BetterPhoto Q&A
Category: New Answers

Calculating DOF

"Ever get ticked off at your DOF preview button because your focusing screen goes so dark you can't tell what's sharp and what isn't? Here's how you can calculate the depth of field without even looking through the viewfinder.

First, find your lens' hyperfocal distance at whatever f-stop you will shoot at.
These formulas are to be used in conjunction with the previous thread about finding the Hyperfocal distance of a lens at a given f-stop.

Some new variables:

H is the hyperfocal distance of the lens.
U is the object distance, or the point being focused on, in distance terms
Dn is the near distance sharpness when focused on U
Df is the far distance sharpness when focused on U

Notice what happens in example 3 when H and U aree the same; everything is sharp from 1/2 H to infinity due to the fact that you cannot divide by zero. (you'll see what I mean.)

The new formula is:

Dn = (H*U) / (H+U)

Df = (H*U) / (H-U)

e.g. if the hyperfocal distance of the lens is 10 feet and the lens is focused at the ten foot mark (U) then Dn = 100/20 or 5 feet and Df = 100/0 or infinity.

e.g. If the hyperfocal distance of the lens is 20 feet and the lens is focused at 10 feet, then Dn = (20*10)/(20+10) or 200/30 or 6.666 feet and Df = (20*10) / (20-10) or 200/10 or 20 feet. So in this case the depth of field is 13.333 feet. Depth of field is Df - Dn

e.g.2 If the hyperfocal distance of a lens is 50 feet and the lens is focused at ten feet (U) then Dn = (50*10) / (50+10) or 500/60 or 8.33 feet and the Df = (50*10) / (50-10) or 500/40 or 12.5 feet. Here, DOF is only 4.22 feet, stretching from 1.66 feet in front of point of focus (U) to 2.5 feet behind point of focus (U).

e.g.3 if the hyperfocal distance of the lens is 10 feet and the lens is focused at the ten foot mark (U) then Dn = 100/20 or 5 feet and Df = 100/0 or infinity.

e.g.4 If the hyperfocal distance of a lens is 11 feet and the lens is focused at 10 feet then Dn is 110/21 or 5.25 feet and Df is 110/1 or 110 feet.

This shows how important it is for critical focus on the hyperfocal distance. A shorter focus of one foot in this example (ten feet instead of the hyperfocal distance of 11 feet) increases foreground sharpness by 3 inches (5.25 feet instead of 5.5 feet) but reduces background sharpness from infinity to 110 feet. "

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July 19, 2005

I think it is very good to bring up the importance of DOF. I have to admit, I know my lenses well enough, that I can usually get a good f stop setting for the amount of DOF I want, but truthfully, I am probably stopping down more than I need to. Why does that matter? Every lens has different settings that it is at which it is sharpest. This is especially true for zoom lenses. On my Canon 28-105, 3.5-4.5, for example, it is sharpest between f8 and f11. So if I am shooting a landscape, and I don't need to have it stopped down to f16 or f22, then I am better off, because the image will be sharper. Also, if shutter speed is a concern, you want to be sure to pick the largest aperture setting you can and still achieve the DOF you need, which will give you a faster shutter speed.

You can almost always find the technical specs of your lenses online, and I would strongly suggest that everyone does this. It usually is in the form of a table, that will tell you the best combination of mm setting (for zooms) and f stop for sharp images. This is not to say you can't get a sharp image with other combinations, it just tells you what is the very best on the lens.

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July 19, 2005