image circle confusiongreenspun.com : LUSENET : Large format photography : One Thread
Recently I took an architectural picture with my 240 Dagor of my 8x10", and I raised the lens as much as I thought possible. I seem to have moved it out of the image circle, but not at the top of the picture (the ceiling) where I would have expected it, but at the bottom (the floor), which should have been well coverec with a lens rise - or at least I thought so. Can somebody explain this to me?
-- Lukas Werth (email@example.com), October 15, 2000
When you raise the lens on a view camera, you are raising the cone of projected light exiting the rear of the lens and moving it up as well. Since the image is inverted on the ground glass, the ceiling being down and the floor up, so to speak, if your circle of coverage is insufficient, vignetting (starting at the corners) of the lower part of the ground glass image, in this case the ceiling, should become noticeable. This is exactly opposite of what you report. Is the cutoff straight across? Do your bellows sag? If you did a substantial rise and the bellows were sagging enough, they could actually crop the image on the gg and the film. And guess which part of the image would it would crop? That's my guess, bellows sag. You're using a relatively short lens and if you have a lot of bellows and they're super flexible, they are probably hanging down in the line of fire. Let us know if this is the case!
-- Robert A. Zeichner (firstname.lastname@example.org), October 15, 2000.
Could be that you moved the lens so much that the bellows was interrupting the light path. As you move the lens up, the bellows sags , and it's quite easy to get the top side of the bellows in the way.
If the 'vignetting' is a ragged line across the bottom of the picture, then this is almost certainly what happened. If it's a circular vignette, then it's still a mystery.
-- Pete Andrews (email@example.com), October 16, 2000.
Thank you for your answers, they helped. The problem was indeed the bellows, but as it may have some significance for other LF users, let me shortly describe it: I used the 240 (or 250??) mm Dagor on a Deardorff camera. The bellows is replaced, but not at all sluggish. The lensboard on which the Dagor is mounted was custom-made by a local carpenter, an it is a little too thick to use the rise of the lensboard which the camera provides. In the - in fact several - cases the problem occured, I had tilted the camera somewhat - using the vertical format - and then set both standarts vertical. This produced the cut-off described, which was sharp, absolute, and SLIGHTLY ROUNDED, which let me astray, or I would otherwise looked at the bellows myself first. Well, the Deardorff bellows is cone-shaped, and in this extreme position it does intrude somewhat between lens and image area at the upper side of the camera. I could only detect this clearly after sticking a small piece of white paper at the inside of the bellows in order to locate its opsition by peeping through the cut corners. It looks like I should be able to mend this problem through building two brackets which lift the bellows somewhat in such extreme positions - but this remains tricky, and needs careful checking in every case.
-- Lukas Werth (firstname.lastname@example.org), October 16, 2000.
I've had the problem of the bellows getting in the way of the image as well, especially with wide lenses, and this thread discusses solutions:
-- David Goldfarb (email@example.com), October 16, 2000.
The usual solution, and the one that a lot of cameras have built-in, is to gather up a portion of the bellows to effectively shorten it.
This is done on my MPP by two little lugs attached to the bellows about 1/3rd of the distance from the front. They fit to two special spring clips on the front standard.
A couple of wire loops stuck to the bellows with gaffer tape would serve the same purpose. They could be designed to hook around the front of the camera in some way. They needn't look too ugly, if you take some care in designing and making them.
-- Pete Andrews (firstname.lastname@example.org), October 17, 2000.