Trap focus is where you prefocus on a fixed physical location, and the camera refuses to fire (even with the shutter pressed all the way down), until the camera recognizes something that is in a particular bracket as being in focus, before it fires.
Different cameras may have different setups to accomplish this, but for a Nikon D700, you can mimic "Trap Focus" with the following settings:
1. Set custom setting #A5 to AF-ON ONLY
2. Set the focus mode to SINGLE SERVO
3. Set Auto focus area mode to SINGLE POINT (9 point, 21 point, 51 point or 51 point 3-D will all work)
4. If your lens has an A/M switch, make sure it is set to A for autofocus mode.
5. Pre-focus the lens for a particular distance. Don't forget that a more convenient way to use this trick is to pre-focus using not the shutter button, but the FOCUS button on the back of the camera can be used. I find this easier and more effective.
6. Press and hold the shutter release ALL the way down, the camera shoots only when the selected focus bracket is in focus.
"Trap Focus" is only effective if you know exactly where your moving object will stop and you yourself do not move or change position from (for example, a baseball player running towards a specific baseball plate or a branch where you expect a bird to land are good examples (but don't forget the hint that having a deeper DOF is also a strong consideration when very close to the subject, this gives you some extra leeway for getting a better shot!).
Another place where "Trap Focus" comes in handy, and not many people know or use it with, is with hand held macro shots. If you are using the trap focus method, you focus on a particular area, are ready to take a shot, breathe perhaps a little, the scene goes ever so slightly out of focus, the camera will refuse to take the shot until you adjust position, camera focuses and the shot is sharp. In this case, "Trap Focus" saved you from taking a blurry picture.
However, it is not very good for use where we want to capture a moving object and we do not know the exact location where it will stop, or in cases where an object moves from sensor backet to sensor backet.
In these cases, continual focus or continuous servo, as Nikon calls it (in conjunction with a Dynamic AF setting and a SINGLE FOCUS AREA ), gives me far more keepers than using "Trap Focus" (ie: bride and groom are moving down the isle and you are moving to keep up with them), as I choose the STARTING focus point and as they move (I may want to recompose the shot on the fly), the focus remains on the subject that I initially set the starting focus point on. It does this because the lens is continually trying to focus as long as I have the shutter pressed instead of waiting on that one point/distance where focus is achieved and the shutter trips to take the shot, and if they move to another sensor, focus tracking is in place and focus is continually modified and locked.
Trap Focus has caused me to lose or miss the moment many times in the past before I learned to understand the D700 focus and exposure system and how it works in detail and when to use each of the proper settings to best effect.
Try them out and see what works best for you under what circumstances!
Enjoy!
