Using ground glass corners???

[doc]

Hi All, I left the corner out of a picture under the following conditions: I had a lot of front rise to include the top of a hill, and I had a little downward t ilt to bring the base of the hill in focus. The above left a corner of sky out o f the negative. My question is, when you use the cut out corners to check your l ens, exactly what should you see? Should you see a round circle of light or elip se??? or what? Thanks

[Pete Caluori]

Greetings,

When you check for coverage by looking through the cut out corners of the ground glass, you should see a "circle" of light through the diaphragm of the lens with no obstructions. As you stop the lens down, the diaphragm gets smaller and the image circle generally increases. I say "circle," because the diaphragm is composed of blades which overlap and never form a perfect circle.

Typically, you'll see the front barrel of the lens, or the lens shade and as you stop down it will disappear (as long as you don't exceed the image circle of the lens.) In practice, I have found that is not always enough. If I stop down just to the point where I see a perfect "circle" of light I will still get slight vignetting, so I usually close down one additional stop.

Regards,

[Gary Frost]

I have seen some backs that leave a sizable chunk out of the corners that looks like it could interfere with composing the image. My camera has the smallest of peepholes cut from the corners that is not a problem. When using movements, the cut out corners are to check for vignette. It is the only reliable way as you cannot accurately estimate light falloff in the corners of the ground glass. You look through the corner openings and make sure that it is the aperture blades that limit the aperture, and not some part of the lens barrel or filter, hood, etc. It still may not appear perfectly round as you are looking at it from an oblique angle, it can appear more oval esp. with wider angle lenses.

[Andrew Cole]

I haven't been doing this as long as most of the other folks posting, but a trick someone once showed me for cameras without the cut out corners, is to do the same in reverse. Look back through the lens and see if you can see the corners of the groundglass through the round hole of the diaphragm. Again, if the view of the corner has the lens flanges intruding into the circle of the diaphragm, you're probably going to lose a bit of light. Same idea, different way of looking at it (no pun intended).