Lens questions/format conversions

[Thomas Ferko]

I am planning to use a 210mm lens made for a 8x10 view camera on a 4x5 view came ra.There will be a huge circle of good definition available for the 4x5 swings a nd tilts. My questions are as follows: I have a focal lenght chart that indicate s a 28mm lens on a 35mm camera covers the same area as the 105mm lens on a 4x5 c amera. 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 wil l be 105mm instead of 210mm? What-if anything- does change in doing this with re gards to depth of field or hyperfocal distance? Contact: talltandj@aol.com

[Ron Shaw]

It will still be 210mm (a mild telephoto)on the 4x5. It will be something like 7 0mm on 35mm (as far as angle of view).

[Alan Gibson]

For lens equivalants across formats, take the ratio of diagonals. For 35mm to 5x 4, this ratio is about 4 (well, actually 3.7). So a 210mm lens on 5x4 has the sa me 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 n egatives 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 20x 24. This will affect the diameter of the image circle, but not the angle of view nor DoF for a given film format.