Nikon D300 User Manual page 259

Now, you might ask yourself, "Why is there such a setting as
Release priority?" Well, many professional photographers
shoot high-speed events at high frame rates—taking hundreds
of images—and use depth of field (or experience and luck) to
compensate for less than accurate focus. They are in complete
control of their camera's systems since they have a huge
amount of practice in getting the focus right where they want
it to be.
There are valid reasons for these photographers not to use
Focus priority. However, most of those same photographers
do not let the Shutter-release button start autofocus either,
since the focus could change every time the Shutter-release
button is pressed. They set AF activation – Custom setting a5
so that the autofocus doesn't even activate until the AF-ON
button is pressed. They then use the AF-ON button
exclusively for autofocus and the Shutter-release button to
take the picture. They separate the two functions instead of
using the Shutter-release button for both.
You need to ask yourself, "What type of a photographer am
I?" If you are a pro shooting hundreds of pictures of fast
racecars, Focus priority may not be for you. However, for the
average photographer taking pictures of his kids running
around the yard, deer jumping a fence, beautiful landscapes,
flying birds, or a bride tossing a bouquet, Focus priority is
usually the best choice. For most of us, it's better to have the
camera refuse to take the picture unless it's able to focus on
our subjects.
When you're shooting at a high frame rate, Focus priority
may cause your camera to skip a series of out-of-focus
images. It will slow your camera's frame rate so that it will
not reach the maximum 6-8 frames per second, in some cases.
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