I'm a little confused about lenses and their circles of coverage. I plan to use a 6x6 roll film back on a view camera. Since I want to use movements I need a lens that covers slightly more than the 80mm (or so) diagonal of the film. However, if a lens covers more than the diagonal, won't part of the image be cropped off, effectively making my standard lens more like a telephoto? Or are the differences so small relative to the film area that they aren't worth worrying about? In order to simplify my lens selection, are there any "rules of thumb" governing how much image circle coverage beyond the film area I'll need in order to allow movements? If the camera allows, for example, a 50mm shift do I simply look for a lens that covers 50mm beyond the film area?

-- Andrew Moore (dmm@bronze.lcs.mit.edu), May 16, 2001

Answers

You can use any size lens you wish if you are willing to move your tripod so that your subject fills the viewing screen in the manner you desire. Pat

-- pat krentz (patwandakrentz@aol.com), May 16, 2001.

Andrew - yes, the same lens will produce different angles of view with different film formats, so you're correct in that on 6X6mm you'll see a "longer" view than with 4X5in. Or more correctly your film is capturing a smaller middle part of the image. As to movements and the lens coverage required, it depends on the amount of movements you plan on using, and which ones. The circle of good definition is expressed in the diameter, and you must compare this circle with a square or rectangular film shape. Try what I did - get an old exposed slide or neg, and place it in the middle of a circle drawn to the diameter of the circle of good definition of the lens you're considering - then move the film around to simulate camera movements. Because of the linear shape of the film and the circular shape of the lens image, you'll find that you'll need more coverage than you think.

-- Mike Mahoney (mmahoney@nfld.com), May 16, 2001.

Andrew, Many of the major manufacturers produce info on their web sites relating to image circle and suitable formats,most also list amount of movement possible, I find them very useful! As a matter of interest, why have you decided on 6x6 as a format? Regards Paul

-- paul owen (paulowen_2000@yahoo.com), May 16, 2001.

Andrew, You should be careful about using such small format, for bellows and focusing considerations. Normal and WA lenses for this format will leave you with pretty short movement possibilities, unless deep boards are used. And not every lens feels confortable on some small and recessed boards. So the matter of angle covering shouldn't be your greatest worry. Good luck.

Cesar B.

-- Cesar Barreto (cesarb@infolink.com.br), May 16, 2001.

Andrew. The image circle of a lens is exactly that, a circle. You can't just say that because a lens has a 100mm circle that your 6x6 frame will allow you 40mm of movement. You have to imagine the 6x6 square (closer to 56 x 56 mm actually), centred inside a circle. This will allow only 10 mm of diagonal movement either way, or about 13.5mm of vertical or lateral movement.
You can calculate the amount of movement by using Pythagoras' theorem of the square on the hypotenuse...etc.
Take the radius of the image circle as the hypotenuse, square it, subtract the square of half the height of the frame (28mm in this case), and take the square root of the remainder. This gives you the distance from the centre of the image circle where the corner of the frame just hits the edge of the circle. Take away half the frame width (again 28mm) from this distance, and you have the amount of movement available.
A square frame makes all this calculation a bit easier, but the same method can be used with any frame dimensions.

A large image circle doesn't necessarily mean that you get a 'telephoto' effect. A lens can be designed to have a large image circle while still having a short focal length.
It's only the focal length which determines the image size, and not the diameter of the image circle.

-- Pete Andrews (p.l.andrews@bham.ac.uk), May 17, 2001.