lense coverage and image circle

[phil sweeney]

Per my recent post on the g-claron 210mm: trying to understand more and looking for a better 210mm for 8 x 10: I understand users find lenses that do better than the manufacturer's specs.

I thought I might evaluate the g-claron 210mm and a rodenstock 210mm sironar-N by putting them on my 7 x 17. (Per sandy king's post on the computar 210(http://largeformatphotography.info/lfforum/topic/498719.html)). If I have this right to just cover 8 x 10 we need a 325mm image circle. The g-claron image circle spec is 260mm. The rodenstock sironar-N is 301mm. So maybe the rodenstock will do well?

Also a 210mm angulon spec is 362mm image circle and I saw a lens repro ad stating it is a 11 x 14 lens - isn't that pushing it?

Last: is schneider more conservative than rodenstock on specifying image circles or is there just no good sense to thses specs?

[Michael Jones]

Phil:

re: the 210 Angulon on 11x14. My version works well with plenty of sharp coverage for significant movements on 11x14. Others' results may vary as QC is sometimes an issue. Good luck.

Mike

[sanking]

Phil,

Bear in mind that most lenses have a much wider circle of illumination than the circle of good definition. Most of the lens specifications provided by manufacturers are based on an aperture of f/16 or f/22 and on the assumption that the image will be enlarged several times. Therefore, the stated angle of coverage is based on the circle of good definition, not on the actual circle of illumination. This makes no sense for ULF work where we contact print ssince we only need about 10-15 lppm for maximum apparent sharpness.

This basically means that most of the lens spcifications are useless for ULF work. A good case in point is the G-Claron. Scheneider gives the angular coverage of this lens as somewhere aroudn 65 degrees but people who use this lens on ULF cameras know that its actual circle of illumination is more on the order of 81 or 82 degrees, which means a 305mm G-Claron will easily cover 7X17" and a 355 G-Claron barely covers 12X20. But you must stop down the lens to f/45 or f/64 for decent performance on the far corners.

The same is true for many other lenses. Red Dot Artars, Apo-Nikkors, etc. have a circle of good definition that is equal to less than 50 degrees of angle, but if you close these lenses down the performance is equal to at least 10-17 lppm out to 65-70 degrees. That is why a 24" Red Dot Artar or Apo-Nikkor, which have a stated circle of coverage of only about 460mm, will easily cover the near 600mm diagonal of the 12X20" format.

[Jim Galli]

Consider also that the original target for the G-Claron lens was Graphic arts reproduction. Those folks requirements for MTF fall-off are entirely different than ours. I'm just guessing here but I'll bet 62 degrees is at a point in the MTF curve where drop off reaches a maximum allowable for flat field reproduction. We can tolerate much farther down that curve for 8X10 contact prints and even enlargements.

[Jim Rice]

As an aside, it's my understanding that modern lenses will not project a larger image circle than is acceptably sharp to the manufactuer. Is that correct? I've toyed with the idea of using my 210 Sironar-N once I aquire the rumored dirt cheap Green Monster. I figured that I'd lose the corners to vignetting.

[sanking]

"As an aside, it's my understanding that modern lenses will not project a larger image circle than is acceptably sharp to the manufactuer. Is that correct?"

I think what you say is true. Most modern lenses do appear to be mechanically vignetted so that most of the circle of coverage is acceptably sharp. This serves to reduce flare. However, it is not true of all modern lenses. For example, the Nikkor-M has a much larger image circle than what would be acceptably sharp for projection printing. In fact, the circle is so large that it will even cover 20"X24" with acceptable sharpness for contact printing when stopped down.

[Jim Galli]

Hey Jim, Noticed a guy over on APUG has one of those C-1's for a pretty fair price. You could use the Sironar for a time. Just pick subjects that are near instead of far.

[phil sweeney]

Based on Sandy's comments I looked at the g-claron 210mm and the 210mm sironar-N on the 7 x 17 at f22. I measured to where there appeared to be no illumination falloff and measured 12 3/4 in for the g-claron and 11 7/8 in for the rodenstock. So with a 8 x 10 diagonal of 12.8 in it indicated what I already knew: the g-claron just covers 8 x 10. Not a highly precise test, I would not recommend the g-claron for 8 x 10!