Under normal shooting, the Nikon bodies by default are configured such that the auto focus is enabled when either the shutter is half pressed or the AF-ON button is pressed: doing either will start the the camera's autofocus sysetm and it will try to lock on to your subject.
However to configure trap focusing on the Nikon bodies (D300 in the following example) you need to do a few things:
- de-couple shutter from focusing - menu a2, "AF activation = OFF", AF-ON only
- set shutter relase on focus priority - menu a5, AF-S priority = focus
To use, you will use the AF-ON button to focus at a particular point (you'll hear the double beep confirming focus lock) and then to press the shutter all the way down. The AF-ON button may be released once focus is grabbed. Note, that simply pressing the shutter all the way down by itself will NOT fire the camera unless there is something at the point of previous focus,
Trap focusing is also configurable in the introductary models such as the D40 - in the D40's case, the AE/AF-lock button is used to initiate the autofocus with the shutter de-coupled as mentioned above.If the camera determines that there is focus locked onto a subject, the camera will fire and record a frame. If not, the camera waits (the shutter is still being pressed all the way down: the AF-ON button is released) until something comes into focus at the pre-determined point before firing.
Now, why would you do such a thing? There are times when you want to do this. Think of catching an object (a ball, a bird, a plane, superman) in flight as it crosses your composition and the camera firing once something reaches the point of previously determined focus. Also, think active child not sitting still for their portrait.
Sure, panning, tracking focus with AF-C can be used by this is a different trick and different tool.
To some, the de-coupled shutter for focus is the way they prefer to shoot but I have become far too used to the half shutter press focus mechanism. For the higher end Nikons, there are useful things called Custom Setting Banks which allow you to save camera settings in different presets which you can name.
For me, I have used custom settings bank A (named "default") configured with the default AF Activation = ON, shutter/AF-ON setting. Custom settings bank B (named "trap focus") is configured for trap focus and when I need to switch between the shooting modes, I simply select the desired custom bank.