Image circle coverage help

[Momentz]

Hi
when a lens says it has an image circle cover of say 235mm does that mean it will cover a film like a 6x17 film?

i am looking at one 75mm lens and it says 170mm@F4.5 and 235@f16 so as long as i stop it down to atleast F16 i will be right and wont get any vignetting?
Cheers

[Nick_3536]

Depends on what you mean by cover.

6x17 is smaller then 5x7. So techincally any lens that covers 5x7 covers 6x17. The question is how much movement will you use? 235mm is more then the requirement for 5x7 but the more movements you use the more you'll want.

[Ron Marshall]

Hi
when a lens says it has an image circle cover of say 235mm does that mean it will cover a film like a 6x17 film?

i am looking at one 75mm lens and it says 170mm@F4.5 and 235@f16 so as long as i stop it down to atleast F16 i will be right and wont get any vignetting?
Cheers

6x17 is about 56x169mm. The diagonal of the two sides is the minimum image circle required to cover (@f16 or smaller aperture), with no movement at all.

diagonal = square root (56 squared + 169 squared) = 179mm.

235mm is ample coverage on 6x17!

The following page lists the amount of movement possible with various lenses on 6x17:http://www.largeformatphotography.info/lenses/LF6x17cm.html

[Brian Ellis]

Just look at the diagonal of the film format. If it's smaller than the stated coverage at a given aperture then at that aperture the lens will cover the format in the sense that you will get no obvious vignetting. However, that doesn't mean the image will necessarily be sharp in the corners. The image may be noticeably soft in the corners. Also, as you have noted, "coverage" in theory is increased as you stop down, by how much depends on the design of the lens. Some lenses (e.g. the G Clarons) continue to increase the more you stop down, others reach a point of maximum "coverage" and don't increase even with further stopping down. I put the word "coverage" in quotes because when you stop down you aren't really increasing the size of the image circle, you're just improving the corners and the edges so the "usable" coverage increases. That's why you'll sometimes see a coverage number at say f5.6 that's less than the diagonal of the film but you'll find that the lens will nevertheless cover the format when stopped down. The 210 G Claron is one example. Its stated coverage is less than the diagonal of 8x10 film but when stopped down to about f16 it covers 8x10 and if stopped down further coverage continues to increase. I'm sure an optical scientist type could explain all this better and more accurately but this is the general idea.

[Bob Salomon]

Be safe. Measure your actual film diagonal, or the opening in the camera. 6x17 is a nominal size not the actual size.

As an example a Linhof 612 camera or roll back is 56 x 120mm - that is a diagonal of 132.4mm.

Most other 612 backs are 56x111 or 55x112mm - they would have diagonals of 124.3 and 125.2mm.

That can make a difference if you need a lens that gives you the maximum amount of movenet.