scott kirkpatrick
Well-known member
telecentricity...
Such a lot of misinformation is shared about telecentricity. First of all, modern rangefinder lenses are far from symmetric. Consider the pair of rangefinder wideangles below. The 1960-ish Canon 19/3.5 on the left extends 19mm in back of the mount, and only 12 mm in front of it. The front element is more than twice the size of the rear element, but at least the exit pupil must be behind the mount frame. The 2000+ Zeiss 21/2.8 on the right extends only 12 mm behind the mount frame, but extends about 50 mm out in front. As a wild guess, the exit pupil might be 30-40 mm away from the image plane. (it takes an optical bench to find out these things; they don't seem to publish the information.) All of the currently manufactured Leica M lenses are similarly much longer than their focal lengths, at least up to 35mm focal lengths and probably beyond as well. The extra length is being used to accomplish the degree of correction now offered.
I think it was corrected far above in this thread, but light rays coming from a lens cannot all be parallel. No focused image would result.
scott
Such a lot of misinformation is shared about telecentricity. First of all, modern rangefinder lenses are far from symmetric. Consider the pair of rangefinder wideangles below. The 1960-ish Canon 19/3.5 on the left extends 19mm in back of the mount, and only 12 mm in front of it. The front element is more than twice the size of the rear element, but at least the exit pupil must be behind the mount frame. The 2000+ Zeiss 21/2.8 on the right extends only 12 mm behind the mount frame, but extends about 50 mm out in front. As a wild guess, the exit pupil might be 30-40 mm away from the image plane. (it takes an optical bench to find out these things; they don't seem to publish the information.) All of the currently manufactured Leica M lenses are similarly much longer than their focal lengths, at least up to 35mm focal lengths and probably beyond as well. The extra length is being used to accomplish the degree of correction now offered.
I think it was corrected far above in this thread, but light rays coming from a lens cannot all be parallel. No focused image would result.
scott