Hyperfocal Distance

Let’s do a quick review of depth of field since hyperfocal distance is the maximum depth of field. Depth of field is inversely proportional to the aperture. That is, if you use a large aperture – say, f/4, then the depth of field is fairly narrow. If you use a small aperture, say f/22, then the depth of field increases.

Here’s an example using a 50mm lens: At f/4, focused at 40 feet, the depth of field is 7.41 feet. Or, to put in a more useful way, the photograph will be sharp in the range from 36.6 feet to 44 feet. At f/22 with the same lens, the depth of field range increases from 26.3 feet to 83.3 feet, a total of 57 feet. (The range of depth of field is not distributed around the focus point evenly. ) The “take away ” from this is that large aperture gives you small depth of field, while small aperture gives you a larger DOF.

Now, however, for landscape work, you would like to maximize the depth of field in some cases. That is, you want the entire scene from flowers in the foreground to snow covered mountains in the background in focus. This is relatively easy to do if you understand the concept of Hyperfocus Distance. In words:

“The hyperfocal distance is the closest distance at which a lens can be focused while keeping objects at infinity acceptably sharp; that is, the focus distance with the maximum depth of field. When the lens is focused at this distance, all objects at distances from half of the hyperfocal distance out to infinity will be acceptably sharp.”

Think of it this way. You’ve trudged 5 miles in and 5000 feet up to a mountain lake. You’re presented the shot of your life. There’s snow covered mountains in the background; a crystal blue lake with water like you’ve never seen in your life, and, would you believe, stunning yellow flowers in the foreground. And the light is fantastic. And you want it all.

Something like 57 feet of depth of field isn’t going to cut it. Ah, but you recall hyperfocal distance. You crank in some settings, and, click, you’ve got the award winning shot – all with “acceptable” sharpness.

OK, so what, exactly, is hyperfocal distance? How do I know what to do? Here’s the equation for Hyperfocal Distance:

HFD 50mm lens Canon 7d


H = (f2/Nc)+f
where
H is hyperfocal distance
f is focal length
N is f-number (f / D for aperture diameter D)
c is the circle of confusion limit – a function of your camera.

I’ve plotted the results of applying this equation for two cases – see the graphs.

Here’s what you want to see: The hyperfocus distance DECREASES as the aperture DECREASES. Or, if you’re one of “those people,” the hyperfocus Distance increases as the aperture increases. Or, in the same manner, the wider angle your lens, the smaller the hyperfocus distance is.

So…let’s say those yellow flowers are only 2 feet from where you must stand. What do you do?

HFD f/16


Easy, you grab your 16-35 mm lens out of your bag, set it to f/22 and 16mm, focus near the flowers if not right on the flowers, and shoot away. Because what you remember is that you want a small aperture and a wide angle lens. That 70-200mm lens is not the right choice right now no matter how much you paid for it. BTW, the actual numbers for that 16-35mm lens @f/22 and 16mm on a 7d would be HFD=2 feet so everything from 1 foot to infinity would be “acceptably sharp.”

And now you have a clue as to why Ansel Adams, Edward Weston and some other photographers formed a group called f/64.

You can, of course, make the calculation for the exact HFD every time. You won’t be surprised to find there’s an app for that. But that seems a little too much for out in the field. I think what you want to remember is the relationship between focal length and aperture. Then, with your depth of field preview, you can set the shot – no calculations required.

This all sounds great, right? Well, unfortunately, TANSTAAFL – There ain’t no such thing as a free lunch. Physics gets in the way to a certain extent here. The problem is something called “diffraction.” Here’s an explanation:

“Ordinarily light travels in straight lines through uniform air, however it begins to disperse or “diffract” when squeezed through a small hole (such as your camera’s aperture).  This effect is normally negligible, but increases for very small apertures.  Since photographers pursuing better sharpness use smaller apertures to achieve a greater depth of field, at some aperture the softening effects of diffraction offset any gain in sharpness due to better depth of field.  When this occurs your camera optics are said to have become diffraction limited. ”

From http://www.cambridgeincolour.com/

Well, that’s cool but does my camera/lens become “diffraction limited?” Yes but the point at which it does depends upon the size of an pixel in the sensor of your camera among other things. My camera starts to show diffraction limiting around f/16 and it’s noticeable by f/22. What this says is that f/22, while desirable for small hyperfocus distance may cost you some sharpness. So you need to test and see what is acceptable for you because that really is the definition of “acceptable,”right? I tend to stick with f/16 unless there’s some overriding reason to go smaller.

There is more information and calculators for this and other things at http://www.dofmaster.com/dofjs.html