Can anyone tell me how to size up the performance of a lens for myself, as opposed to trusting what someone else thinks is good. Specifically, (on a 4X5) I have an Ilex Paragon Anastigmat F 4.5 E.F. 5" Series S on an Ilex optical co. #1 universal shutter: tested out as 1 3/4 1/5 1/10 1/25 1/50, or as the guy who tested it for me said, this is a good one. F ranges from 4.5 to 45. Glass is uncoated. Has what appears to be one rear element and two front elements, although I couldn't unscrew the front elements to see hwo they were made. Subjectively, it seems to me that for some reason the lens is sharp and performs better at f 8 than at f 16 or 22. In theory this shouldn't be the case. Then I read that a tessar design, which this seems to me to be, shouldn't be used but under f 11. Sometimes this old lens seems sharp and at other times it seems really bad. How do you determine what's what with a test. Or how do you make optimal use of a half-way decent lens? Also, how can you test a lens for circle of illumination? Would shutter vibration be a factor, and how would you know you had it if it was a problem? Or what used lenses are famous for their tonality range? Thank you, David

-- david clark (doc@ellensburg.com), October 16, 1998

Answers

>> Can anyone tell me how to size up the performance of a lens for myself, as opposed to trusting what someone else thinks is good.

Use it. It's the only way. Sure, you can also get lens testing charts if you are interested in objective tests, but subjective testing may be more important.

IMHO, lens testing, whether subjective or objective, is most useful when comparing lenses. Here, you might test the Ilex alongside another lens that you already know, even if it is a different format. Then you can directly compare sharpness, contrast, bokeh, or whatever you are most interested in.

>> Subjectively, it seems to me that for some reason the lens is sharp and performs better at f 8 than at f 16 or 22. In theory this shouldn't be the case.

The lens may have been made for a hand-held camera, and optimised for a relatively large aperture. In any case, if f/8 is sharper than f/16, then accept that fact.

>> Sometimes this old lens seems sharp and at other times it seems really bad.

If this happened to me, I would check that there wasn't anything loose in the lens,and it wasn't fogging up in humidity. Then I would check my own technique.

>> Also, how can you test a lens for circle of illumination?

Test it. Put it on a 10x8 camera, or use the 5x4 and shift the lens or back or both to get maximum displacement, and photograph a brick wall, or a series of lens charts, parallel to the lens and film. Look at the negative or slide, and determine where the limit that is acceptable to you lies.

>> Would shutter vibration be a factor, and how would you know you had it if it was a problem?

It's usually only a factor when hand-held or on a flimsy tripod, and the vibration should be very much lower than a 35mm SLR. Again, you can do a practical test. With a tripod, photograph a lens chart at different shutter speeds, varying the illumination of the chart to get a constant aperture. If the negs with the faster speeds are sharper than those of the slower speeds, then you have camera shake.

-- Alan Gibson (gibson.al@mail.dec.com), October 16, 1998.