Focusing problems on 40d

Twizzel

Twizzel

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Rach
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Another thread from me hehe... Just got back from a horse event where towards the end my 40d seemed to be playing up with the auto focus. Had it set on ai focus which normally works well, and would pre focus on the jump wing/pole, moved camera to allow space for horse to jump into frame and then release shutter as the horse jumped into the frame.

The problem I had was that after focusing on the jump wing and setting up the frame, the camera would unlock the focus whilst my finger was still half on the shutter button and jump to focus on something else... Which resulted in out of focus images and a very frustrated twizzel. Going to take it into shop today but need to have an idea of what it could be, does anybody have any suggestions please? Luckily i have a second back up body but I really need thia body back in action too...
 
move the AF to the * or AF-ON button. solves the problem (although not a problem as such, the camera is just refocusing as asked when you half press the shutter).

but personally i wouldnt lock focus on the jump, use the SERVO mode and pan with the horse/rider.
 
neil_g said:
move the AF to the * or AF-ON button. solves the problem (although not a problem as such, the camera is just refocusing as asked when you half press the shutter).

but personally i wouldnt lock focus on the jump, use the SERVO mode and pan with the horse/rider.
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Surely though when I focus on the jump it should then lock the focus and not search for more focus points? If it was in servo mode it would be able to keep refocusing but I thought ai focus meant the focus was locked until the shutter was released?
 
AI focus, thats the one that focuses then shifts to servo on movement isnt it. you want one shot then keep the half press held.

otherwise when you half press the shutter it will auto focus on your selected AF point. the only other way to "lock" would be to move the focus to the */AF-ON or focus then switch the lens to MF which would be a little time consuming.

id expect more keepers from AI servo however.
 
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It sounds like an operational issue. AI Focus doesn't have many fans (see handbook for how it operates).

In that situation, given that you know almost exactly where the horse is going to be (and depth of field should cover small changes) I would use centre-point AF, pre-focus and hold it there. Switching to back button AF might be easier.

Or, use AI Servo and hope the camera/lens keeps up - should do if you can give it a decent target to get a fix on. The thing I'd be concerned about with servo is that if you're shooting at an angle, you can track the horse on the approach, then just before the critical moment the fence post gets in the way and diverts the focus. Pre-focusing just removes any possibility for error there - providing you can estimate the horse's position accurately enough.
 
Ok cheers, Ive looked into this back button af, is it easy to get the hang of? In terms of horses jumping and back button af, do I track the horse or wait for it to come into the frame like I am used to doing? It seems to have a lot of praises sung by sports photographers so im wondering if its worth giving it a go?
 
Give it a whirl. I was a bit unsure about it, but tried it and have never switched back! It just means that you can focus and it will stay there no matter what you do with the shutter release. Not sure it's so good if you're on servo though.

On servo, you want to pick up the subject early so the camera gets a fix on it. It needs a moment or two to lock on and behind the scenes it uses other AF points to estimate speed and distance travelled to predict the right position when you release the shutter. Accuracy depends on how well it calculates this (not easy when you think about it) and then the ability of the lens to do what it's told very quickly. Depending on the subject and circumstances, sometimes it works a dream and other times it's just not good enough - hence my suggestion of pre-focusing which cannot go wrong, provided you know where the horse will be and can time your shot precisely.

Try AI Servo with single shot, and also on continuous drive. Then try pre-focusing and see which method has the best success rate. You might use all three methods in different circumstances.
 
I do a fair amount of sport and only ever use back button, do a search as lots of threads on it. Does take some getting used to and would recommend either use it all the time or not at all.
 
Twizzel said:
After using a nikon this weekend if i had the money i would...


Never thought i would say that lol :suspect:
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Canon 7D or 1D4 are probably the best cameras you can get for AF tracking. Though I believe one or two Nikons have been known to take a stab at it :D
 
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