Using ground glass corners???greenspun.com : LUSENET : Large format photography : One Thread
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 tilt to bring the base of the hill in focus. The above left a corner of sky out of the negative. My question is, when you use the cut out corners to check your lens, exactly what should you see? Should you see a round circle of light or elipse??? or what? Thanks
-- david clark (firstname.lastname@example.org), May 29, 2000
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.
-- Pete Caluori (email@example.com), May 29, 2000.
Another view is that the cut out corners interfere with your ability to see to the edges of the frame which means some surprises come time to view the developed image. Viewing the whole image on the ground glass without the corner cutouts is very important to some photographers while others like the cutouts for some reason.
-- Dan Smith (firstname.lastname@example.org), May 29, 2000.
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.
-- Gary Frost (email@example.com), May 30, 2000.
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).
-- Andrew Cole (firstname.lastname@example.org), May 30, 2000.