Doug Kerr
Well-known member
In a recent thread regarding wide-angle lenses for use on a Sony A7r camera, Ben Rubinstein was kind enough to cite for us this passage from a Sony marketing site, referring to the Sony A7r camera (the emphasis is added):
But as to the description, note that in fact it is not the flange-back distance which (of itself) has any affect on this phenomenon. The optical laws "know nothing" about where the mount interface plane is, compared to the principal points of any given lens design.
What matters to the matter of mean obliquity is primarily the location of the exit pupil of the lens.
And it is in fact this which typically makes the obliquity phenomenon more potent for short focal length lenses. That is because, in a "classical" lens design, the exit pupil will be (for focus at infinity) located in front of the focal plane by exactly the focal length. Thus short focal length "classical design" lenses will have their exit pupil close to the focal plane, and the average obliquity of the rays landing on points well off the center of the frame will be high.
But it may well be that lens design regimes for a lens system with a small flange-back distance may tend to lead to, for a lens of any given short focal length, an exit pupil closer to the focal plane than for other lens design regimes.
Many short-focal length lenses employ "invented telephoto deign". This means among other things that the exit pupil lies (perhaps substantially) forward of the second principal point of the lens. This is often one of the design "tools" used to gain large back-focus distance (not at all the same thing as a large flange-back distance, but often going with it in the practicalities of lens design) for, for example, SLR cameras, where the rear node of the lens must be clear of the mirror.
In a mirrorless camera lens system, that requirement is relieved. The designer may consider short focal-length lenses with a small back-focus distance. That may in turn reduce the degree to which the principle of "invented telephoto design" for short focal-length lenses needs to be employed.
This in turn may result in an exit pupil farther to the rear in such lenses, and in turn a greater exposure to obliquity of the rays landing on the outer reaches of the sensor frame.
Maybe.
Best regards,
Doug
Gapless On-chip Lens Design
Every gapless on-chip lens (OCL) is designed and positioned to cover every pixel and eliminate the spaces between the micro-lenses to collect significantly more light. Moreover, each on-chip lens is optimally positioned depending on its location to accommodate the sharper angle of light entering the periphery, which is caused by larger sensor dimensions being teamed with the E-mount's short flange-back distance.
Of course what this is discussing is a change of the "angle of acceptance pattern" of the photodetector plus its individual micro lens as we move from the center of the sensor frame. This in turn is intended to minimize exposure effect fall off from the greater mean obliquity of the light on photodetectors farther from the center of the frame. And that in fact seems like a worthwhile thing to do.Every gapless on-chip lens (OCL) is designed and positioned to cover every pixel and eliminate the spaces between the micro-lenses to collect significantly more light. Moreover, each on-chip lens is optimally positioned depending on its location to accommodate the sharper angle of light entering the periphery, which is caused by larger sensor dimensions being teamed with the E-mount's short flange-back distance.
But as to the description, note that in fact it is not the flange-back distance which (of itself) has any affect on this phenomenon. The optical laws "know nothing" about where the mount interface plane is, compared to the principal points of any given lens design.
What matters to the matter of mean obliquity is primarily the location of the exit pupil of the lens.
And it is in fact this which typically makes the obliquity phenomenon more potent for short focal length lenses. That is because, in a "classical" lens design, the exit pupil will be (for focus at infinity) located in front of the focal plane by exactly the focal length. Thus short focal length "classical design" lenses will have their exit pupil close to the focal plane, and the average obliquity of the rays landing on points well off the center of the frame will be high.
But it may well be that lens design regimes for a lens system with a small flange-back distance may tend to lead to, for a lens of any given short focal length, an exit pupil closer to the focal plane than for other lens design regimes.
Many short-focal length lenses employ "invented telephoto deign". This means among other things that the exit pupil lies (perhaps substantially) forward of the second principal point of the lens. This is often one of the design "tools" used to gain large back-focus distance (not at all the same thing as a large flange-back distance, but often going with it in the practicalities of lens design) for, for example, SLR cameras, where the rear node of the lens must be clear of the mirror.
In a mirrorless camera lens system, that requirement is relieved. The designer may consider short focal-length lenses with a small back-focus distance. That may in turn reduce the degree to which the principle of "invented telephoto design" for short focal-length lenses needs to be employed.
This in turn may result in an exit pupil farther to the rear in such lenses, and in turn a greater exposure to obliquity of the rays landing on the outer reaches of the sensor frame.
Maybe.
Best regards,
Doug
