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In the field I find a trick like this is much quicker, though not perfectly acurate: hold out your hand at arm’s length.
– 1 finger width = 800mm
– fist width = 120mm
– index to pinkie streched out = 80mm
– span (thumb to pinkie stretched out) = 50mm
– two spans (same as above but both hands, pinkie to thum) = 25mm
This website makes it really easy to get the exact number, assuming you are using only rectilinear lenses. (so no fisheyes) It also calculates depth of field. Note: the site contains ads and I have not looked at the other pages on the site. https://www.pointsinfocus.com/tools/depth-of-field-and-equivalent-lens-calculator/
For those interested in the mathematical formula, it’s simple trigonometry. You do have to use a different formula depending on whether or not your lens extends when changing focal distance. https://www.pointsinfocus.com/tools/depth-of-field-and-equivalent-lens-calculator/#angle-and-field-of-view
- This reply was modified 3 years, 5 months ago by David Frazer.