Lens questions/format conversions

greenspun.com : LUSENET : Large format photography : One Thread

I am planning to use a 210mm lens made for a 8x10 view camera on a 4x5 view camera.There will be a huge circle of good definition available for the 4x5 swings and tilts. My questions are as follows: I have a focal lenght chart that indicates a 28mm lens on a 35mm camera covers the same area as the 105mm lens on a 4x5 camera. Also the same area as a 210mm lens on a 8x10 camera. Since I will use the 8x10/ 210mm lens on a 4x5 camera does that mean I my effective focal lenght will be 105mm instead of 210mm? What-if anything- does change in doing this with regards to depth of field or hyperfocal distance? Contact: talltandj@aol.com

-- Thomas Ferko (talltandj@aol.com), May 05, 1998

It will still be 210mm (a mild telephoto)on the 4x5. It will be something like 70mm on 35mm (as far as angle of view).

-- Ron Shaw (shaw9@llnl.gov), May 05, 1998.

For lens equivalants across formats, take the ratio of diagonals. For 35mm to 5x4, this ratio is about 4 (well, actually 3.7). So a 210mm lens on 5x4 has the same angle of view as a 57mm lens.

For depth of field, take the square root of the ratio. sqrt(3.7)=1.9, so a 210mm lens on 5x4 has the same DoF as a 110mm lens on 35mm format. This assumes the negatives from the two formats will be enlarged to the same final size.

It doesn't matter whether the 210mm lens was designed for 5x4, 5x7, 10x8, or 20x24. This will affect the diameter of the image circle, but not the angle of view nor DoF for a given film format.

-- Alan Gibson (gibson.al@mail.dec.com), May 06, 1998.