flat field (symetrical)lenses and focus

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I recently got into a "discussion" with my photo professor about flat field lenses, specifically the G Clarons. I have a 150 Claron, and when I researched flat field lenses before I bought it the only potential problem I found with the symetrical design of these lenses was that there was a sharp drop off of the area of sharpest focus from the plane of focus. Presumably, this is why Schneider recommends that you should stop down to f22 or smaller if you use the lens for shooting anything beyond its optimum range. When I tried to explain this to my prof. he said he had no idea what I was talking about and implied that I had no idea what I was talking about either. I no longer have the info I got about this issue and can't seem to find it on the web. If anyone knows where I can find it or can tell me about it, it would be appreciated. It was not fun being made to look like a fool in front of the class and I would like to go to next class (4/16/01)armed with some data. Thanks.

-- John Laragh (jwl@taconic.net), April 11, 2001


John, Could you explain more clearly what kind of focus drop you've found? It's on the borders or merely short depth-of-field?

Cesar B.

-- Cesar Barreto (cesarb@infolink.com.br), April 11, 2001.


There is an article by Ron Wisner titled "The Myth of Flat Field Lenses" at www.wisner.com/myth.htm. Whether it supports you or your prof, I don't know.

-- Chris Patti (cmpatti@aol.com), April 11, 2001.

I'll throw my 2 bits in too (hopefully it makes sense; I think it does but it probably won't tomorrow).

Wisner's article is quite good and I think describes flat field lenses and their Petzval sums well but is verbose. Many years ago a lens designer explained it to me this way: if we assume that the object and the image are both flat & parallel to each other and also to the lens then the only thing that prevents the light wavefront from reaching the image plane all at the same time is the glass. Most of the lens elements, as you know, have varying thicknesses from center to edge with the thicker part in the center (convex). This greater glass thickness slows down the light more causing the light thru the center to be delayed giving an unflat field. (This neglects the concave surfaces and the elements with different Nd and also the air spaces)

Now, I'm not completely sure I buy this anymore because I also understood that each part of the lens surface contributes to each part of the image. (That's why a scratch doesn't affect the image the way we think it should). However, if we do agree with it then stopping down the lens should help reduce the field curvature because a smaller area of the lens is used which has proportionately less curvature. It also increases the depth of field so that whether or not the image is focussed on the film or not, it'll be good enough.



-- Duane K (dkucheran@creo.com), April 15, 2001.

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