Aperture and Depth of Field
A little review from Part One. Aperture refers to the physical opening where light enters the lens. Aperture settings are measured by f-stops: f1.0 up to f32 and even greater. Generally speaking, a super-wide aperture is f1.0; a super-narrow aperture is f32. Most of us don’t use either of these extremes in bird photography. If you look into the lens while it is attached to your camera, you can see the circular opening formed by a metal apparatus made from thin fins. As you change the aperture on your camera you can see the circle opening get smaller and larger as it is changed. The widest aperture (maximum aperture) for your lens is the f-stop that corresponds to the largest circle your lens can make.
Maximum aperture is the largest opening of which you lens is capable. The larger the maximum aperture, the faster the lens is said to be. A bird lens (400mm or longer) with a max. aperture of f2.8 is considered fast. A bird lens with a max. aperture of f5.6 – f6.3 is considered pretty slow.
If you’ve studied photography basics, you probably know that the wider the aperture, the shallower the depth of field (DOF). DOF is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in an image (Wikipedia). The DOF is not only affected by the aperture setting but also by how close the lens is to the subject that is being focused on and also the focal length of the lens.
Say I have an f2.8, 400mm lens and I’m shooting wide open (f2.8). If I am 30 feet from a bird, my DOF will be from 29 feet 11 inches to 30 feet 1 inch (taken from DOF table). Anything in that distance range from the lens will be in reasonably sharp focus. The DOF in this case is 2 inches.
Say I now have the same lens with the same settings but I move back to 50 feet from the bird (wouldn’t do that in real life probably!). Now my DOF is from 49 feet 9 inches to 50 feet. The DOF has grown to 3 inches just because I’m farther away from the bird.
Say I change my lens to a 400mm f5.6 lens and I set its aperture wide open at f5.6. If I’m 30 feet from the bird, the DOF is 29’ 10” to 30’ 2”. DOF is now 4 inches with the f5.6 aperture compared to 2 inches with the f2.8 aperture.
Say I have the same 400mm f5.6 lens wide open and I move back to 50 feet from the bird. DOF is now 49’ 6” to 51’. That’s a DOF of 7 full inches compared to the f2.8 aperture at 50 feet. It was only 3 inches when I used f2.8.
You can see how shallow DOF is when you use an f2.8 lens. If you are shooting a large bird like an eagle from 30 feet away, there will surely be parts of the bird that are not in acceptable focus because an eagle’s beak to the back of the head is longer than 2 inches. So maybe the eye would be in focus but the tip of the beak may not be. So it may not be the best choice of aperture to use in this situation unless you’re looking for a very special type of shot. Stopping down to f4 would give you an extra inch of DOF. (As a note, good bird photos do not require that the entire bird be in focus! But I would prefer at least the bill and the eye(s) to be in focus. It depends on the shot and what the photographer wants to communicate to viewers of the photo).
Note that a lot depends on how the bird is posed. If the bird is posed at a profile angle to you, then that f4 setting might work out fine. But if the bird is facing you, and the eagle has a bill to tail length of 24 inches say, only 3 inches of that length will be in focus—which 3 inches depends on what part of the bird you focus on.
“Luckily,” most beginner and intermediate bird photographers don’t have to worry about shallow DOF at f2.8 because many can’t afford a 400mm f2.8 lens! But I wanted to use it to demonstrate the DOF comparisons above. Most hobbyists can’t justify buying an f4 bird lens. We usually have to settle for an f5.6 to f6.3 lens, especially as we are starting out in the hobby. I listed some lens prices back in Part One. There’s a pretty big jump in prices once you get faster than f5.6 at 400mm and greater!