Large aperture ratio interchangeable lens for a single lens reflex camera

A large aperture ratio interchangeable lens for a single lens reflex camera comprises a lens body, a preset aperture ring provided for rotation with respect to the lens body, a first signal member operatively associated with the preset aperture ring to transmit a corrected difference signal (f-f.sub.O -.DELTA.f) to the camera body, and a second signal member for transmitting a corrected open value signal (f.sub.O +.DELTA.f) to the camera body.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an interchangeable lens for a single lens reflex 
camera, and more particularly to a large aperture ratio interchangeable 
lens for use with a single lens reflex camera of the TTL open photometry 
type. 
2. Description of the Prior Art 
An interchangeable lens for use with a single lens reflex camera of the TTL 
open photometry type is known in which a first signal member for 
transmitting to the camera body a difference signal (f-f.sub.O) 
corresponding to the number of stages by which a preset aperture value (f) 
is stopped down with respect to an open F-value (f.sub.O) is provided on a 
preset aperture ring formed with equidistant aperture divisions, and in 
which a second signal member for transmitting the open F-value (f.sub.O) 
inherent to the lens to the camera body is fixed onto the lens body. 
The construction for such signal transmission is basically directed to 
lenses in which the relation between the number of stop-down stages of the 
aperture diaphragm and the quantity of light passed through the lens 
exhibits the linearity as shown in FIG. 1 of the accompanying drawings 
(usually, dark lenses having open F-values of 2 or greater). However, in 
bright lenses having open F-values of 1.2, 1.4 and so on, the quantity of 
light reaching the surface of a light-receiving element and the surface of 
a film in the vicinity of a fully opened aperture is decreased below the 
theoretical value due to vignetting or the like, as shown in FIG. 2 of the 
accompanying drawings. For this reason, in these bright lenses, the first 
signal member is installed with a deviation of .DELTA.f corresponding to 
the decrease in the quantity of light received from the theoretical 
quantity so as to produce a signal (f-f.sub.O -.DELTA.f) as a corrected 
difference signal representing the number of aperture stages, i.e., stops, 
from the open F-value to the preset aperture value (f), and thereby 
prevent occurrence of a metering error. 
If the preset aperture value is displayed within the finder or transmitted 
to a speed light, i.e., an electronic flash, through the camera body by 
the use of a difference signal (f-f.sub.O) and an open value signal 
(f.sub.O) obtained from the first and second signal members, respectively, 
then it is possible in the dark lenses to obtain an accurate preset 
aperture value f by combining the difference signal (f-f.sub.O) and the 
open value signal (f.sub.O). In the bright lenses, however, the first 
signal member produces a corrected difference signal (f-f.sub.O -.DELTA.f) 
and it is therefore impossible to obtain an accurate preset aperture value 
by simply synthesizing the open value signal (f.sub.O). 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a large aperture ratio 
interchangeable lens for a single lens reflex camera in which an accurate 
preset aperture value can be transmitted to the camera body side. 
Such object of the present invention is achieved by providing a second 
signal member for producing a corrected open value signal (f.sub.O 
+.DELTA.f), in addition to a first signal member for producing a corrected 
difference signal (f-f.sub.O -.DELTA.f). 
The invention will become fully apparent from the following detailed 
description thereof taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The invention will hereinafter be described and contrasted with the prior 
art by reference to the drawings. FIG. 3 illustrates the positional 
relationship between first and second signal members in the 
interchangealbe lens of the present invention (typically, an 
interchangeable lens having open F-values of 1.4 and 2). The prior art 
will first be described. In FIG. 3, a first signal member 32 is provided 
on a preset aperture ring 31. A first transmitted member 33 on the camera 
body side is biased counterclockwise and when an interchangeable lens is 
mounted on the camera body by clockwise rotation thereof, the first 
transmitted member is engaged by the engaging end 32a of the first signal 
member 32, which maintains an operative association therewith so that the 
first transmitted member is rotated when the preset aperture ring is 
rotated. A resistance brush (not shown) is operatively associated with the 
first transmitted member 33 and the resistance value thereof is linearly 
varied in accordance with the rotation of the preset aperture ring to a 
desired aperture value to produce a difference signal having a magnitude 
of (f-f.sub.O). 
A second signal member 34 is provided on the body 30 of the interchangeable 
lens, for example, on a protecting member secured to the inner periphery 
of a lens mount and projected beyond the surface of a rearward lens 
element. A second transmitted member 35 on the camera body side, biased 
counter clockwise, is engaged by the engaging end 34a of a second signal 
member 34 and rotated clockwise with the clockwise rotation of the 
interchangeable lens during mounting of the same on the camera body, which 
displaces the second transmitted member to a predetermined position when 
the mounting operation is completed. A resistance brush (not shown) for 
producing an open value signal (f.sub.O) is operatively associated with 
the second transmitted member 35 and the resistance value thereof is 
linearly varied in accordance with the clockwise rotation of the lens 
during mounting. 
The engaging end 32a of the first signal member 32 of the interchangeable 
lens having an open F-value of 2 is spaced apart by a predetermined angle 
.theta. from a fixed index mark 37 on the lens body when a division f:2 
(representing the open F-value) of aperture value divisions 36 
equidistantly provided on the aperture ring is registered with the fixed 
index mark 37 as the first standard position on the lens body after 
completion of the mounting of the interchangeable lens. The dark 
interchangeable lens having an open F-value of 2 or greater is designed 
such that this angle .theta. is always maintained when the aperture 
division representing the open F-value has been registered with the fixed 
index mark after the mounting of the lens onto the camera body. 
In contrast, the engaging end 32a' of a first signal member 32' of an 
interchangeable lens having an open F-value of 1.4 is provided so that it 
is spaced apart by an angle (.theta.+.DELTA..theta.) from the fixed index 
mark 37 as the first standard position. That is, in the case of an 
interchangeable lens having a theoretical open F-value of 1.4, if the 
substantial open F-value is only of the order of 1.7, a correction is made 
as if the metering were being effected by a lens having an open F-value of 
1.7. 
On the other hand, the second signal member 34 for transmitting to the 
camera body the theoretically determined open F-value inherent to each 
interchangeable lens body is adapted to be spaced apart from a second 
standard position (index mark), after completion of the mounting of the 
lens 38 by an angle resulting from multiplying an angle .beta. 
corresponding to one stage of the open F-value by the number of stages of 
the open F-value inherent to the interchangeable lens. According to the 
construction of the present invention, in a large aperture ratio 
interchangeable lens having an open F-value of 1.4, the second signal 
member 34 is provided with a further deviation of .DELTA..beta.in addition 
to the angle .beta.. 
Reference is now had to FIG. 4 to describe the operation of the present 
invention by substituting the displacement of the first and second 
transmitted members 33 and 35 by the variation in resistance values for 
the difference signal (f-f.sub.O) and for the open F value signal 
(f.sub.O). 
FIG. 4 shows, in overlapped relationship, the displacement of the 
difference signal (f-f.sub.O) by the first signal member and the 
displacement of the open value signal (f.sub.O) by the second signal 
member. The ordinate represents the resistance value, showing the standard 
by a straight line R=O. The abscissa represents the aperture value, and 
the lower portion of FIG. 4 shows the displacement of the difference 
signal (f-f.sub.O) by the first signal member with respect to 
interchangeable lenses having open F-values of 1.4, 2 and 2.8, 
respectively, while the upper portion of FIG. 4 shows the displacement of 
the open value signal (f.sub.O) by the second signal member. A straight 
line a in FIG. 4 indicates the variation in resistance value corresponding 
to the difference signal (f-f.sub.O) and a straight line b indicates the 
variation in resistance value corresponding to the open value signal 
(f.sub.O). Resistance values R(1.4), R(2) and so on, corresponding to the 
preset aperture values 1.4, 2 and so on, are predetermined proportionally 
varying values. By the combined value of the resistance for the difference 
signal (f-f.sub.O) and the resistance for the open value signal (f.sub.O) 
the preset aperture value corresponding to this combined resistance value 
is represented. 
Description will first be made of an interchangeable lens having an open 
F-value of 2. When the aperture division f:2 is registered to the fixed 
index mark 37 as the first standard position, the first transmitted member 
33 is displaced in response to the first signal transmitting member 32 and 
the value of the resistance corresponding to the difference signal 
(f-f.sub.O) exhibits the value of .gamma..sub.1 on the straight line a in 
FIG. 4. On the other hand, due to the displacement of the second 
transmitted member 35 in response to the second signal member 34 during 
the lens mounting, the value of the resistance for the open value signal 
(f.sub.O) exhibits the value of .gamma..sub.2 on the straight line b 
because the open F-value is 2. The sum (.gamma..sub.1 +.gamma..sub.2) of 
these resistance value represents a resistance value R(2) corresponding to 
the preset aperture value f:2. The resistance value corresponding to the 
preset aperture value varies along a straight line c.sub.1 which represent 
the straight line a plus the resistance value .gamma..sub.2 of the open 
value signal f.sub.O. 
A large aperture ratio interchangeable lens having an open F-value of 1.4 
will now be described. When the aperture division f:1.4 is registered to 
the fixed index mark 37, the resistance for the difference signal 
(f-f.sub.O) exhibits the value h.sub.1 on the straight line a because the 
first signal member 32 is deviated by an angle .DELTA..theta.. The 
resistance for the open value signal f.sub.O exhibits the value h.sub.2 on 
the straight line b when the second signal member 34 is only deviated by 
an angle .beta. from the second standard position 38. As can be seen, this 
resistance value (h.sub.1 +h.sub.2) is less than the resistance value 
R(1.4) representing the preset aperture value f:1.4. This is because the 
value of the resistance for the difference signal (f-f.sub.O) is reduced 
by (.gamma..sub.1 -h.sub.1) corresponding to the angle .DELTA..theta.. 
Therefore, according to the present invention, the position of the engaging 
end 34a of the second signal member 34 is deviated clockwise by 
.DELTA..beta. from the standard position 38 and the resistance value 
corresponding to (.gamma..sub.1 -H.sub.1) has been added to the resistance 
for the open value signal f.sub.O. By doing so, the value of the 
resistance for the open value signal (f.sub.O) becomes h.sub.2 ' and 
(h.sub.1 +h.sub.2 ')=R(1.4). 
Accordingly, the resistance value h.sub.2 ' for the open value signal 
(f.sub.O) is invariable as long as an interchangeable lens is mounted on 
the camera body and therefore, when an aperture value is arbitrarily 
selected by rotating the preset aperture ring 31, the resistance for the 
difference signal (f-f.sub.O) is varied along the straight line a and the 
preset aperture resistance is varied along a straight line c.sub.2 which 
is the straight line a plus h.sub.2, thus it is possible to obtain an 
accurate preset aperture value. That is, by combining the corrected open 
value signal (f.sub.O +.DELTA.f) transmitted by the second signal member 
34 with the corrected difference signal (f-f.sub.O -.DELTA.f) transmitted 
by the first signal member 32', (f-f.sub.O -.DELTA.f)+(f.sub.O 
+.DELTA.f)=f is achieved. 
Incidentally, in the case of an interchangeable lens having an open F-value 
of 2.8, the situation is substantially similar to that in the case of the 
above-described interchangeable lens having an open F-value of 2, but 
since the resistance for the open value signal (f.sub.O) by the second 
signal member 34 exhibits the value k.sub.2 on the straight line b, the 
resistance value corresponding to the preset aperture value is varied 
along a straight line c.sub.3 which is the straight line a plus k.sub.2. 
Although the construction has been made such that the resistance values 
vary following each signal member, it is also possible to employ a 
mechanical combined signal or an electrical digital signal. 
According to the present invention as described above, even in bright 
lenses having F-values of 1.2, 1.4, etc., an accurate preset aperture 
value can be transmitted to the camera body or to the speed light, which 
is effective for the display of the correct preset aperture value or for 
the control of the guide number in the speed light.