Device for adjusting movement of photographic lens and field of viewfinder

Attachment devices for automatic focusing cameras which permit the taking of exposures having a visual field different from that attainable with the fixed photographic lens of the camera comprise a conversion lens of different focal length, e.g., wide angle or telephoto, than the camera lens adapted to mount on the front of the camera lens, and refraction means to compensate the automatic movement of the camera lens for the mounting of such conversion lens including a prism to mount on the camera to cover a window on the camera through which light passes to control automatic focusing of the camera lens, such prism being structured to change the path of light passing through the window by a angle D that corresponds to the required alteration in the range of movement of the camera lens when the conversion lens is mounted on the camera.

BACKGROUND OF THE INVENTION 
The present invention relates to a device which allows automatic focusing 
of an automatic focusing camera even when various conversion lenses are 
used. 
Automatic focusing cameras have been recently commercially available which 
automatically detect a distance between the subject and the camera to 
focus the photographic lens on the subject. There are two main automatic 
focusing methods: a method in which light from a subject is received to 
measure the distance between the subject and the camera; and a method in 
which light is emitted by a camera to the subject, and the reflected light 
from the subject is received to measure the distance between the subject 
and the camera. In both these methods, a distance measuring mechanism is 
interlinked with a photographic moving mechanism to automatically focus 
the photographic lens on the subject, since a focal length of a lens 
mounted on an automatic focusing camera (to be referred to as a 
photographic lens hereinafter) is set to be constant. 
However, such a device cannot be used when a conversion lens such as a 
telephoto lens or a wide-angle lens is attached to the camera. This is 
because these conversion lenses have different focal lengths from that of 
the standard lens. 
Therefore, the focal length of the photographic lens may not be kept 
constant. When a conversion lens is attached to the photographic lens, a 
synthetic or effective focal length results which is the product of the 
focal length of the standard lens and the magnification of the conversion 
lens. In this case, the required range of movement of the photographic 
lens to provide focusing at a given distance between the subject and the 
camera is altered. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a lens 
movement adjusting mechanism which changes the range of movement of a 
photographic lens of an automatic focusing camera to automatically focus 
the photographic lens on the subject even when one of a variety of 
conversion lenses is attached to the standard lens. 
A prism (usually called a "D prism") is provided just in front of and 
covering a range finder window of the automatic focusing camera. Light is 
emitted through the prism to the subject and reflected light enters the 
prism. Alternatively, light emitted from the subject enters the prism. The 
angle of the D prism is so determined as to alter the range of movement of 
the photographic lens to provide focusing. When light is emitted from or 
enters the window, it passes through the "D prism". The incident angle or 
the transmission angle is changed and transferred to the photographic lens 
to move it by the appropriate amount to provide automatic focusing even 
when a conversion lens is used. 
Also, when a conversion lens, particularly a telephoto lens or a wide-angle 
lens is used, an image photographed on the film is in practice either 
smaller or larger then the field seen through the viewfinder. It is 
desirable to adjust the field seen through the viewfinder so as to 
correspond to the image photographed on the film. 
It is another object of the present invention to provide a viewfinder field 
adjusting mechanism for the automatic focusing camera as well as 
photographic lens movement adjusting mechanism. When a telephoto lens is 
used, the image actually photographed on the film is smaller than the 
field seen through the viewfinder. This problem can be solved by 
providing, just in front of the viewfinder window, a frame having a 
smaller area than the viewfinder window corresponds to the actual image 
photographed on the film. Conversely, when a wide-angle lens is used, the 
image actually photographed on the film is larger than the field seen 
through the viewfinder. This problem can be solved by providing a concave 
lens just in front of the viewfinder window. Such a viewfinder field 
adjusting mechanism comprising a frame or a concave lens is provided 
integrally with the photographic lens movement adjusting mechanism. A 
proper device may be selected according to the type of conversion lenses 
for adjusting the movement of a photographic lens and the field of the 
viewfinder. 
It is still another object of the present invention to provide an 
inexpensive device of the type described above which is manufactured by 
monolithic molding of relatively rigid synthetic resins such as an acrylic 
resin and which has a good outer appearance. 
Other objects, features and advantages of the present invention will be 
apparent from the following detailed description taken in conjunction with 
the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a conventional automatic focusing camera I; a photographic 
lens II a viewfinder window formed at the front of the camera I, and a 
window IV through which light is emitted to a subject and reflected light 
enters; alternatively, through which light emitted by the subject enters. 
These two windows are usually provided side by side. Reference symbol V 
denotes a conversion lens, for example, telephoto lens or a wide-angle 
lens, mounted on the standard lens. 
Further, also in FIG. 1 reference numeral 1 denotes a rectangular main 
body, such as an acrylic sheet, of an adjusting device made of a 
relatively rigid synthetic resin. On one side of the main body 1 (on the 
right-hand side in the figure) is a photographic lens movement adjusting 
mechanism, which comprises a prism 2 usually called a D prism large enough 
to cover the light-emitting/light-receiving window IV of the camera I. 
Among several possible shapes of the prism 2, major shapes may be selected 
from a saw-toothed or wedge shape and a Fresnel lens type shape having a 
series of wedges or ridges from its periphery toward its center, as is 
shown in FIGS. 5(i) and 5(ii), respectively. 
The angle of the D prism is so determined that it corresponds to the 
required alteration in the range of movement of a photographic lens when 
the conversion lens V is attached to the camera I. 
On the other side of the main body 1 (on the left-hand side in the figure) 
is a viewfinder field adjusting mechanism. FIG. 1 shows a case where a 
telephoto lens is used as the conversion lens V. The main device 1 also 
comprises a through hole 3 which is of a similar shape as but provides a 
reduction of the range tinder window III of the camera I, thereby 
providing a frame 4. The degree of reduction is so determined that the 
field seen from the through hole 3 coincides with that actually 
photographed on the film using a telephoto lens. 
The main body 1 of the device which has a structure as described above will 
have a shape corresponding to the shape of the camera to which it is 
attached. For example, when the photographic lens interferes with the 
attachment of the main device 1 since the two windows III and IV are 
located too close to the photographic lens II, corners of the main body 1 
of the device may be cut off. The method of attaching the main body 1 to 
the camera I may also vary. 
FIG. 1 shows an example wherein an elongated, flat mounting portion 5 is 
fixed substantially at the center of the main body 1 of the device and is 
perpendicular thereto. The mounting portion 5 extends backward for a 
distance identical to the width of the camera I and has a bending portion 
6 extending downward by a short distance. 
The mounting portion 5 is thus firmly mounted on the upper side of the 
camera I when the conversion lens V is attached over the photographic lens 
II of the camera I by clamping the camera I between the main body 1 and 
the bending portion 6 of the mounting portion 5. In this case, the prism 2 
of the main body 1 and the through hole 3 of the frame 4 are located 
immediately in front of the window IV from which light is emitted or 
received and the viewfinder window III of the camera I, respectively. 
Therefore, even when the conversion lens V is attached to the automatic 
focusing camera I, light passing out or received through the window IV of 
the camera I passes the D prism 2 of the main body 1. Since the angle of 
this prism corresponds to the required alternation in the range of 
movement of the photographic lens II, the appropriate incident angle or 
emitted light to provide automatic focusing is transferred to a 
photographic lens moving mechanism (not shown) in the camera I. 
FIG. 1 shows the case where telephoto lens is attached as a conversion 
lens. The area of subject seen through the viewfinder window III via the 
through hole 3 which is defined by the frame 4 of the main body 1 is the 
same as that actually photographed on the film, thus preventing errors 
such as failing to photograph desired parts of the subject. 
FIGS. 3 and 4 show a case where a wide-angle lens is used as the conversion 
lens V in which the same reference numerals as used in FIGS. 1 and 2 
denote the same parts. The concave lens 7 ajusts the field seen through 
which the viewfinder window III so as to correspond it to the image 
actually photographed on the film, in the similar manner as the frame 4, 
thus preventing errors such as photographing unnecessary parts of the 
subject.