Photo reconnaissance and angular size

I went out for a scout this afternoon to find some new places to take pictures. As well as finding good locations, I needed to know which lenses to bring next time. None of the stuff in this article is ground-breaking but I think it’s a useful rule of thumb (literally).

For now, let’s forget about lens focal lengths in millimetres and think of the angular field of view of a lens. Hold your arm out straight in front of you. You can judge the angular size of a distant object by comparing it against the angular size of different parts of your hand.

Image from http://chandra.harvard.edu/photo/scale.html
Little finger, or Index fingernail
Thumb at its widest part
First, second and third fingers together
Width of palm or knuckles 10°
Distance between index and little fingers 15°
Full handspan 25°
Double handspan 50°

So when you get home, you’ve got a list of places and the field of view you need to take each photo. Now you can use this table to figure out which lens you’ll need to give you that coverage. I’ve rounded the focal lengths to the nearest commonly-found focal lengths for APS-C DSLRs, 35mm full-frame (D)SLRs, and medium format cameras.

Field of view[1] Focal length
APS-C DSLR 35mm FF SLR 6×7 Medium Format
50° 24mm 35mm 65mm
25° 50mm 85mm 150mm
15° 85mm 135mm 300mm
10° 135mm 200mm 400mm
250mm 400mm 800mm
600mm 1000mm 2000mm
1200mm 2000mm 4000mm

So to give a real example, if you are out on your scouting walk, and you note that you can cover part of a landscape with the palm of your hand, you know the angle subtended by the landscape is 10° and that you’ll need a 135mm lens for your DSLR when you return.

I hope this is useful to someone – I’ll certainly be using it from now on.

  1. Usually the diagonal angular value is given, but in this example I think it makes more sense to use the horizontal dimension.

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