Choosing the right focus method makes a difference.

Possible savannah sparrow.

It's difficult to identify this sparrow species given the fact that most of it is obscured by foliage.  However, the point of the photograph isn't so much the bird or the composition but rather the method obtained to achieve a sharp focus on the subject.

Long lenses at wide-open apertures have a notoriously short depth of field, especially at close distances.  You can see that in the photo above.  Below is an image copied (and somewhat adjusted for size) from a website hosting a depth of field calculator (click here) that gives some empirical parameters to justify that first statement.

Depth of field calculator with values used in the sparrow photo above.

The numbers verify what was just said.  To simplify, long lenses (500 mm) at wide open apertures (f/5.6) at close distances (450 cm or 4.5 m) have a very narrow depth of field (in this case, the depth of field is only 1.63 cm or just over half an inch).  If you look closely at the photo of the sparrow (click on the image and it should render a full-sized frame), you will notice that the bird itself is in focus but the rest of the image is blurred to varying degrees.

The key was in selecting center-point focus with single-point (S) when the shutter button is pressed halfway down).  In the field I use this focus parameter most of the time, occasionally toggling to 3-D with continuous focus (C) when shooting a moving subject.  It is possible to select where in the frame the focus point is located, but I mostly use the center position as the camera functions at its best here.  Then all I have to do is to line the critical part of the image up on the focus point, press the shutter halfway down, reframe, and shoot.  It is important not to remove your finger from the lock position as the focus will shift if you do.

Although photography, especially the digital variety, deals with numbers and mathematical relationships, the simple truth is that none of those concepts need to be understood in order to take nice photos.  The general principles behind them are important, but not the values in themselves.  Knowing that depth of field is very small with long lenses, wide apertures, and close distances is enough.  You just have to have a plan to deal with those facts.

Thanks for reading.

Eric Svendsen     www.ericspix.com