Spotting vignetting and cut off groundglass cornersgreenspun.com : LUSENET : Large format photography : One Thread
I'm new to large format photography and I've been shooting to learn, setting myself little problems to be solved by camera movements. Yesterday I developed some negatives and found my first case of vignetting due to a front rise that went too far and cut off the upper corners of the frame. Trouble is that my groundglass, like many as I understand it, is cut off at the corners. How are you supposed to be able to check vignetting if you run out of ground glass to see it with? I did read somewhere about peering through the corner gap and looking at the lens opening to check vignetting, but I don't know what I'm looking for if I do so. Can someone help?
-- Tony Galt (firstname.lastname@example.org), February 18, 2002
If the edge that you see through the corner is curved, you are looking at the rim of the lens, and this will result in vignetting of your image. If you are seeing no edge at all, you should get a good image.
-- Graeme Hird (email@example.com), February 18, 2002.
The groud-glass corners are cut so you can prevent vignetting. The procedure is to compose the image and execute your movements. Then, peer through the corners (usually the bottom two only, as most vignetting movements will be rises) - if you can see the lens opening, you are half-way there (if you cannot see the opening, you care fully vignetted, and need a serious correction).
If you can see the lens opening, pay attention to the shape - if it is a full circle, then there will be no vingetting. If, however, part of the opening is obscured by the rim of the lens (or the bellows, as sometimes happens) then you need to ease off on the movement until you can see the entire opening - then there is no vignetting. A quick practice run with this will give you the idea, and if yuo always remeber to check this way, you will not have the problem again.
-- Eric Boutilier-Brown (firstname.lastname@example.org), February 18, 2002.
Tony: This is often easier to do the other way around. Stop the lens down to taking aperture. Go around the front of the camera and look through the lens. Can you clearly see the corners of the ground glass? If not, you're in trouble.
-- Kevin Crisp (KRCrisp@aol.com), February 18, 2002.
Just to emphasize the point made in the previous post: Check for vignetting at the taking aperture. There is less likelihood of vignetting than with the lens wide open.
-- Henry Friedman (email@example.com), February 18, 2002.
To make it easier to learn what this looks like, take the back off the camera and move your eye to the corners. By doing this you'll quickly get a sense of whether you are in peril of vignetting. This is especially useful if you're having trouble seeing the corners due to an awkward camera position. Then put the back on and check the corner notches as the others have described, giving particular attention to your potential trouble corner(s).
-- Ted Kaufman (firstname.lastname@example.org), February 18, 2002.
Thanks for all your kind help. This forumn is a perpetual view camera workshop and it is extraordinarily valuable to those of us who are just starting out. I will try all the suggestions you've given me.
-- Tony Galt (email@example.com), February 18, 2002.