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Old Oct 4, 2006, 12:44 PM   #21
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OK. I have spoken to Sigma and they have suggested getting the camera checked by Canon (AF, focus plane etc) and then send the lenses and body to them for calibration. This seems a bit of a faf and will mean I am without the kit for over a month!!!

I am going to try the lenses on my friends 5D to see how everything performs to see if that will shed any light on the situation.
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Old Oct 4, 2006, 1:27 PM   #22
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Do try as NHL suggests and switch to "One shot" AF.

I have never had any luck getting sharp shots with AI Servo on my 20D.

On the other hand I have never had any problem getting sharp shots even tracking fast moving objects with "one shot".

Do the tests again with one shot mode.

And another link to the focus-recompose issue for reference, although it's unlikely to be the problem here...

http://visual-vacations.com/Photogra...pose_sucks.htm
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Old Oct 4, 2006, 1:45 PM   #23
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Definitely try your tests with one shot focusing since you are mainly checking for front/back focus. Don't throw AI Servo out the window though because when your ready to take a serious series of shots, your just not going to do it with single shot. I personally have no problems with the AI on my setup and use it more than single shot.
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Old Oct 4, 2006, 3:25 PM   #24
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It all depends on how big your subject is... with AI-servo

If your subject is rather large and can cover several AF points (on the 20D/30D it needs to be rather large since the AF points are spreaded further apart) - On the MrkII the 45 AF points are dynamic and packed tighter together so the subject can be smaller, but even with small birds I still have problem with AI-servo because the leaves or the branches will get in the way... (unless they are in the wide open)

-> But to check for front/back focus, unless it's on a tripod the results are unpredictable, even in one-shot AF you risk of moving your head back/forth after the camera has locked (especially up close with narrow DOF lenses - People who do Macro run into this all the time)!

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Old Oct 4, 2006, 4:57 PM   #25
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I completely forgot to mention that the test shots taken of the memory card test for front/back focus were taken on single shot. I have just been working through all the pics taken last week at the hockey match to see if I can salvage any of them for the website/sale and there might be a few, however on the vast majority of them it does seem to be showing a backfocus (not only when the subject is moving towards the camera). I hope to find out if this is from the lens or the camera when I try this on my friends 5D.

AI Servo is essential as I shoot sports 90% of the time.
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Old Oct 4, 2006, 8:36 PM   #26
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Mark1616

Like peripatetic said, single shot AF do not preclude you to only still shots. It's whatever tickle you - I shoot mostly high-speed actions and always in one-shot AF

o Theses were taken with the Sigma 120-300 f/2.8 EX on a 10D: http://www.stevesforums.com/forums/v...mp;forum_id=11

o With AI-servo I'll get mostly the background instead (especially on the last shot in the link where the background was busy and overwhelmed the tiny main subject): http://www.stevesforums.com/forums/v...mp;forum_id=11

-> The size of the subject matter... in which AF mode you pick :idea:
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Old Oct 5, 2006, 2:25 AM   #27
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Yes, using one shot AF mode you can still take action shots.

The frames-per-second rate is completely independent of it, as I'm sure you know.


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Old Oct 5, 2006, 3:56 AM   #28
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OK now I am confused (it does not take a lot), with one shot AF I know that shooting multiple shots is possible but for shooting sports where refocusing is needed how does this work? Do you have to keep releasing the shutter for each one (or if using customfunctions then press the * button) so can this work for taken shots in series?
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Old Oct 5, 2006, 4:16 AM   #29
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Nope, keeping the shutter down it will continue to take shots at 5fps as long as it can maintain focus lock, just the shutter will not release if it loses focus, so obviously your frame rate drops at that point.

As I understand it the AI Focus is simply for predictive focus and the shutter will trip whether it has a focus lock or not.

AI Servo is a weird hybrid between the two.

It will depend a lot on how good the AF prediction is, so on the 45point AF of the 1 series it will work nicely withmedium and large objects. On the 9point AF it can only really be expected to work with large objects that substantially fill the frame, and give the algorithm as many AF points to work with as possible. I would expect it would need to cover at least 3 points to have any reasonable chance of the predictive AF working. Also I think the algorithm would be far more successful with objects that are moving in a plane where the distance to the camera is more or less constant i.e. panning shots. Where an object is approaching or receding directly from the camera I would expect the algorithm to struggle because the object will be changing relative size. That would mean the algorithm will have trouble recognising it as the same object. With a panning shot it has a relativley constant sized object moving across the frame to work with and so can predict which AF point to use next.

I think for that sort of predictive AF we are still a couple of generations away from top performance. Digic III has face recognition, after that with general object recognition we might get to a situation where it can accurately track the rate of change of apparent size of objects approaching the camera.

What I do with action stuff (I'm no expert mind you) is to switch to centre-point AF only and one shot focus with multi-shot. As long as you can keep the AF point on target you will get close to 5fps, depending on :

1. how rapidly it is changing distance to the camera and

2. how easy it is to focus on (object contrast, lighting conditions,etc) and

3. the max aperture of the lens (f2.8 is good with the centre point) and

4. the speed of the focus motor (USM or HSM rules).

I have done this trying to track birds in flight with moderate success, but I think this is how NHL works.

If you lose your target then you will certainly know straight away, the shutter won't trip until it refocuses.

I find this is better than simply letting AI Servo decide you wanted to focus on the background and do that without telling you.

I expect there is a skill that can be learned - how to accurately use the AI Servo mode to get the results you want, but it is very much that, not something that one can simply use without thinking about it and get professional results. Of course if you can nail it with the 30D moving up to the 1 Series would be a piece of cake.
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Old Oct 5, 2006, 4:23 AM   #30
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So do you think that me using a single focus point (I usually use either the center of if I want to shot to have room for the player to 'run into' then would use one of the outside or diagonal ones is causingor at leastmakingthis problem worse? I always selected thefocus point when usingmy KM5D and it seemed to work fine.

Thanks
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