Backbutton focusing: how and why?

[n8]

I've been hearing about this quite a bit lately. I understand how to set it up on my camera, and understand how to push the button for AF of course, but I don't quite understand the benefit for the mechanics behind it. Anyone?

[Greg McCary]

I use to setup up my E3 to do this it makes it easier to focus on a specific spot and recompose after focus and the take the shot. Otherwise if I lifted my finger off of the shutter I had to start all over.

[armando_m]

What Greg said
and there is also the af/ae lock , focus on a spot , exposure from a second one, recompose, shoot

[n8]

Thanks guys. I think it just took a little bit of practicing with it last night to get the hang of it.

[nekonelac]

Back button focusing is great for trap focus (this can only be done with nikons)

This is a neat way to use auto focus especially for fast sports/wildlife action.

Custom (pencil) menu:

Autofocus set to AF-S
AF area mode set to single
AE-L/AF-L set to AF ON

Compose your shot and set the focus by aiming the centre focus icon at a definite target at the precise distance you want, (Say a tree branch where a bird is about to land, or on second base where the baseball player's foot will land) and pressing the "AE-L/AF-L" button near the viewfinder. This will focus the lens, let go of AE-L/AF-L button. Turn away,
press and hold the shutter button all the way down. Point camera at pre-focused point when subject approaches, and your camera will take the shot (or shots, if you are in continuous shooting mode.)

Much faster than Human reflexes.

[armando_m]

Much faster than Human reflexes.

or any AF system

why do you say is only doable with nikons?
I can prefocus my point & shoot and do the same thing, or .... what am I missing ?

[nekonelac]

or any AF system

why do you say is only doable with nikons?
I can prefocus my point & shoot and do the same thing, or .... what am I missing ?

You are missing quite a lot- with trap focusing, once you press the back button, focus is locked-and by holding down the shutter button,and pointing camera at the pre-focused area, the shutter will automatically take the shot,when the subject appears

Her's what dp review forum says about trap focusing
:Trap focus is a camera feature that allows you to manually focus and have the camera delay firing the shutter until you have pressed AND something is in focus. Originally intended to capture photos of the wedding party as they walk down the aisle, it works for many sports if you pre-focus where the action is headed. With a manual-focus lens, you can also use it to confirm focus by holding down the shutter button while you focus -- it will take the image as soon as focus is achieved.

And again from Ken Rockwell;
you can get most AF Nikon cameras to wait and release the shutter only when an object has moved into a preset focus distance if you set the AE-L/AF-L button to AF ON (set in a custom function) and then keep the shutter pressed all the way down while NOT pressing the AE-L/AF-L button. This is called Trap Focus. How does this trick work? Presuming you have the camera set to AF-S mode the camera won't shoot until it thinks the subject is in focus. By setting the AE-L/AF-L button to AF-ON the camera won't focus itself unless you press that button. If you don't press the AF button the camera has to wait for the subject to move and when the "in focus" dot lights up the camera will shoot so long as you've been holding the shutter button all the way down. This trick works so long as you have the camera and lens set to AF-S mode and keep holding the shutter all the way down. If you set the camera to manual focus or touch the focus ring on an AF-s lens while holding down the shutter it will shoot at the wrong time. It's a primitive hack and I doubt it's smart enough to predict subject motion for good results photographing something like horses jumping over a fixed object, but worth a try. It also doesn't work if the subject is moving so fast that the camera doesn't notice it's in focus as the object flies by. Of course you need to prefocus the lens where you want it while not holding the shutter down.