Sharp or Fuzzy? That Is the Question
Depth of Field: How it works
Techniques like depth of field, leading lines, frame-within-a-frame, angle, etc. are tools you can use to make your images have more visual appeal and impact—to get the viewers to concentrate on the thing (or things) in the frame that you think important (the thing that made you want to make the photograph in the first place). Depth of field is one of the most creative and useful of all these tools—we can’t always get a different angle on our subjects, and leading lines aren’t always available. With depth of field, you can deemphasize elements in the fore- and/or back-ground that are distracting and make your subject stand out from them. Or you can make it blend in.
It’s all about aperture and lens length.
Depth of field is the apparent zone of sharpness in front of and behind the subject you focus the lens on—that is to say, other things in the frame that look sharp. Sometimes you want a little, sometimes a lot. Here’s an example:
Basically, the smaller the aperture, the greater the depth of field. So f22 = lots of depth of field, and f2.8 = little. (Remember that the numbering of f-stops is kinda backwards—the higher the number the smaller the hole.) But it varies by focal length of the lens. A really wide-angle lens (like a 15mm) has lots of depth of field at any f-stop; a long telephoto (like a 600mm) has very little at any f-stop.
In ancient times, when you changed the aperture by turning a ring on the lens, you could actually see the range of depth of field. Little color-coded lines showed the range for different f-stops, like this:
As you can see, depth of field shrinks rapidly as the focal length of the lenses grows. When focused on a subject 5 feet away, the 24mm lens has depth of field from just over 2 feet to well past infinity (if there is such a thing). The 35mm lens has depth of field from just over 3 feet to just under 15 feet. My 300mm lens will not focus on anything as close as 5 feet. When focused on something 13 feet away, it has only a few inches of depth of field at f16, and only slightly more at its smallest aperture, f22 (orange markings). You can also see from the lens markings that depth of field is greater behind the subject than in front of it. And it matters how close the subject is too. The nearer the subject, the less depth of field you have; the farther away, the greater the depth of field.
If your camera has one, you can use the depth of field preview button to check the depth of field before you make the photo. It’s located just to the right of the lens mount (as you are looking through the view finder). Pressing it closes down the diaphragm to whatever f-stop you have selected. The problem is that at small f-stops like f16 or f22, very little light is getting through the lens, and it’s hard to see much of anything, much less check to see what’s sharp.
Okay. Enough of that stuff.
You need to know it to control it. In this image of farmers in the Himalayas, I wanted both the children in the foreground and their parents to be sharp. But I couldn’t use too slow a shutter speed because of the movement of the parents winnowing grain. Because I understood all that stuff above, I knew which lens to use for the combination of shutter speed and f-stop that would both freeze the motion and yield the depth of field I wanted. (Hint: it helps to have lots of sunshine.)
Getting great depth of field is pretty easy if you’re using a wide-angle lens and/or you’re shooting in bright sunshine. You can close down the aperture and still get a decent shutter speed—one at which you can hand hold the camera without blur. But if you can’t stop down far enough without crossing that threshold, here are a couple of easy tricks. Use a wider lens (more depth of field at same shutter speed, f-stop). Increase the ISO. (This once meant changing the film in the camera. Now it’s just a spin of the dial.)
But sometimes having everything sharp can be a problem—too visually confusing. You may want shallow depth of field to make your subject stand out. You can back away from your subject and use a longer lens (less depth of field at same shutter speed, f-stop). Or you can take advantage of the really fast shutter speeds on today’s cameras. Open up the diaphragm and crank ‘er up.
MORE DEPTH OF FIELD:
- Shorter lens
- Smaller aperture (aka diaphragm)
- Subject farther away
LESS DEPTH OF FIELD:
- Longer lens
- Bigger aperture
- Subject closer
Actual Info: Text © 2010 Robert Caputo
Photos © 2010 Robert Caputo, © 2010 Cary Wolinsky
All Actual Info and Tips Flix