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Hey all,
I read how to do this on dpreview so those of you that read that site probably already know how to do it, but I'll repeat it here for those that don't.

What is trap focusing?

Trap focusing is taking a picture by prefocusing at a particular distance, holding down the shutter and waiting until a subject comes into focus using a selected "focus point". As soon as something comes into focus the camera will "automatically" take the photo.

What needs to be set up first?

I'm pretty sure this only works in P, A, S, and M modes, it might work in the others, but to be honest, why bother. (Even P mode is stretching it a bit...) So set your camera to one of these modes first.

Now, go into the Custom Function Menu. Make sure it's in "detailed" mode and the following are set:

02 - Autofocus: AF-S
03 - AF-area mode: Single Area
15 - AE-L/AF-L: AF-ON

Now, compose your picture as normal. Find something that is in line with what you are going to shoot (for example if you are waiting for a car to come round the corner and are going to use trap focusing to get the car in focus, find a tree next to the road or try focusing on the road where the car will be).

Next, hold down the AE-L/AF-L button on the back of the camera (to the right of the viewfinder) until focus lock is achieved (the green dot illuminates in the viewfinder). Now, LET GO OF THE BUTTON (I was holding it down for a while, and it will take a photo instantly if you press the shutter so let it go!).

If you now move the camera so that you are not pointing at the already in focus object, and press the shutter button completly, the camera will not take a photo. When something comes into focus (try this and just pan the camera back onto the original object), the camera will automatically take a photo as if by magic!!!

This method will also work if you have the camera set to take multiple frames at one time, so long as the object stays in focus.

Also note, you don't need to keep refocusing inbetween each photo. So you can take one photo, take your finger off the shutter, and press it down again waiting for the next opportunity.

Well, I hope that this tutorial was helpful to someone. If you have any comments or feedback, please don't hesitate to comment!!

DragonStar.

Trap focusing is an awesome trick up the D70's sleeve. It functionally emulates the professional film bodies (I assume pro digital bodies too) with separete AF engage button.

However, it uses that ONE customisable button on the D70 - the AE-L/AF-L button. While the ability to program the button to a variety of functions is highly versatile, it does mean that only 1 function can effectively be used at once.

eg. setting up the camera for trap focusing will mean not being able to use the button as flash value lock (another trick of the D70, avoiding the iTTL pre-flashes by independently engaging the pre flash metering with the press of the button).

Shooting with the kit lens, I'd rather flip the M/A - M focusing switch on the lens to M to keep the lens from focusing and saving the AE-L/AF-L button for exposure lock.

But each to their own...

It's a big pity that you have to keep your finger on the shutter, as this reduces its usefulness by 90%. Some of the older Canon cameras could be set up this way and you could leave the camera unattended. The shutter would trip by itself when something moved into the focus area. Great for candids of wild animals!