Written By onci on Monday, September 21, 2009 | 4:19 PM
A Word on Focus – Getting Razor Sharp Images: Part 2
by Matthew Studebaker
Limiting Your Focal Range
Many modern auto-focus lenses allow the photographer to limit the range of focus. For example, when photographing large birds far away, one could theoretically switch the lens to only focus 20 meters to infinity rather than include the closer ranges. Because auto-focus is not very good at quickly searching the entire focal range for the subject, some bird photography teachers advise their pupils to use the range limitation feature on the lens to limit the range of focus in situations like this. The theory is that the lens will only have to search a more limited range to find the subject. The problem with this theory is two fold.
First, these teachers assume that they won’t have a surprise encounter with another bird species. For example if you were photographing a distant heron nesting colony and set your lens range to distant subjects, what happens when a small warbler lands right next to you with no warning? You try to focus but your range is limited to only distant objects. You fumble to switch your range to close subjects but by then the warbler is gone. Or what happens when a peregrine falcon starts flying directly towards you, starting in your distant range, but in a matter of seconds moves into your close range? The camera will lock the focus and you’ll miss your shot.
Second, limiting your focal range assumes that the best way to focus is to let the lens find the bird. As we already discussed, we don’t want the lens to do the range searching – that’s not what auto focus is good at. It’s only good at small micro adjustments for locking on to the subject. So if we are doing 95% of the range searching manually anyway, there’s no need to limit the auto focusing range. Keep the range unlimited. Do the major focusing adjustments by hand. Depress the shutter half way and only let your auto focus take over and lock on at the last second.
There is one and only one situation where I limit my focal range. The qualifying situation is as follow:
• photographing birds in flight,
• I can predict the bird’s trajectory (like daily trips to their nest).
• There is not much light outside so the camera could use a little help locking on to the subject faster
• I don’t care about getting any random photos in the field, I’m content to limit myself to a single subject
A situation that comes to mind is my local wood ducks leaping off a platform feeder near sunset. I know exactly where they are flying from and exactly where they will land in the pond next to the feeder, the light is low, and there’s nothing else in the area to photograph.
Posted by onci at 4:19 PM