How to use focus points

[Toby]

I'm a bit confoozled here.

I generally set my cam to use the centre focus point only rather than all of them. What I can't wrap my head around is how exactly things work with multiple focus points.

If I use them then, when I focus, some of them will get a "lock" and flash red but what exactly does this mean? The camera can only focus at one distance so is it just pot luck which focus points it chooses?

I can see how multiple points could be useful when using the A-DEP setting (which I don't) to allow the camera to work out the aperture required to keep all the focus points within the DoF but, in other modes, how does it work or help?

Any advice appreciated, sorry for the noobish question :)

[divine]

I dunno about others but my camera you choose the focus point first and then focus, not focus and then the camera picks one.

[Toby]

well yeah I can choose a single focus point (typically the centre one) but I can also leave it on "all" of them and it's this mode that I just don't understand

[divine]

Oh right ok, mine doesn't do that, can't see any point to that, what camera is it?

[Mark]

It'll be an EOS of some kind, having A-DEP, maybe. :)

All I can say is RTFM. :p

It should say in there. One thing to note however is that DPreview say you're better off not using it because it can be a bit random. My own experience has backed this view up.

Evaluative metering
This is an all-around metering mode suited for portraits and even backlit subjects. The camera sets the exposure automatically to suit the scene. This metering mode is set automatically in the Basic Zone modes.
Take that as you see fit, or read this:

http://en.wikipedia.org/wiki/Metering_mode#Multi-zone_metering

:)

Edit - yup, it's an EOS alright. Forum skills. CBA to find out which one though. :)

[divine]

Metering isn't focussing, so either he's getting confused or that's something entirely different.

[Matblack]

The canon will tell you which focus points it considers to be within focal tollerance if you leave it to do its thing. It will try (and probably fail) to choose the most appropriate to prioritise. I tend not use one shot centre focus and hold the shutter half closed to maintain off centre focus although if using auto focus servo I'll let the camera decide as it usually gets it right then.

MB

[Toby]

It's a Canon 40D

The manual isn't much help but I've been reading the field guide I downloaded which is a bit more useful.

From what I can gather, the only advantage of using the "all" focus points setting is in AI Servo focus mode, where it will initially use the centre point but then track the focused object around the frame using the other eight points.

Mat, not entirely sure I follow you but it sounds like you're on about attaining focus lock using a specified focus point then recomposing, or have I misunderstood?

[Mark]

The last two of your paragraphs pretty much cover MB's post. :)