[rollei_list] Re: Hello again, and a lens question

  • From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
  • To: <rollei_list@xxxxxxxxxxxxx>
  • Date: Fri, 11 Dec 2009 13:28:46 -0800
----- Original Message ----- From: "frank deutschmann" <fdeutschmann@xxxxxxxxxxxxx> 
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Friday, December 11, 2009 8:40 AM
Subject: [rollei_list] Hello again, and a lens question



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
I've seen this stated before and there is a trick for obtaining stereo images with a single large lens using an obstructing stop with two apertures but its been so long since I've seen it I don't know where to find it. The effect requries a very large lens,much larger than any of the Rollei lenses. Interocular distance is the distance between the eyes, normally about 62mm. However, a strereo effect can be gotten with smaller separation. The lenses Sidney Ray was talking about were probably about four or five inches across. These very large lenses have a slight stereo effect on the ground glass due to the imperfect diffusion. One can actually see a slight change in the image when moving across the back of the camera. However, on a single film the effect is to blur the background more than may be apparent on the GG. The effect on the Rollei lens may be from a depth of field effect but the lens diameter is not large enough to produce the effect Ray is writing of, at least to any great extent. For more on lenses that is not too technical find two books by Rudolf Kingslake: _A History of the Photographic Lens_ and _Lenses in Photography_ The latter went into more than one edition but the basic text is the same. Kingslake was head of Kodak's lens department from 1939 until his retirement in 1961. Over this period Kodak was making some of the best lenses in the world. He is a very good writer and anything he wrote is worth reading. 

--
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 
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