flat field (symetrical)lenses and focus

[John Laragh]

I recently got into a "discussion" with my photo professor about flat field lens es, 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 syme trical 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 recom mends that you should stop down to f22 or smaller if you use the lens for shooti ng anything beyond its optimum range. When I tried to explain this to my prof. h e said he had no idea what I was talking about and implied that I had no idea wh at 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 li ke a fool in front of the class and I would like to go to next class (4/16/01)ar med with some data. Thanks.

[Cesar Barreto]

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.

[Chris Partti]

John,

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.

[Duane]

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.

Cheers,

Duane

[redu]

I would like to resurrect this topic instead of puting up a new one for the sake of integrity of this flat field discussion. I have been doing some test for some time now on some lenses (repro/process, enlarger, proper macro and taking) for close up work around 1:2.5 magnification. With every lens, I am shooting the same object at different apertures (max, f11, f22, f45) with the same image size as much as possible.

Actually i believe for a lens it shouldn't matter whether the object is 2D or 3D. A bokeh is a bokeh and i have not seen a "type of lens" parameter in the depth of field calculation. But the lenses differ. I notice that so called flat field lenses (I guess these are process and enlarger lenses) are mostly single coated. This is because they are working only in controlled lighting conditions i believe. Besides most of them have simple aperture mechanisms (6 blades) since i think they are not intended to result a creamy bokeh at all. Also most of them have f9 as maximum aperture since you have more than enough light to work on the GG in a process room. Other than that i think they are perfectly good macro / table top product taking lenses eventhough one shouldn't be surprised to notice chromatic abberations when they are used to focus at itfinity. Similarly but on the contrary i have noticed that CA starts walking in when a Sironar N 210 5.6, which is a diamond for general purpose photography, is used for 1:2.5 table top product work.

I have not completed my tests yet but according to the initial results i can say that 150mm f9 G Claron is as sharp as the 120mm f5.6 Macro Symmar HM at f11 and f22. Only slightly less contrastier. I expect the compendium that i have ordered to improve the contrast on G Claron. Once i finish the tests properly i hope to share the results with you in an elegant way. I do the tests on a 8000x6400 Powerphase scanback which completely eliminated errors due film handling and film scaning.