Understanding lens projection circle

KayGee14x17-ATL · Jan 21, 2025

Good day fellow ULF-ers. I am building up a 14x17 due to the realization that I have had a ULF lens for years. It is the Schneider Kreuzuach 360/620. The specs I found on this lens stated a 500mm images circle, with means I should be getting fall off in the corners. Much to my surprise I see no fall off on the ground glass, and seem to have a crazy amount of rise and fall with no visible fall off on the ground glass. Yahoo! So does this mean that the factory images circle of 500mm if for optimal projection, and the quality of image will start to diminish outside of that 500mm while the actual edge of the full projection is much greater?

xkaes · Jan 21, 2025

That's pretty much it. The image circle (IC) is not static. First, it varies with the aperture -- so it's not the same size when the aperture is open or stopped down. Most IC sizes are listed at f22. The IC also varied with the subject distance. The listed IC is typically at infinity, but it grows dramatically the closer the subject is to the film. The other factor is the image quality. This is based on the manufacturer's standard. The farther away from the center of the image the light falls off and the resolution decreases. If an IC is listed as 500mm, that does not mean there is no image passed 500mm. It means that the quality does not meet the lens maker's standard -- but it might meet yours. And how far out you can go is up to you -- on some lenses it's not very much at all, but on others it's quite a bit.

KayGee14x17-ATL · Jan 21, 2025

xkaes said:
That's pretty much it. The image circle (IC) is not static. First, it varies with the aperture -- so it's not the same size when the aperture is open or stopped down. Most IC size is listed at f22. The IC also varied with the subject distance. The listed IC is typically at infinity, but it grows dramatically the closer the subject is to the film. The other factor is the image quality. This is based on the manufacturer's standard. The farther away from the center of the image the light falls off and the resolution decreases. If an IC is listed as 500mm, that does not mean there is no image passed 500mm. It means that the quality does not meet the lens makers standard -- but it might meet yours. And how far out you can go is up to you -- on some lenses it's not very much at all, but on others it's quite a bit.

Thank you very much for the insight! looking forward to sharing images soonish. Cheers!

Ian Grant · Jan 21, 2025

That 500mm IC is at f16, later Schneider change to give the ICs of their LF lenses at f22. The later 360mm Symmar S was a f6.8 lens and smaller, and a reduced image circle.

I'd agree with @xkaes, assuming you are contact printing it's unlikely you would actually notice any quality drop off at the image corners. I use a couple of Schneider lenses that technically don't quite cover 5x4, or have very limited movements, but in practice with care there are no problems. It's just a case of any tilt is used with corresponding rise/fall to stay within the image circle.

Have fun and let us see the results.

Ian

Understanding lens projection circle

KayGee14x17-ATL

xkaes

KayGee14x17-ATL

Ian Grant

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