[6BQ5]

Most film-era lenses are sharp in the center because the light is tele-centric (i.e. the light is projected into the well of the photo site straight on). The corners tend to suffer because the light is oblique, hitting the well at an angle. We never noticed this effect on film because any point on film can accept light from any angle.

How do we think the FA Limited lenses will perform on the upcoming FF body? Are the designs tele-centric for projecting as straight-on as possible the corners and borders? If they are not then I wonder if we'll all see an updated FA Limited line of lenses soon after release of the FF body.

I'd love to see some test data from a FA Limited lens mounted on a FF body like the A7.

[stevebrot]

I'd love to see some test data from a FA Limited lens mounted on a FF body like the A7.

I believe there are anecdotal accounts and example photos on the non-Pentax section of this site. As for tele-centric lenses, the key is the location of the exit pupil. With long flange distance cameras, this is essentially a non-issue. It is more of a problem with mirrorless cameras mated to rangefinder (e.g. Leica M or thread mount) wide angle lenses, many of which have rear elements mere millimeters from the sensor surface. I don't believe there are any true tele-centric lenses made for SLR cameras (image circle diameter must equal exit pupil diameter).

BTW...I believe the issue with angle is vignette, not sharpness and is a problem on film as well*. Light fall-off is a factor of both distance and angle and may be calculated as cos θ ^4.


Steve

* Huge problem for view cameras where radial ND graduated filters are sometimes required

[HYS]

Most film-era lenses are sharp in the center because the light is tele-centric (i.e. the light is projected into the well of the photo site straight on). The corners tend to suffer because the light is oblique, hitting the well at an angle. We never noticed this effect on film because any point on film can accept light from any angle.

How do we think the FA Limited lenses will perform on the upcoming FF body? Are the designs tele-centric for projecting as straight-on as possible the corners and borders? If they are not then I wonder if we'll all see an updated FA Limited line of lenses soon after release of the FF body.

I'd love to see some test data from a FA Limited lens mounted on a FF body like the A7.

The telecentric lens has exit pupil at infinity. So the chief ray is parallel to the optical axes of the system.
And the cone of rays strikes the sensor normally. This is necessary for measurements. In photography
the oblique ray cones cause larger volumes of activated corns in the classical film and in this way
the silver halide "image of a point source" is greater. The resolution is worse. But on the other hand
to realize a telecentric lens one needs larger diameters of the lenses (usually). This is unpractical in
photography. So they make (the firms) some piece of compromise.
And don't publish much information considering this lens property.