Photos Spare Cycles MythBusters

Trading DOF for hyperfocal distance, aka calculating DOF for 70-200 vs. hyperfocal for 16-35

another crosspost from spare cycles

For my Canon 70-200 f/4 lens I keep in my head some approximate equations for calculating depth of field (DOF) at f/4:

200mm: 11.5 / (100/distance)2
70mm: 89 / (100/distance)2

These equations look difficult, but if you keep to easy distances you can rough it out. For example:

200mm @ 10ft: 11.5/(100/10)2 = 11.5/100 ~= 0.1 ft
200mm @ 25ft: 11.5/(100/25)2 = 11.5/16 ~= 0.7 ft

To convert to other f-stops, you simply multiply (e.g. f/8 is twice f/4).

When shooting with the 70-200, the DOF tells you whether you're going to be shooting a lone rider, a pack, or a really-in-focus gear shift. My Tour of California photos from last year are full of examples of overly optimistic DOFs for my slow Digital Rebel and trigger finger.

Chris Horner nears the finish-1 Ekimov nears the finish-1

above left: Horner's chest logo and thigh are in focus, but not much else. above right: I got luckier catching Ekimov's face, but the DOF is only about the length of his hands. If I had used a larger DOF, I wouldn't have tossed away as many shots. An alternative is to get a better camera, which I did.

My new 16-35 is adjusting my way of thinking on this. I can't extend my previous equation because at 100 ft, the DOF is infinity. In fact, at f/2.8:

16mm @ 10ft: DOF 21.4ft
16mm @ 5ft: DOF 3.52
16mm @ 1ft (minimum distance): DOF 0.12ft

At less than 5ft, it might be worth remembering, but a more useful calculation will probably be the hyperfocal distance:

When the lens is focused on the hyperfocal distance, the depth of field extends from half the hyperfocal distance to infinity. -- Photography, Phil Davis, 1972. source

Instead of trying to put one thing in focus, this is the distance to think about for putting everything in focus. For my 16-35mm this breaks down to:

16mm @ f/2.8: 15.7ft
35mm @ f/2.8: 74.9ft

This is much easier to remember and calculate than DOF. To get hyperfocal distance for other f-stops you divide (f/5.6 = half f/2.8).

This isn't field tested, but I'm thinking that it will be more useful to trade DOF for hyperfocal distance when I'm getting my 16-35mm shots. We'll see how it all works out after its inaugural run this weekend.

Thanks to the ever useful online DOF calculator

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