A while back I mentioned using trap focusing on this website. I think it was before the big meltdown so the original post is probably lost. Anyway, I havenít had a chance to really try it out till now.
For those who donít know, trap focusing is where you keep autofocus switched on but disable the association between the autofocus and the camera shutter, fix the camera focus at a certain distance, hold the shutter button down continuously, and wait till the moving subject arrives at the preset distance at which point the camera takes the photo itself. On a Nikon, this is achieved by setting the AR-L/AF-L menu option to AF-ON (as opposed to the default AE/AF Lock), and tweaking a few other minor settings. The focus distance is thus set by pressing the AE-L/AF-L button, and remains constant until you deliberately change it again. You then simply hold the shutter down continuously, move the camera around as you please and the shutter will trip whenever the camera is able to get good focus on something. Naturally this will only occur when the subject is at exactly that preset distance.
Why would you do this ? Well, in theory the response time of even the clunkiest camera electronics is many times faster then a humanís entirely pedestrian shutter finger. In short, if a moving subjectís trajectory is even mildly predictable then you should get sharper photos of moving objects using trap focus.
Did it work ? A couple of days ago I went down to the Parramatta flying fox colony. This spot is well worth a visit. Bad weather (for bats) in the north of NSW has pumped up the numbers from the usual 5-6,000 to 17,000, and you can stand on the rock platforms in the river and observe everything. Even in daylight there are bats in the air almost constantly Ė perfect for flight shots. And the trap focus worked producing lots of keepers versus none at all using conventional shutter techniques. However I should point out that I doubt the difference would always be this clear cut, as in these light conditions it was difficult to get shutter speeds up and my ageing camera doesnít really tolerate ISOs above 400.
Is it a good way to take photos though ? Yes and no. Though flight shots were almost invariably sharper using trap focus, the problem arises when you want to take a photo of a bat that is not at the critical distance. Perhaps a bat does something fascinating just beyond the critical focus distance (I like to catch them when they swoop down to drink in the river) Ė you cant easily convert the camera back to conventional shutter mode to take a quick photo. I guess whether it is worth using depends upon how predictable and how repeatable the subjectís trajectory is.
Anyway, its another trick to store away so I hope people find it useful.
Fairly typical photo taken with trap focus is attached.