Canon 350D AI FOCUS & One shot

311

I nearly always have my camera on one shot focus ,but I'm not clear about AI focus .Something I've noticed though is that when I use One shot with one of my sigma lenses it won't autofocus sometimes.
But when I change the camera to AI focus it works all the time.
Why is this happening?

Paddy@CIRL

One Shot - is exactly that, it focues on one spot and locks to that until you release the shutter button.

AI Focus - The Camera decides between One shot and AI Servo

AI Servo - Camera is continually focusing,on the points selected, it never locks on just keeps making adjustments to itself.

Try it yourself on the different modes and you'll see what I'm on about

311

cheers paddy ,I've no need for servo ,I use it with my 70-200 canon and the lens turns into a machine gun,handy for sports.
My problem is more why this problem only happens with one paticular lens ,is there software in the camera that calculates the light and judges where the subject is ???Why wont the lens focus at all when in one shot??

It's the very lens thats very hard to manual focus with ,although lately I've had to teach myself by looking at the distance reading on the lens in relation to real world distance.

elven

The AI servo is so that you can use the AF on a moving subject. Given, of course, that your lens will focus fast enough to track it too

311

I probably asked the wrong question ,what makes the camera autofocus on a certain point in One Shot autofocus.?
And why would the camera not autofocus in One shot?

elven

Are you using all the focus points, or just the centre one?

If you're using them all, in one shot mode, it could be trying to find which one is best to use - but in AI servo mode, it uses the centre one first of all then when it detects movement, it will decide which one to use from there on. That could explain it, if you're using all the focus areas.

I found an interesting thread from another forum that discusses the one shot/AI focus modes - just ignore it when they talk about the third mode, it isn't on the 350D and I don't think that the people that have it even use it anyway...

Fenster

Tag for a friend

jlang

Fenster wrote:

Tag for a friend

???

Paddy@CIRL

Are you getting any error readings on the wee LCD when the lense won't focus ? If not its what Elven said, the camera can't find what it wants in the centre point only so set the AF mode to all points !

Never knew that about Servo mode elven, re: the centre point AF'ing first, cheers !

311

the problem I'm having is when all the points are on ,it just won't focus on any point. It doesn't happen all the time ,just certain shots ,but once it happens in a shot it keeps on happening.

elven

See if it happens less when you have it set to the centre AF point only. Is it in low light, or on something with low contrast? When you have a problem with a particular scene, change to a different lens and see if it happens with that one on AF one shot too.

311

thanks elven ,I'll stick at it ,wasn't sure if something causes the problem other than light etc.

Cheers
Brian.

elven

Oh and I stupidly forgot the link from earlier, and just had to spend 10 minutes searching for it again. Don't say I'm not good to you...

http://photography-on-the.net/forum/showthread.php?t=81536

Fenster

jlang wrote:

???

My friend can go through my post history and find this thread if it drops off the front page before she sees it. :]

darragh.delany

The point of the AI Servo mode is to handle moving subjects that will go out of focus during the composition and shooting time whereas the One Shot mode is for still subjects or at least those that are unlikely to alter focus. AI Servo can misbehave is the motor on your AF lense is not fast enough to keep up and either mode can be tricked by low light or eratic movement. I have one Canon lense that is prone to 'getting lost' on my 20D and D300 bodies whilst focusing and usually find that releasing the lense part way as though about to take it off and then locking it back into place seems to reset it.

DjD

311

Thanks for that daragh ,I've noticed sometimes if a lens starts behaving stupid ,I relock it and it's ok.

sometimes though ,I don't turn off the camera when taking off a lens ,which I think is like turning off windows without shutting down.