axial distance from said entrance surface to said
exit surface of said prism system, np is a refractive index of a
medium of said prism system, fE is a combined focal length of said
finder system on a pupil side of a position of the image formed
by said imaging optical system, L.sub.1 is an axial distance from
said entrance surface to said first reflecting surface of said prism
system, L.sub.2 is an axial distance from said second reflecting
surface to said exit surface of said prism system, and .theta..sub.1
is an angle made by an optical axis from said entrance surface to
said first reflecting surface, of said prism system with a normal
line to said first reflecting surface.
3. A finder according to claim 1, wherein said ocular optical system
includes, in order from an object side, a positive lens having an
object-side surface that is convex toward the object side and a
negative lens having a pupil-side surface that is concave toward
a pupil side, and
satisfies the following conditions:
where R.sub.31 and R.sub.32 are radii of curvature of the object-side
surface and a pupil-side surface, respectively, of said positive
lens and R.sub.33 and R.sub.34 are radii of curvature of an object-side
surface and the pupil-side surface, respectively, of said negative
lens.
4. A finder according to claim 3, wherein said ocular optical system
has aspherical surfaces and is constructed so that said negative
lens is movable for diopter adjustment, said ocular optical system
further satisfying the following condition:
where fE is a combined focal length of said finder system on a
pupil side of a position of the image formed by said imaging optical
system.
5. A finder according to claim 4, wherein said prism system for
erecting an image is constructed so that said exit surface thereof
is shaped into a curved surface which is convex toward the pupil
side and satisfies the following condition:
where RP is a radius of curvature of the exit surface of said prism
system for erecting the image.
6. A finder according to claim 1, wherein said finder system is
placed on a transmission side of said reflecting plane for splitting
a light beam from said imaging optical system.
7. A finder according to claim 1, wherein said finder satisfies
the following condition:
where H is a longitudinal dimension of a finder field and V is
a lateral dimension of the finder field.
8. A finder for single-lens reflex type digital cameras, comprising:
a reflecting plane for splitting a light beam from an imaging optical
system of a camera into two beams directed toward an image sensor
and a finder system;
a prism; and
an ocular optical system,
wherein said prism and said ocular optical system are arranged
along a traveling direction of light in said finder system placed
after a position of the image formed by said imaging optical system,
wherein said prism comprises, in order along the traveling direction
of the light, a first reflecting surface, a second reflecting surface,
and a third reflecting surface,
wherein a normal line to said first reflecting surface and a normal
line to said second reflecting surface lie in a plane and intersect
one another,
wherein said plane intersects an optical axis of said imaging optical
system,
wherein an axial light beam from said imaging optical system is
reflected by said third reflecting surface, to be substantially
parallel with the optical axis of said imaging optical system,
wherein said ocular optical system includes, in order from an object
side, an object side surface of a biconvex lens that has a larger
absolute value of curvature than an absolute value of curvature
of a pupil-side surface thereof and a pupil side surface of a biconcave
lens that has a larger absolute value of curvature than an absolute
value of curvature of an object-side surface thereof, and
wherein said finder satisfies the following conditions:
where l.sub.1 is an axial distance from said first reflecting surface
to said second reflecting surface of said prism, l.sub.2 is an axial
distance from said second reflecting surface to said third reflecting
surface of said prism, L is an axial distance from an entrance surface
to an exit surface of said prism, np is a refractive index of a
medium of said prism, and fE is a combined focal length of said
finder system on a pupil side of a position of the image formed
by said imaging optical system.
9. A finder according to claim 8, wherein said ocular optical system
has aspherical surfaces and is constructed so that said biconcave
lens is movable for diopter adjustment, said ocular optical system
further satisfying the following condition:
where fE is a combined focal length of said finder system on a
pupil side of a position of the image formed by said imaging optical
system, R.sub.32 is a radius of curvature of the pupil-side surface
of said biconvex lens, and R.sub.33 is a radius of curvature of
the object-side surface of said biconcave lens.
10. A finder according to claim 9, wherein said prism is constructed
so that said exit surface thereof is shaped into a curved surface
which is convex toward the pupil side and satisfies the following
condition:
where RP is a radius of curvature of the exit surface of said prism.
11. A finder according to claim 8, wherein said finder system is
placed on a transmission side of said reflecting plane for splitting
a light beam from said imaging optical system.
12. A finder according to claim 8, wherein said reflecting plane
for splitting a light beam from said imaging optical system is constructed
with a half mirror, said half mirror reflecting the light beam toward
said image sensor and transmitting the light beam toward said finder
system, and a reflecting surface is interposed between said half
mirror and said position of the image formed by said imaging optical
system.
13. A finder according to claim 8, wherein said finder satisfies
the following condition:
where H is a longitudinal dimension of a finder field and V is
a lateral dimension of the finder field.
14. A finder for single-lens reflex type digital cameras, comprising:
a reflecting plane for splitting a light beam from an imaging optical
system of a camera into two beams directed toward an image sensor
and a finder system;
an image-erecting prism system; and
an ocular optical system,
wherein said prism system and said ocular optical system are arranged
along a traveling direction of light in said finder system placed
after a position of the image formed by said imaging optical system,
wherein said ocular optical system includes, in order from an object
side, an object side surface of a positive lens which is convex
toward the object side and a pupil side surface of a negative lens
which is concave toward the pupil side, and
wherein said finder satisfies the following conditions:
where R.sub.31 and R.sub.32 are radii of curvature of the object-side
surface and the pupil-side surface, respectively, of said positive
lens and R.sub.33 and R.sub.34 are radii of curvature of the object-side
surface and the pupil-side surface, respectively, of said negative
lens.
15. A finder according to claim 14, wherein said ocular optical
system has aspherical surfaces and is constructed so that said negative
lens is movable for diopter adjustment, said ocular optical system
further satisfying the following condition:
where fE is a combined focal length of said finder system on a
pupil side of a position of the image formed by said imaging optical
system.
16. A finder according to claim 15, wherein said image-erecting
prism system is constructed so that an exit surface thereof is shaped
into a curved surface which is convex toward the pupil side and
satisfies the following condition:
where RP is a radius of curvature of the exit surface said image-erecting
prism system.
17. A finder according to claim 14, wherein said finder system
is placed on a transmission side of said reflecting plane for splitting
a light beam from said imaging optical system.
18. A finder according to claim 14, wherein said finder satisfies
the following condition:
where H is a longitudinal dimension of a finder field and V is
a lateral dimension of the finder field.
Digital cameras description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a finder for single-lens reflex type
digital cameras which uses an electronic image sensor such as a
CCD.
2. Description of Related Art
In recent years, special attention has been devoted to a digital
camera (electronic camera) as an alternative, used in the next generation,
to a 35 mm silver halide film (usually called a Leica size) camera.
The size of a CCD used as an image sensor in this digital camera
varies in diagonal length from a fraction to the order of tenth
of a 35 mm silver halide film. Thus, where a single-lens reflex
system is applied to the camera of this type, it is necessary to
increase the magnification of an ocular optical system used in a
finder system. In doing so, the focal length of the finder system
must be reduced as a matter of course. Furthermore, it is also necessary
to interpose an image erecting optical system between the first
imaging plane and the ocular optical system of the finder system.
However, the pupil diameter and the eyepoint position of the finder
system must be constant, irrespective of the focal length of the
ocular optical system of the finder system, and a sufficiently wide
field angle must also be secured for a single-lens reflex camera.
As such, an effective sectional area of the image erecting optical
system tends to increase. In order to construct the image erecting
optical system with a prism under such circumstances, its optical
path length must be increased. This makes the power distribution
of the ocular optical system very difficult and develops a tendency
that aberrations affecting the view of the finder are considerably
deteriorated.
Consequently, in order to solve the above problems, it is necessary
to take account of a prism design prepared to have the shortest
possible optical path length and the construction of an ocular optical
system with a power distribution and a lens configuration which
are advantageous for correction for aberrations.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide
a finder for single-lens reflex type digital cameras which is equipped
with an ocular optical system favorably corrected for aberrations
and having a high magnification and a wide field angle.
In order to achieve this object, according to one aspect of the
present invention, the finder for single-lens reflex type digital
cameras is provided with a reflecting plane for splitting a light
beam from an imaging optical system of a camera into two beams directed
toward an image sensor and a finder system, and has a prism system
for erecting an image and an ocular optical system which are arranged
along a traveling direction of light in the finder system placed
after the position of the image formed by the imaging optical system.
The prism system for erecting the image includes, in order along
the traveling direction of light in one prism block, an entrance
surface, a reflecting surface consisting of one plane, a reflecting
surface of roof shape, and an exit surface. An optical path between
the entrance surface and the reflecting surface consisting of one
plane is practically perpendicular to an optical path between the
reflecting surface of roof shape and the exit surface in the prism
block.
According to another aspect of the present invention, the finder
is provided with a reflecting plane for splitting a light beam from
an imaging optical system of a camera into two beams directed toward
an image sensor and a finder system, and has a prism having of a
first reflecting surface, a second reflecting surface, and a third
reflecting surface and an ocular optical system which are arranged
along a traveling direction of light in the finder system placed
after the position of the image formed by the imaging optical system.
The normal lines of the first and second reflecting surfaces are
included in the same plane. This plane is practically perpendicular
to the optical axis of the imaging optical system, and the normal
line of the first reflecting surface is practically perpendicular
to that of the second reflecting surface. The third reflecting surface
reflects an axial light beam from the imaging optical system so
as to be practically parallel with the optical axis of the imaging
optical system. The ocular optical system includes, in order from
the object side, a biconvex lens whose object-side surface has a
larger curvature than in its pupil-side surface and a biconcave
lens whose
pupil-side surface has a larger curvature than in its object-side
surface. The finder also satisfies the following conditions:
where l.sub.1 is an axial distance from the first reflecting surface
to the second reflecting surface of the prism, l.sub.2 is an axial
distance from the second reflecting surface to the third reflecting
surface of the prism, L is an axial distance from the entrance surface
to the exit surface of the prism, np is the refractive index of
the medium of the prism, and fE is the combined focal length of
the finder system on the pupil side of the position of the image
formed by the imaging optical system.
According to still another aspect of the present invention, the
finder is provided with a reflecting plane for splitting a light
beam from an imaging optical system of a camera into two beams directed
toward an image sensor and a finder system, and has a prism system
for erecting an image and an ocular optical system which are arranged
along a traveling direction of light in the finder system placed
after the position of the image formed by the imaging optical system.
The ocular optical system includes, in order from the object side,
a positive lens directing a convex surface with relatively large
curvature toward the object side and a negative lens directing a
concave surface with relatively large curvature toward the pupil
side. The finder also satisfies the following conditions:
where R.sub.31 and R.sub.32 are radii of curvature of the surfaces
on the object and pupil sides, respectively, of the positive lens
and R.sub.33 and R.sub.34 are radii of curvature of the surfaces
on the object and pupil sides, respectively, of the negative lens.
This and other objects as well as the features and advantages of
the present invention will become apparent from the following detailed
description of the preferred embodiments when taken in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is sectional view, developed along the optical axis, showing
the arrangement of the finder of a first embodiment;
FIG. 2 is a view showing the case where the finder of the first
embodiment is mounted to a single-lens reflex type digital camera;
FIGS. 3A, 3B, and 3C are views showing aberration curves in the
finder of the first embodiment;
FIG. 4 is a perspective view showing the arrangement of prisms
used in the finder of a second embodiment;
FIG. 5 is a sectional view, developed along the optical axis, showing
the arrangement of the finder of the second embodiment, looking
from the direction of an arrow A in FIG. 4;
FIG. 6 is a sectional view, developed along the optical axis, showing
the arrangement of the finder of the second embodiment, looking
from the direction of an arrow B in FIG. 4;
FIG. 7 is a sectional view, developed along the optical axis, showing
the arrangement of the finder of the second embodiment, looking
from the direction of an arrow C in FIG. 4; and
FIGS. 8A, 8B, and 8C are views showing aberration curves in the
finder of the second embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The finder of the present invention is mounted to the single-lens
reflex type digital camera and can be constructed with a roof prism
or a Porro prism. What follows is a description of the case where
either prism is used.
Where the roof prism is used, an ocular optical system with high
magnification is reduced in focal length. Thus, when such an ocular
optical system is used to construct the finder, the optical path
length of an image erecting optical system must also be reduced.
Consider now an effective sectional area of the prism to be decreased
in the image erecting optical system in order to reduce the optical
path length of the image erecting optical system.
When a so-called pentaprism which has been widely used as the image
erecting optical system in a single-lens reflex camera is employed
to construct the finder, it is conceivable to diminish the size
of the pentaprism in accordance with the focal length of the ocular
optical system used together. Even though this has been done, a
distance from the position of an observer's pupil or the ocular
optical system to the pentaprism is constant, irrespective of the
focal length of the ocular optical system, and thus the sectional
area on the exit surface side of the pentaprism is insufficient
as a matter of course.
The smallness of the sectional area on the exit surface side of
the pentaprism is attributable to the fact that its cross section
assumes a roof shape (the shape of an isosceles triangle). The sectional
area on the entrance surface side of the pentaprism may be relatively
small because it is governed by the size of an imaging plane. In
addition, the sectional area on the entrance surface side is not
restricted by the roof shape. As such, in the present invention,
the pentaprism of the single-lens reflex camera of Leica size is
constructed so that the entrance surface side and the exit surface
side of the pentaprism mentioned above are reversed. By doing so,
the size of the effective sectional area of the pentaprism is maintained,
and at the same time, the compactness of the entire finder is achieved.
Specifically, the finder of the present invention is provided with
a reflecting plane for splitting a light beam from an imaging optical
system of a camera into two beams directed toward an image sensor
and a finder system, and has a pentaprism (an image erecting optical
system) and an ocular optical system which are arranged along a
traveling direction of light in the finder system placed after the
position of the image formed by the imaging optical system. The
pentaprism includes, in order along the traveling direction of light
in one prism block, an entrance surface, a reflecting surface consisting
of one plane, a reflecting surface of roof shape, and an exit surface.
An optical path between the entrance surface and the reflecting
surface consisting of one plane is practically perpendicular to
an optical path between the reflecting surface of roof shape and
the exit surface in the prism block.
In this case, it is necessary to satisfy the following conditions:
where L is an axial distance from the entrance surface to the exit
surface of the prism, np is the refractive index of the medium of
the prism, fE is the combined focal length of the finder system
on the pupil side of the position of the image formed by the imaging
optical system, L.sub.1 is an axial distance from the entrance surface
to the reflecting surface consisting of one plane, of the pentaprism,
L.sub.2 is an axial distance from the reflecting surface of roof
shape to the exit surface of the pentaprism, and .theta..sub.1 is
an angle made by an optical axis from the entrance surface to the
reflecting surface consisting of one plane, of the pentaprism with
the normal line of the reflecting surface consisting of one plane.
Condition (1) defines the optical path length of the pentaprism.
If the value of L/(np fE) exceeds the upper limit of Condition (1),
it is difficult to construct an ocular optical system with high
magnification. If, on the other hand, the value of L/(np fE) is
below the lower limit of the Condition (1), it is difficult to construct
the pentaprism.
Conditions (2) and (3) refer to conditions for optimizing the shape
of the prism. If these conditions are not met, even though Condition
(1) is satisfied, principal rays will be eclipsed to cause a shade
on the periphery of a finder image or ghost will be liable to occur,
which is unfavorable.
Where the Porro prism is used as the image erecting optical system,
on the other hand, it is desirable that the Porro prism is constructed
as described below, in view of the mounting spaces of an electronic
image sensor and a circuit board which are mounted in the camera
and the layout of optical elements.
The finder is provided with a reflecting plane for splitting a
light beam from the imaging optical system of the camera into two
beams directed toward an image sensor and a finder system, and has
a prism having a first reflecting surface, a second reflecting surface,
and a third reflecting surface and an ocular optical system which
are arranged along a traveling direction of light In the finder
system placed after the position of the image formed by the imaging
optical system. The normal lines of the first and second reflecting
surfaces are included in the same plane. This plane is practically
perpendicular to the optical axis of the imaging optical system,
and the normal line of the first reflecting surface is substantially
perpendicular to that of the second reflecting surface. The third
reflecting surface reflects an axial light beam from the imaging
optical system so as to be substantially parallel with the optical
axis of the imaging optical system.
In the digital camera, the circuit board is mounted closer to an
observer's eye than to the electronic image sensor, and thus some
space is required. Furthermore, a prism used in the camera of this
type is such that restrictions on its design and space vary very
greatly, depending on the aspect ratio of a finder field. For most
of the digital cameras, such aspect ratios range from 0.72 (in a
rectangle) to 1 (in a square). Thus, in order to hold an eyepoint
at a proper position in the assembly of the camera, the finder of
the present invention is constructed as mentioned above, so that
it is avoidable that the light is bent at a plane corresponding
to the first reflecting surface to be parallel to the optical axis
of the imaging optical system and to travel toward the object side.
In addition, a condition for designing the prism in this case is
as follows:
where l.sub.1 is an axial distance from the first reflecting surface
to the second reflecting surface of the prism, l.sub.2 is an axial
distance from the second reflecting surface to the third reflecting
surface of the prism.
Here, if the value of l.sub.2 /l.sub.1 passes the upper limit of
Condition (4), an effective portion of the third reflecting surface
of the prism becomes liable to interfere with an effective portion
of the reflecting plane for splitting the light beam from the imaging
optical system in the vicinity of the third reflecting surface,
and thus it is difficult to construct the prism. If, on the other
hand, the value of l.sub.2 /l.sub.1 is below the lower limit of
Condition (4), the effective portions of the second and third reflecting
surfaces becomes liable to interfere with each other, and thus it
is also difficult to construct the prism.
Hence, the finder of the present invention further satisfies Condition
(1) even in the case where the Porro prism is used, thereby obviating
the above defect.
Moreover, in the finder of the present invention, the power distribution
of the ocular optical system is taken into account and thereby a
proper optical path length of the prism system for erecting an image
is ensured. Specifically, the principal point of the ocular optical
system is located on the object side of a surface, closest to the
object side, of lenses constituting the ocular optical system. It
is good practice that this ocular optical system is constructed
with: (a) a single positive meniscus lens whose convex surface is
directed toward the object side or (b) a positive lens and a negative
lens which are arranged in this order from the object side. Case
(a) is favorable because its placement constitutes a so-called landscape
type from a relation with a pupil position. In Case (b), where the
two lenses are fixedly used or they are integrally moved along the
optical axis to thereby make a diopter adjustment, an arrangement
such that the negative lens is shaped into a meniscus lens whose
concave surface is directed toward the pupil side is advantageous
for paraxial rays and for correction for aberrations (notably spherical
aberration) as well.
Also, it becomes necessary for the ocular optical system constructed
as mentioned above to satisfy the following conditions:
where R.sub.31 and R.sub.32 are radii of curvature of the surfaces
on the object and pupil sides, respectively, of the positive lens
of the ocular optical system and R.sub.33 and R.sub.34 are radii
of curvature of the surfaces on the object and pupil sides, respectively,
of the negative lens of the ocular optical system.
Conditions (5) and (6) define the shapes of the lenses constituting
the ocular optical system. If the values of (R.sub.31 +R.sub.32)/(R.sub.31
-R.sub.32) and (R.sub.33 +R.sub.34)/(R.sub.33 -R.sub.34) are outside
the lower limits of Conditions (5) and (6), respectively, an ocular
optical system with high magnification cannot be constructed. Beyond
the upper limit, it becomes difficult to favorably correct astigmatism
and spherical aberration, and the view of the finder will be impaired.
In the ocular optical system, where either the positive lens or
the negative lens is moved along the optical axis to make the diopter
adjustment, one surface of either lens is configured to be aspherical
or the following condition is satisfied, and thereby the fluctuation
of astigmatism produced by the diopter adjustment can be controlled.
If the value of (fE/R.sub.32)-(fE/R.sub.33) is outside the limits
defined by Condition (7), the fluctuation of astigmatism due to
the diopter adjustment becomes prominent.
Furthermore, in the present invention, when the exit surface of
the prism system for erecting the image is shaped into a curved
surface which is convex toward the pupil side, the principal point
of the entire ocular optical system can be easily shifted to the
object side. The powers of individual optical elements can also
be distributed, which is advantageous for correction for aberrations.
In this case, it is desirable that the prism system for erecting
the image satisfies the following condition:
where RP is the radius of curvature of the exit surface of the
prism system for erecting the image. If the value of RP/fE passes
the upper limit of Condition (8), this will bring about little effect
on correction for aberrations. If, on the other hand, the value
is below the lower limit, the power of the entire ocular optical
system will be too much shifted toward the prism side, and thus
correction for aberrations becomes difficult. This is unfavorable.
The ocular optical system, which satisfies Conditions (5)-(8),
is used together with the prism system for erecting the image which
includes the roof prism or the Porro prism, thereby bringing about
a further effect.
Where the Porro prism is used in the finder of the present invention,
it is desirable that the reflecting plane for splitting the light
beam into two beams directed toward the image sensor and the finder
system is constructed with a half mirror. This is because the half
mirror reflects the light beam toward the image sensor and transmits
it toward the finder system and thereby another reflecting surface
can be provided between the half mirror and the imaging plane of
the imaging optical system in the finder system to facilitate the
layout of the image sensor and the circuit board as well as the
design of a camera body.
When the longitudinal and lateral dimensions of the finder field
are represented by H and V, respectively, the finder of the present
invention satisfies the following condition:
In the present invention, as mentioned above, the finder satisfies
Conditions (1)-(3) and (5)-(9) when using the pentaprism and Conditions
(1) and (4)-(9) when using the Porro prism. In this way, excellent
performance is achived.
The finder of the present invention brings about an improvement
when Conditions (1)-(8) are replaced by the following conditions:
##EQU1##
Also, the finder of the present invention is further improved when
Conditions (1')-(8') are further replaced by the following conditions:
##EQU2##
The embodiments of the present invention will be described below.
First Embodiment
In FIG. 1, a finder 1 in this embodiment includes, in order from
the object side, a pentaprism 2 and an ocular optical system 3.
The pentaprism 2 has an entrance surface 2a, a reflecting surface
2b, a roof surface (reflecting surface) 2c, and an exit surface
2d. The reflecting surface 2b consists of one plane. The ocular
optical system 3 includes, in order from the side of the pentaprism
2, a positive lens 3a and a negative lens 3b. The positive lens
3a is disposed so that its convex side with relatively large curvature
faces the pentaprism 2. The negative lens 3b is placed so that its
concave side with relatively large curvature faces the pupil.
In the pentaprism 2, an optical path from the entrance surface
2a to the reflecting surface 2b is nearly perpendicular to an optical
path from the roof surface 2c to the exit surface 2d. The ocular
optical system 3 is designed so that the positive lens 3a and the
negative lens 3b can be moved along the optical axis for diopter
adjustment.
FIG. 2 shows a single-lens reflex type digital camera in which
the finder 1 is mounted. This camera is adapted to photograph in
such a way that a light beam from an object, entering an imaging
optical system 4, is imaged on a CCD (image sensor). Between the
imaging optical system 4 and the CCD, an instant-return mirror 5
is removably mounted. When the instant-return mirror 5, as shown
in the figure, is introduced into the optical path, the light beam
from the object, passing through the imaging optical system 4, is
reflected by the instant-return mirror 5 and then is imaged on a
finder screen 6. In order to determine the picture composition of
the object before photographing, an object image formed on the finder
screen 6 is magnified by the finder 1 of the first embodiment and
this magnified image is observed at the position of an eyepoint
Ep. Specifically, the object image formed on the finder screen 6
is transmitted through the entrance surface 2a of the pentaprism
2, reflected by the reflecting surface 2b, and further reflected
by the roof surface 2c to reach the exit surface 2c. The light beam
transmitted through the exit surface 2d is introduced through the
ocular optical system 3 to an observer's pupil.
The following are various numerical data relative to the finder
of the first embodiment.
L/(np fE)=1.122
L.sub.2 /L.sub.1 =1.242
.theta..sub.1 =28.000
(R.sub.31 +R.sub.32)/(R.sub.31 -R.sub.32)=-2.705
(R.sub.33 +R.sub.34)/(R.sub.33 -R.sub.34)=8.953
(fE/R.sub.32)-(fE/R.sub.33)=-0.754
RP/fE=-13.093
H/V=0.75
Height of the image observed by the ocular optical system 3=5.00
(mm)
______________________________________ r.sub.1 = .infin. (imaging
plane) d.sub.1 = 1.2963 r.sub.2 = .infin. (transmitting surface)
d.sub.2 = 12.0700 nd.sub.2 = 1.60311 .nu.d.sub.2 = 60.68 r.sub.3
= .infin. (reflecting surface) d.sub.3 = -12.5000 nd.sub.3 = 1.60311
.nu.d.sub.3 = 60.68 r.sub.4 = .infin. (reflecting surface) d.sub.4
= 15.0000 nd.sub.4 = 1.60311 .nu.d.sub.4 = 60.68 r.sub.5 = -288.0113
(transmitting surface d.sub.5 = 0.500 r.sub.6 = 7.6141 d.sub.6 =
4.0000 nd.sub.6 = 1.60311 .nu.d.sub.6 = 60.68 r.sub.7 = 16.5480
(aspherical surface) d.sub.7 = 1.8283 r.sub.8 = 10.5578 (aspherical
surface) d.sub.8 = 1.2000 nd.sub.8 = 1.80518 .nu..sub.8 = 25.43
r.sub.9 = 8.4362 d.sub.9 = 15.4907 r.sub.10 = .infin. (eyepoint)
______________________________________
Conic constants and aspherical coefficients
Seventh surface
K=0
A.sub.4 =2.4415.times.10.sup.-5, A.sub.6 =1.5628.times.10.sup.-6,
A.sub.8 =0
Eighth surface
K=0
A.sub.4 =-3.2789.times.10.sup.-4, A.sub.6 =-5.8462.times.10.sup.-6,
A.sub.8 =0
FIGS. 3A, 3B, and 3C show aberration curves in the finder of the
first embodiment.
Second Embodiment
The finder of this embodiment uses a Porro prism as an image erecting
optical system. The arrangement of the finder of the second embodiment
will be described below with reference to FIGS. 4-7.
As shown in FIG. 4, a Porro prism 10 used in the finder of the
second embodiment is constructed with a first prism 11 and a second
prism 12. A finder screen 13 is interposed between the first prism
11 and the second prism 12. The finder of the second embodiment
including these prisms is also mounted in the single-lens reflex
type digital camera such as that shown in FIG. 2. In this case,
the CCD (image sensor) is placed below the first prism 11.
When the finder of the second embodiment is used in the digital
camera, a light beam from the imaging optical system of the camera,
after entering the first prism 11, is split into two beams for a
photographing optical path and a finder optical path by a half mirror
11a. The light beam from the imaging optical system to be introduced
into the photographing optical path is reflected by the half mirror
11a and reaches the CCD. On the other hand, the light beam from
the imaging optical system to be introduced into the finder optical
path is transmitted through the half mirror 11a and then is reflected
by a reflecting surface 11b to reach the finder screen 13. An object
image produced by the imaging optical system is formed on the finder
screen 13. Subsequently, the light beam forming the object image,
after being incident on the second prism 12 from an entrance surface
12a thereof, is reflected in turn by a first reflecting surface
12b, a second reflecting surface 12c, and a third reflecting surface
12d and reaches an exit surface 12e. After that, the light beam
emerges from the exit surface 12e and is conducted to the observer's
pupil through an ocular optical system 14 shown in FIGS. 6 and 7.
Also, in FIGS. 6 and 7, the entrance surface 12a is blocked by the
first reflecting surface 12b, and the first reflecting surface 12b
by the second reflecting surface 12c, respectively.
In the second prism 12 of the finder of the second embodiment,
the normal lines of the first and second reflecting surfaces 12b
and 12c are included in the same plane, which is nearly perpendicular
to the optical axis of the imaging optical system. In addition,
the normal lines of the first and second reflecting surfaces 12b
and 12c are nearly perpendicular to each other, and an axial light
beam introduced from the imaging optical system is reflected by
the third reflecting surface 12d so as to be practically parallel
with the optical axis of the imaging optical system.
In the finder of the second embodiment, the ocular optical system
14 includes, in order from the side of the second prism 12, a positive
biconvex lens 14a whose surface with relatively large curvature
is directed toward the second prism 12 and a negative biconcave
lens 14b whose surface with relatively large curvature is directed
toward the pupil. The positive lens 14a and the negative lens 14b
are moved along the optical axis, thereby making the diopter adjustment.
The following are various numerical data relative to the finder
of the second embodiment.
L/(np fE)=1.102
(R.sub.31 +R.sub.32)/(R.sub.31 -R.sub.32)=-0.296
(R.sub.33 +R.sub.34)/(R.sub.33 -R.sub.34)=0.338
(fE/R.sub.32)-(fE/R.sub.33)=-0.353
RP/fE=-1.203
H/V=0.75
l.sub.2 /l.sub.1 =1.118
Height of the image observed by the ocular optical system 14=5.00
(mm)
______________________________________ r.sub.1 = .infin. (imaging
plane) d.sub.1 = -1.0017 r.sub.2 = .infin. (transmitting surface)
d.sub.2 = -4.4000 nd.sub.2 = 1.52540 .nu.d.sub.2 = 56.25 r.sub.3
= .infin. (reflecting surface) d.sub.3 = -11.9000 nd.sub.3 = 1.52540
.nu.d.sub.3 = 56.25 r.sub.4 = .infin. (reflecting surface) d.sub.4
= 13.3000 nd.sub.4 = 1.52540 .nu.d.sub.4 = 56.25 r.sub.5 = -.infin.
(reflecting surface) d.sub.5 = 8.400 nd.sub.5 = 1.52540 .nu.d.sub.5
= 56.25 r.sub.6 = -27.1980 (transmitting surface) d.sub.6 = 0.3000
r.sub.7 = 14.9070 d.sub.7 = 1.8283 nd.sub.7 = 1.49241 .nu.d.sub.7
= 57.66 r.sub.8 = -27.4380 (aspherical surface) d.sub.8 = 2.3000
r.sub.9 = -48.0090 (aspherical surface) d.sub.9 = 1.3000 nd.sub.9
= 1.58423 .nu.d.sub.9 = 30.49 r.sub.10 = 23.7540 d.sub.10 = 15.5000
r.sub.11 = .infin. (eyepoint) ______________________________________
Conic constants and aspherical coefficients
Eighth surface
K=0
A.sub.4 =1.0531.times.10.sup.-4, A.sub.6 =-7.7711.times.10.sup.-7,
A.sub.8 =4.5344.times.10.sup.-9
Ninth surface
K=0
A.sub.4 =5.1686.times.10.sup.-5, A.sub.6 =-7.8733.times.10.sup.-7,
A.sub.8 =4.1723.times.10.sup.-9
FIGS. 8A, 8B, and 8C show aberration curves in the finder of the
second embodiment.
In the numerical data shown in the above embodiments, r.sub.1,
r.sub.2, . . . represent radii of curvature of the surfaces of individual
optical elements such as lenses; d.sub.1, d.sub.2, . . . represent
thicknesses of individual optical elements, or spaces therebetween;
nd.sub.1, nd.sub.2, . . . represent refractive indices of individual
optical elements at the d line (567.56 nm); and .nu.d.sub.1, .nu.d.sub.2,
. . . represent Abbe's numbers of individual optical elements at
the d line. Also, when X is taken as the coordinate in the direction
of the optical axis, Y is taken as the coordinate in the direction
normal to the optical axis, K denotes a conic constant, and A.sub.4,
A.sub.6, and A.sub.8 denote aspherical coefficients, configuration
of each of the aspherical surfaces in the embodiments is expressed
by the following equation: ##EQU3##
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