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Old December 13th, 2013, 02:08 PM
Bart_van_der_Wolf Bart_van_der_Wolf is offline
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Quote:
Originally Posted by Jerome Marot View Post
How would you know that from the cross section?
Hi Jerome,

The last/rear lens is a strongly negative element. It needs to be there if the exit pupil is close to the sensor/film to cover the sensor with its image circle. It also reduces the apparent size of the exit pupil.

When you check the fora for lenses that did not perform well, especially the A7r received a lot of coverage, they in general share that (strongly negative lens(es) at the rear of the design) property. They are also often symmetrical lens designs made for small/shallow rangefinder cameras.

The lenses that did perform well, all have deeper barrel designs with positive elements at the rear, because they were designed for a camera with a deeper mirror box. I've checked with some of the lenses I have, mostly Canon, and they are much more retro-focus designs with their apparent exit pupil position even further away than the mount (which is already 43mm in front of the sensor on Canons).

In general, I'd be careful with lenses that were not designed for SLRs, like the Zeiss lens designs for rangefinder film cameras. Instead I'd look for lenses designed for SLRs with asymmetric retro-focus designs, or specialized par-focal designs (e.g. for Cine applications which tend to not change image magnification when changing focus distance, to avoid 'focus breathing').

Cheers,
Bart
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