checking for vignetting with frount risegreenspun.com : LUSENET : Large format photography : One Thread
I have a 4x5 camera with 135mm f/4.5 Tessar type lens. The image circle should be very limited for movements yet when I give about 1 inch of frount rise, I don't see any vignetting on the ground glass. This is when viewing the image at f/4.5 (wide open). Why doesn't the ground glass show the vignetting? Certainly there should be some since I am rising the frount more than 6mm. How does one explain this?
-- John Dorio (firstname.lastname@example.org), February 17, 2000
John, I don't know exactly how to explain why the vignetting is not visible on your ground glass but there is a simple trick to avoid unseen vignetting affect your image. I had on my camera a Sinar ground glass with cut corners, and now a Bosscreen that has it too, very handy to make this simple checking: Looking through the hole from the corners of the ground glass into the lens, make sure you can see a perfect circle of light from the lens. If you see an oval, try closing the lens more until it gets circular. If the lens barrel (or the shade) are still in the way, lessen your rise or fall until you get the perfect circle. If you have a plain GG, you will have to remove your back to make the checking.
-- Paul Schilliger (email@example.com), February 17, 2000.
Vignetting is often only visible when the lens is stopped down. Wide open, the edge of the image circle is too poorly defined to make out vignetting. The above-mentioned trick of looking at the lens diaphragm has proved fool-proof for me since I started using it. Works to avoid problems with lens hoods as well.
-- Nathan Congdon (firstname.lastname@example.org), February 17, 2000.
I assume that the 6mm figure you mention derives from the image circle the lens is supposed to have. Keep in mind that the published image circle is at infinity focus. As you focus closer, the image circle increases in diameter. At 1:1 (double extension of the bellows), the image circle is twice the infinity diameter. I dont think I have ever taken a shot when focused at infinity. Usually some hyperfocal focus is used, increasing the image circle. So, basically, you probably do have enough image circle to employ an inch of rise without vignetting.
-- Ron Shaw (email@example.com), February 18, 2000.