Checking lens coverage with clipped corners

[Noah A]

Ok, I know this is a stupid newbie question but I'm having problems with a new lens and I'm not getting the answers I need in the other thread I started, which may have been to specialized, so here goes...

I've always thought that one could check the coverage of a lens by looking through the clipped corners of a groundglass. I was taught this in college and have been doing it ever since with various cameras and lenses. I was under the impression that if you could see a round aperture then you're good to go. And if you see an oblong aperture, like an American football, that you didn't have sufficient image circle.

I know that in some cases this may not be perfect, since a lens may illuminate a larger area than it's designed to cover with good image quality. But in my case I'm having the opposite problem. I have a lens, a 120 Apo-Symmar L, that should allow for 20-25mm of rise at f/22 and infinity, but I only get a round aperture with one or two millimeters of rise.

And yes, I've checked the alignment of my cameras (TK45S and MT2000), the lens is properly mounted on a genuine Linhof lensboard, there are no hoods or filters mounted, all other movements are zeroed, etc.

I've also tried looking through the lens instead--I can see the corners of the groundglass but when my eye is in that position, the aperture is football-shaped instead of round.

What's blocking the image and making the aperture oblong is the front barrel of the lens, I can see that clearly.

So is my basic premise flawed--do some lenses provide good image quality in the corners even if they don't appear to cover using the clipped-corner test?

[ic-racer]

It is always oblong. When the sliver of light goes away, coverage is nil. The thing to remember is that it is only demonstrating coverage to the diameter of the clip in the glass. So even if it is 'good to go' by visual inspection through the clipped corner the corner can still be cut off depending on how much glass you have clipped off.

[Mark Sawyer]

Pretty much what ic-racer said. When you can see the entire aperture through the corner openings, you have full illumination, regardless of whether the aperture is round or oblong. When the barrel or hood starts to cover part of the aperture, you get into degrees of mechanical vignetting, which is what you seem to be hitting. It sounds like you've checked all the right things, and your lens just doesn't have as much coverage as advertised because the front barrel is in the way. (I'd be tempted to pop it on an 8x10 and shoot a paper negative at f/22 and see if you really get the advertised 189mm circle.)

Keep in mind that full illumination doesn't mean the corners are getting as much light as the center, especially with short lenses. That's why there are center filters.

[Drew Wiley]

If the aperture looks oblong, it means you're nominally covered, but with some distortion - typical of wide-angle applications perhaps, but not ideal.