using back button for focus

[GeoffS]

i dont see much of an advantage over just holding the button half way down

The disadvantage of using the half-press shutter button is that it starts the auto-focus and locks the auto-exposure. 
If you use servo-AF and track a plane for a long time it's likely that the exposure that was set when you first half-pressed the shutter button is wrong for the conditions when you take the shot.  Putting the AF control on a separate button allows you to have focus tracking but set the exposure at the moment you take the shot.

As an aside, the default behavior of the * button is to fix the exposure, and then the half-shutter sets the focus.

Cheers,

Geoff S.

[Photo Op?]

When the D30 was first shipped to me, it was set to the back button focus.  You can imagine how confused I was when the normal half-depressed shutter button didn't work!!  Fortunately I figured it out (by some stroke of sheer luck) and set it back to the normal mode.   

However, at the Manitowoc Airshow a few weeks ago I decided I'd try it with the back button focus.  I think I like it!  It takes a bit of getting used to but since the button's in a convenient position, not too much.  It was pretty handy to prefocus a spot and (at cloudy icky Janesville) to lock in an exposure based on the sliver of blue sky so the camera didn't leave me with silhouettes. 

I'm glad this topic was brought up! Thanks! 

[PVNick]

I've recently started playing with the back focus button. As far as I'm concerned......it's the shizznit.

nick

[DanG (m1)]

i am now a convert. By keeping the focus control on the *button i can always have it on AF servo and still have MF overide. By this i mean, i can focus on the subject (often a bird) unclick the *button, fine tune if needed then take the shot. Then, if the bird decides to fly, i dont have to curse a lot as i the bird flys at of the depth of field before i can refocus. This is because  all i have to do is push the button again and it will turn on the focus. If i leave it on servo the conventional way, i have to switch focus mode or the switch on the lense to fine tune if the camera back focuses or wont focus on the head etc.

[GeoffS]

When the D30 was first shipped to me, it was set to the back button focus. [...]

Ah, now we another of Jim's secrets... 

Cheers,

Geoff S.