Noah A
17-Sep-2013, 13:15
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?
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?