|Bald Eagle on the bank of the Columbia River, near Golden, BC.|
Whether a subject is changing its position relative to you, or you are changing your position relative to it, or even both at the same time, there is a problem with focusing. Most of the time I try to keep still while peering through my viewfinder. I always prefer my subject to be stationary and compliant, but the truth is there are plenty of exceptions to both situations. In the case regarding the above photo, I was traveling downstream. As I moved forward the distance between my subject and I gradually decreased. This creates a problem for focusing.
The issue is that once focus lock is achieved and you have reframed the camera to get the desired shot, the distance has changed and the subject is no longer in crisp focus. This is less of an issue with normal and wide-angle lenses, but telephotos have a very narrow depth of field, especially long ones, and it doesn't take much before your subject is blurry. This is where continuous focus can help.
Continuous focus allows the camera to alter focus continuously as long as your finger is pressed halfway down on the shutter button. Choosing the correct focus point is important to allow you to capture the image without reframing. If you attempt to reframe it is likely your subject will snap out of focus as it is no longer in line with the active sensor. You can get away from this using the AF-L button, but I prefer not to do this. Instead, I chose a focus spot and keep my subject lined up on it as I am paying attention to the many variables of composition.
This is also a useful procedure when setting the drive mode to obtaining more than one shot at the press of a button. Using CH (continuous-high) on the drive selector, with the focus mode being set to continuous, and the appropriate focus zone locked in, I can get multiple images, all in focus, of whatever dynamic situation I find myself in.
A really interesting focus mode is 3-D focus-tracking, which I use in sports and for active wildlife such as flying birds. That is a discussion for another time though.
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