Computar/Kyvytar/Graphic Kowa Field Curvature

[mrossano]

I posted this question on photo.net, but haven't uncovered any first-hand reports, so please be kind enough to indulge me here:

It has been discussed many times that the lenses seen in various guises as Computar f/9, Apo Kyvytar and (sometimes) Graphic Kowa have exceptionally large coverage. It has also been mentioned that field curvature becomes significant as we approach the limits of the 95 or so degree coverage. Based on the experience of actual users, at what point (75, 80, 85, 90 degrees?) does this become detectable (as opposed to objectionable, which is a looser and more subjective criterion)?

I'm trying to decide on one of four options in the 240mm-250mm range for use as a semi-wide angle on 8x10:

240mm Computar/Kyvytar/Graphic Kowa
250mm (10") Wide Field Ektar
240mm Docter Germinar-W
250mm f/6.7 Fujinon W

I expect to use the lens for architectural exteriors, among all else. Past experience has shown that for my work habits, noticeable field curvature gives me fits, so I'm trying to pick a lens that exhibits little or none within an 80 to 85 degree field. Anyone care to relate their experience?

Thanks!

[Jason Greenberg Motamedi]

I have been using a 210/9 Computar for 6 years or so. I have noticed field curvature on that lens, however only at the very ends of its image circle, between 90 and 95 degrees. Practically this means that there is no significant effect with 8x10, but there is with 11x14. My brief use of a 240/9 Computar on 11x14 indicated that the field curvature started at roughly the same degree of coverage.

[Jim Galli]

I don't have a complete answer for you, but once upon a time (before I had my web site and scanned stuff, so no evidence exists) I tested several of the f9 process 210mm lenses next to each other. Schneider G-Claron, Computar, E. German Ultragon, and Whatever Rodenstock called theirs. Of the 3 plasmats (exclude the Computar, it is 6 elements 6 group) they were all very similar and reached out to + or - 370mm. The G-Claron was by far the best. The other 2 got pretty mushy. The Computar was in a class all it's own. Seems like I remember it reaching out to about 410mm and it was sharp to within 4 or 5 mm from the edge. Quite a good performance. The other more recent contender is the Germinar that Kerry Thalmann helped import and distribute.

[Rick Moore]

I have not used the Computar, Ektar, or Fujinon lenses you list, but my Docter 240 Germinar-W is one of my very favorite 8x10 lenses. Small, light, very sharp, and excellent coverage.

[Dan Fromm]

Mark, how are you measuring field curvature?

Given the dependence of curvature of field on aperture (stopping down reduces it) and field angle (wider increases it), you may have to stop down more than desired or use a longer lens than desired (and stand back farther) to get more-or-less the results you want.

[mrossano]

Thanks to all for the commentary.

My assessment of field curvature is entirely empirical. I align the lens and film planes parallel to my favorite large, flat wall. Staying at least 100 focal lengths away (preferably more) I focus on the center of the image circle using my 4x loupe. I then check focus at both edges of the image circle, using shift, if necessary, to get there, and confirming parallelism. If the focus difference is within my expected range of focus error, I'm calling the field flat.

I agree that the 240 Germinar W is a sweet lens. I bought one from Kerry, mounted it in a Prontor Professional shutter, and later sold it. I kick myself regularly for doing so. If anyone here decides to sell theirs, I would like the chance to make an offer.