Optical: systems and elements – Prism
Reexamination Certificate
1999-08-09
2002-08-27
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Prism
C396S384000
Reexamination Certificate
active
06441977
ABSTRACT:
This application is based on an application No. 10-225861 filed in Japan, the contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical apparatus such as a camera, and particularly relates to an optical system thereof which has a lens, a prism, and the like.
2. Description of the Related Art
Accompanying a miniaturization (or compactness, or thinnerization) of cameras which have been provided in recent years, there has been a tendency that a length in a direction of optical axis in an optical unit also becomes short. Under this tendency, it becomes difficult to arrange a light projecting/receiving optical system of an active AF, or an AE optical system, with a sufficient focal length. If the focal length of each of these optical systems is made relatively shorter, undesirable influences, such as a deterioration of precision in AF focussing, a difficulty in AE spot photometric measuring, and the like, may be brought to a camera performance.
Firstly, a description is made below upon the deterioration, or lowering, of precision in the AF focussing.
FIG. 1
is a view showing an optical relation among a photographing optical system, a light projecting lens (or a light emitting lens), a light receiving lens, and so on, of a camera which employs an active AF as a focussing device. An AF beam emitted from a light projecting element (or a light emitting element) such as a LED, is reflected on an object (i.e. a subject to be photographed), and then is projected, or incident, upon a light receiving surface of a photo sensor (or a light receiving element) such as a PSD. Therefore, it is possible to know a position of center of gravity of light distributed over the light receiving surface, such as a distance from a left edge of the photo sensor to the position of center thereof. Therefore, if a relation between the position of center thereof and a distance “D” up to the object to be photographed (i.e. a distance “D” between the object and the camera) is tabled, or memorized, beforehand, it is possible to calculate the distance “D” up to the object. Alternatively, if a formula which represents a relation between the position of center thereof and the distance “D” up to the object, it is also possible to calculate the distance “D” up to the object by calculation, or operation, due to the position of center thereof.
The aforementioned table or formula is prepared on the assumption that all the projected AF beam returns to the photo sensor. That is, in order to accurately measure the distance with the aforementioned manner, it is necessary that all the projected AF beam returns to the photo sensor.
FIG. 2B
shows a situation in which the AF beam is all projected on the object. That is, the horizontally elongate and shaded region in
FIG. 2B
shows a region on the object in which the AF beam is projected. In this case, all the projected AF beam is reflected on the object, and it is possible to measure the distance (i.e. to focus) up to the object with high accuracy.
Meanwhile, in
FIG. 2A
, only a part of the projected, or emitted, AF beam can hit the object. In this case, what reflects on the object is not all of the projected, or emitted, AF beam (hereinafter, this phenomenon is referred to as “partial reflection” or “vignette (or vignetting)”), and the accuracy of the distance measuring is naturally lowered or reduced. The degree of error of the distance measuring becomes larger as the degree of the vignette increases.
Therefore, for the purpose of accurate distance measuring, it is preferable that there is no “partial reflection” at all. However, even if the “partial reflection”, or “vignetting”, occurs, and even if the actual location of the object and its focused location (namely, the distance up to the object measured by means of active AF) are not exactly coincide with each other as a result, the focussing error does not become a substantial problem as far as the object exists within a depth of field relative to the location thus focussed.
Referring to
FIG. 1
, it can be understood that if the depth of field is “dL”, the width “dX” of the region on the photo sensor corresponding to the depth of field can be determined geometrically. Even if the center location of the light distributed on the photo sensor is deviated due to the “partial reflection”, the deviation is not a substantial problem as far as it exists within the region “dX”. Therefore, supposing that the resolution of the photo sensor is fixed or constant, the wider the “dX” becomes, the higher the accuracy of the distance measuring becomes substantially. As explained below, the “dX” can be represented by a formula including the focal length “fr” of the receiving lens.
Generally, the following “FORMULA 1” is established between a permissible circle of confusion (or a permissible derangement circle) and a depth of field, where the “F
no
” is a f-number of the photographing optical system, and the “n” is a permissible coefficient which can be determined optionally in compliance with a specific design (the “n” can be 0.333, for example).
1
/
L
-
1
/
(
L
+
d
L
)
=
n
·
F
no
·
σ
/
ft
2
(
FORMULA
1
)
On the other hand, from the geometrical relationship shown in
FIG. 1
, the following “FORMULA 2”, “FORMULA 3” and “FORMULA 4” are established.
(
L
+
d
L
)
:
B
=
fr
:
X
1
(
FORMULA
2
)
L
:
B
=
fr
:
X
2
(
FORMULA
3
)
d
x
=
X
2
-
X
1
(
FORMULA
4
)
Elimination of X
1
and X
2
from the “FORMULA 4”, using the “FORMULA 2” and “FORMULA 3”, brings a “FORMULA 5”, as follows.
d
x
=
B
·
fr
/
L
-
B
·
fr
/
(
L
+
d
L
)
=
B
·
fr
·
{
1
/
L
-
1
/
(
L
+
d
L
)
}
(
FORMULA
5
)
Finally, substitution of the “FORMULA 1” into the “FORMULA 5” brings a “FORMULA 6” as follows.
d
x
=
(
n
·
F
no
·
σ
·
B
·
fr
)
/
ft
2
(
FORMULA
6
)
By the way, in the case that the photographing optical system is a zoom lens which is constituted by a pair of lens groups, the diameter of entrance pupil &phgr; does not change, even if the magnification is changed by zooming. Therefore, using a relationship F
no
=ft/&phgr;, the “FORMULA 6” can also be expressed as a “FORMULA 7” as follows.
d
x
=
(
n
·
σ
·
B
·
fr
)
/
ft
·
φ
(
FORMULA
7
)
From the “FORMULA 6” and “FORMULA 7”, it can be understood that the “dx” is proportional to “fr”. That is, it can be understood that the longer the focal length “fr” of the light receiving lens becomes, the higher the accuracy of the distance measuring (i.e. the accuracy of focussing) becomes.
As to the focal length “fs” of the light projecting lens, the longer “fs” becomes, the higher the accuracy of the distance measuring becomes. Next, an explanation thereof is made below.
That is, the shorter the focal length “fs” of the light projecting lens becomes, the projected, or emitted, AF beam diverges from the projecting lens or element with a relatively wider angle. As a result, the beam projected area on the object also becomes relatively larger, supposing that the distance to the object is fixed.
FIGS. 3A and 3B
show this situation explanatorily.
Namely,
FIG. 3B
illustrates a situation in which the focal length of the light projecting lens is relatively longer; therefore, the beam projected area is relatively smaller. On the other hand,
FIG. 3A
illustrates a situation in which the focal length of the light projecting lens is relatively shorter; therefore, the beam projected area is relatively larger.
As apparent from
FIGS. 3A and 3B
, if the beam projected area relative to the object is relatively larger, there increases the possibility that the aforementioned “partial reflection” occurs, so
Cherry Euncha
Minolta Co. , Ltd.
Sidley Austin Brown & Wood LLP
Spyrou Cassandra
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