135mm on 8x10

[uphereinmytree]

So I mounted a 135mm xenar f4.7 on an 8x10 and am suprised to find more image circle than most of the info on this site leads me to believe. I measured a 200mm circle of illumination when focused at infinity. Granted, it's not very resolved out to the edges, but it's much more than the 160mm of coverage described in numerous threads. Stopped down, the edges aren't sharp, but usable. It is very comparable to the fujinon 125 f5.6 that I compared along side the xenar on an 8x10. The fujinon is sharper closer to the edges, but has nearly the same circle of illumination. Did I miss something in the less than scientific test

[Kevin Crisp]

The circle of illumination does not equate with the circle you can get in focus. When most on this forum speak of "coverage" they are talking about useful image circle -- meaning reasonably sharp across the circle. The little 90mm Angulon will shine light on an 8X10 ground glass, but for most it is a 4X5 lens.

If you find that "edges that aren't sharp" are usable for what you are doing, go for it. For many people, even slight rise on the 135mm xenar would result in sharpness they don't consider acceptable. Since 'sharp enough' is entirely subjective, what matters for you is what you consider acceptable.

[Steve Hamley]

No you aren't missing anything, in fact your curious and observant. I've noted the same thing.

However, I've come to believe that image circle figures can and are specified differently. IIRC, process lens coverage was frequently specified by distortion, because when one made maps for example, the roads had to line up between images and it took very, very, little distortion to make them miss. That would explain why people using process lenses as camera lenses frequently find usable coverage considerably in excess of that expected from manufacturer's data.

The attached image is of a Linhof Bi-Kardan ground glass, made with a Canon G10. Think Linhof made curved ground glass sides and lines? They didn't. If this were a process lens, the IC would be as small as it took to get a believable rectangle, yet we'd find it covers so much more...

Sharpness of tessars is known to fall of more rapidly than some other designs, so possible tessar coverage was specified by resolution needed for a certain enlargement of a given format. So IC could be specified by a given resolution needed for a certain enlargement. If you're selling lenses and want satisfied customers, such an approach would make the world of sense.

And if you're contact printing, you can get away with a lot, so maybe the IC for a tessar (or whatever) could be much larger if the negative and print resolution are essentially the same.

BTW, I have a 125mm Fuji CM-W that's supposed to cover 204mm, or about 6mm short of 5x7. In fact, I find it covers with a few mm of rise possible. I wouldn't really call it a 5x7 lens although I occasionally use it for such - and contact print.

Cheers, Steve

[uphereinmytree]

as a side note, the 135 xenar seems a bit more contrasty than the 125 fujinon at least on the ground glass