Film lenses vs. exit pupil + sensor stack thickness

Started Jun 15, 2014 | Questions thread
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ProfHankD Veteran Member • Posts: 5,726
Film lenses vs. exit pupil + sensor stack thickness

I was reading through the always excellent LensRentals articles on sensor stack thickness issues (especially the tables in this article ). I had been vaguely aware of such issues before, but I had always assumed that sensor stacks had about the same optical thickness across all cameras. Knowing now that they don't, I'm thinking about some of the stranger designed-for-film lens behaviors I've seen on digital cameras....

The articles argued that resolution and SA (Spherical Aberration) can be significantly altered. Could the following be due to variations in the exit pupil and sensor stack optical thickness?

  1. I've noticed that some lenses have infinity consistently very slightly off relative to other lenses on the same adapter. I had assumed that this was from wear or was literally designed-into some lenses to take advantage of DoF wide open (i.e., making the infinity stop be where DoF just covers infinity, rather than centered on infinity). Could it be slight focus shift due to sensor stack interaction with the exit pupil?
  2. In my testing, the dreaded PF (Purple Fringing) usually seems to be associated with sensor NIR sensitivity (which varies with different sensors and NIR-blocking filters but is generally higher than most films) and SA in the NIR. I had assumed variations using the same lens on different bodies were primarily due to differences in NIR sensitivity. Could the exit pupil/stack thickness interaction actually be a major cause of PF by increasing NIR SA?

Offhand, I'd bet it at least contributes to #2, but I don't know what's happening in #1. I'm also open to other possible explanations for #1.

Also, is there a quick and easy way to accurately measure the exit pupils of my 130+ lenses so I could check for correlations myself?


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