[rollei_list] Hello again, and a lens question

  • From: frank deutschmann <fdeutschmann@xxxxxxxxxxxxx>
  • To: rollei_list@xxxxxxxxxxxxx
  • Date: Fri, 11 Dec 2009 11:40:18 -0500 (EST)

Hi everyone,
It's been a long time since I posted here (though I will confess to occasional internet stalking); to much other stuff going on to keep up with this busy info-heavy list.

Recently, I've been trying to improve my meager optical knowledge (knowledge of optics actually); to that end, I've been flipping through Ray's _Applied Photographic Optics_, mostly just looking at the pictures because math really isn't my thing. (Hey, I'm a quant working on subprime mortgages, what can you expect?!) As I know there are a few of you on here deeply immersed into optics, I thought I would drop by and ask a question....

Anyway, Ray makes an interesting comment in Section 23.3 (pg 239 in the softcover 3rd edition): "When portrait lenses for large formats were of very long focus, up to 1m or more, a working aperture o f/16 gave an entrance pupil diameter comparable to the human interocular distance (IOD) of 63.5mm. This pupil area provides a large number of different viewpoints which integrate in the film plane to give a diffused image. This effect of stereo parallax is therefore not a true plane perspective but gives a psychological effect of 'roundness' or 'plasticity' which many people consider to be more natural than an accurate centeral perspective of the sitter. The small depth of field, giving progressive loss of image contrast to facial features and reduced sharpness, also contributes to this effect."

Alas, this large number of integrated viewpoints and the IOD is not discussed anywhere else in this tome that I can find, so I am left with quite a few questions! (Incidentally, this is emblematic of this book; as large as it is, for me the book has inspired more new questions than answered existing ones, so quite an excellent book....)

Avoiding the dreaded OT mark in the subject line, I will comment that I have particularly noticed the photos, particularly people, pet, and object portratits, that I have taken with my 180mm/2.8 tele-xenar (6008) often show this wonderful 3D quality, especially when looking at the bare chrome through a loupe on the light table. I get this effect with this lens more than any other, and on reading this passage I realized that shot wide open (typical for these portrait situations) the entrance pupil is within the realm of the IOD as Ray mentioned. This lens is certainly not the absolute sharpest I own, nor is it soft; stopped down, it is often overly harsh for subjects other than children and perhaps furry friends. Even now, seeing this 3D effect on the light table, through one eye, still impresses and fascinates me; it is at once pleasing and intriguing.

So I'm wondering, is this roundness effect largely a result of the entrance pupil diameter, combined with pleasingly shallow and well-allocated focus depth? And, if so, given the multiple viewpoint aspect, am I right to assume that this is an effect concentrated in the part of the image which is just short of sharp focus? And, it seems to me that to maximize this effect, there's no substitute for large formats, as the distance to the subject needs to be appropriate for a pleasing perspective (so 180mm is pretty much the limit for MF portraits)?

Can anyone point me to further reading on this fascinating subject?

Thanks very much for your time, and I'm glad to see this list still exists!
-frank
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