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Old Oct 5, 2006, 5:26 AM   #31
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Hmm, NHL will have something valuable to say here I think.

But IMO if you are using AI Servo you should be using all the focus points.

For one shot you can use all or just the centre. I would use just the centre one with one shot AF for action work.
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Old Oct 5, 2006, 8:12 AM   #32
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Just to clear some things up:

One shot vs. Servo:

One shot takes a single focus lock for first frame and maintains that same focus lock for your entire burst. So, if your subject changes focus planes - i.e. is moving toward you this focus mode will not work. This is where you will see true 5 fps speed. The first shutter will not occur until the camera has achieved focus - but as long as you keep the shutter down, the focus is locked at the same point as the first shutter - the camera DOES NOT REFOCUS. If you want to track a subject that is changing focus planes you must release the shutter completely and regain focus - so your frame rate is not going to be 5fps - more like 1fps or worse. So, if you want to track a subject moving toward or away from you AND you want a fast frame rate you need to use SERVO.

In AI Servo - this is the predictive focus mode. First frame happens whether camera is focused or not - it is shutter priority. Every shot thereafter is FOCUS PRIORITY - the camera refocuses and then fires. Because the camera is refocusing (UNLIKE IN THE ABOVE EXAMPLE) your frame rate can go down. In one-shot mode above with the shutter depressed you only focus once for a 5 shot burst. In this mode the camera focuses 5 times. That takes time and sometimes the camera has more trouble than others predicting and focusing so your frame rate will go down. The key is to make sure youo focus and track before firing, because the first shot is shutter-priority, track your subject for a split second until you get a focus lock first then fire.

AI-FOCUS is the hybrid mode - I have yet to hear from any reliable shooters that use this mode - I myself have never used it.

Single point vs all points in AI Servo mode. If using all focus points in this mode, the camera aquires the target with the center focus point. The camera will then allow the subject to pass to other focus points and continue to track it. The downside to this approach is that when other subjects enter the focus area the camera can get confused as to which one it is supposed to track. You also have to realize that the focus area is larger than the red dot - so when your target is small in the frame, the focus area may cover it and the background behind it. If there is more contrast in the background guess what the camera chooses to focus on .

Using single focus point in SERVO mode - this gives you tighter control to ensure the camera focuses and tracks only the subject you want and doesn't get confused by another subject (think football where bodies are all over the place - keep your focus point on the right head and you are OK). The down side to this approach, of course, is that it requires a finer tracking technique on your part - if that focus point slips off your target it will re-focus on something else.

So, the key to successful servo use - especially for sports - is to have a larger subjecct in the frame. The more of your subject that covers the focus point the easier it will be for you and the camera to track the subject and the more contrast the camera will be able to pick up and get a finer lock. Also, as you know - you can't do heavy cropping of a sports shot anyway and still get a crisp crop.

So, both focus point approaches have their strengths and weaknesses. But, while tracking a bird going left to right in the same focus plane may work with one shot - if that subject is coming toward or away from you at a decent clip (let's say a person running or a bird flying) and you take a 5 shot burst in one-shot mode - no way you're going to get multiple shots in focus. How could you? The camera only focused once? (Again, unless you release focus entirely and reaquire - even if using the * button you're not going to be fast enough to come anywhere close to 5fps).




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Old Oct 5, 2006, 9:42 AM   #33
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I agree with most everything JohnG wrote except:
Quote:
In this mode the camera focuses 5 times. That takes time and sometimes the camera has more trouble than others predicting and focusing so your frame rate will go down. The key is to make sure youo focus and track before firing, because the first shot is shutter-priority
What was neglected is that the camera only allows certain amount of time-out per focus: 200us for USM/HSM and 250us for non-USM lens which translate to 5 or 4 shots/seconds...(there's a link somewhere). Again do the lens cap test. The camera will keep up the frame rate even if nothing is in focus - Just check it everyone and convince yourself of this shutter-priority mode

Now the MrkII can do 8fps - and here I agree with JohnG that in one-shot AF the camera can achieve that rate, but it will fall down to 5fps if the lens can't achieve focus (it will try but the time-out will overide it) - again check it with the lens cap on... especially with non-USM lens (i.e. 4 vs 8fps)!

The bottom line is this:
o With one-shot AF at least the first frame will be in focus (the shutter won't releas otherwise) - and yes you can do a burst as long as the subject stays in the DOF field and you'll will be kosher
o In servo-AF, the first frame is not guaranteed to be in focus to maintain the shutter-priority, but the camera will try to AF between shot to track the subject if it can (i.e. under the time-out period) otherwise the camera will try to maintain the frame rate even if it can't focus!

... Yes I dabble with Sport once in a while (again in one-shot AF): http://www.stevesforums.com/forums/v...mp;forum_id=82
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Old Oct 5, 2006, 11:15 AM   #34
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NHL,

I don't doubt the veracity of what you're saying I just have a hard time figuring out why sometimes my camera gives me close to 5fps and other times while keeping the shutter pressed it will take 1 or 2, long pause then resume. Unless it's a hiccup in the buffer handling. But I use Extreme III card and I almost never take more than 5-6 shot burst. And, when shooting sports I shoot in JPEG.

So, let me rephrase my original statement to this:

based on my experience of about 30,000 frames shooting action sequences while using an EXTREMe III cf card and taking typically 2-6 shot burst, always in AI-Servo mode I have experienced different levels of burst rate performance. Sometimes I get around 5fps. Other times I get half that. Cause of this is unknown to me. Whether this same scenario would occur in single shot I can't say - my experience there is somewhat less (I do use it for golf where the player's face typically remains in the same focal plane - so based on a thousand or so shots I consistantly got the 5fps performance).
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Old Oct 5, 2006, 11:30 AM   #35
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John & NHL - Excellent thread guys.

That I guess confirms my experience even though my understanding of the underlying principles was wrong.

I have used one-shot mode (with and without) multiple shots with sufficient success to keep me happy.

In general with action shots I try to keep my aperture at a value that gives me a decent DOF (f8 by default). My burst depth would only usually be 2-5 shots, so in practice I have had very few out-of-focus shots using this technique.

I also tend to use the centre-point only for action.

I guess one of the best ways to experiment is to stand by the side of a road and practice on cars going by, or a Sunday football game.

Of course it MAY still be a problem with the lens.




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Old Oct 5, 2006, 12:08 PM   #36
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peripatetic wrote:
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In general with action shots I try to keep my aperture at a value that gives me a decent DOF (f8 by default).

My guess is your success rate has more to do with the f8 value than in using single shot. Especially in tracking say a bird in flight. Heck, even something coming right at you. You have a lot more leeway in your DOF for a subject to move and still be in focus. When you shoot wide open at 2.8 or 2.0 it doesn't take much motion for your subject to be OOF.
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Old Oct 5, 2006, 12:14 PM   #37
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The next instalment.......

I have just been to see a friend of mine who is a pro photographer (and waiting for an award ceremony next week as she is down to the final 3 for the British Institute of Professional Photography and the Master Photographer's Association's Dennis Hylander Award for wedding photography - so much fun having someone around who it a lot better than me), to try out some of her lenses and also try my lens on her 5D to see if the results are similar.

I am going to spend some time looking at them properly later however it does seem that there is a back focus problem with the 70-200, however also with her 24-104 f4 on my 30D there also seems to be some focus issues so I am now thinking that I might have a2 fold issue here. Once I have finished work for the day I will look more closely at the results and come back to you.
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Old Oct 5, 2006, 3:06 PM   #38
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peripatetic wrote:
Quote:
In general with action shots I try to keep my aperture at a value that gives me a decent DOF (f8 by default). My burst depth would only usually be 2-5 shots, so in practice I have had very few out-of-focus shots using this technique.
Peri, this is my practical side:
1 Mile = 5280 ft
1 yard = 3 ft

How fast can a person run?
Let assume Superman can do 10 Mph...
10 Mph = 52800 ft/hr or ~15 ft/s

The DOF of a 300mm @ f/2.8 is 32 ft at 100 yard
Assuming your focus is dead on you can shoot Superman in action @ 5fps and still be within 1/2 the DOF with one-shot AF regardless of direction

With the same lens @ f/8 the DOF is now 94 ft
which mean you can shoot up to 3s burst without having to refocus!!!

-> This why I stay in one-shot AF for nature shots...
May be when I start shooting race cars or motorcycles then I'll use AI-servo :idea:
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Old Oct 5, 2006, 5:20 PM   #39
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Conversely in my situation that same 300mm lens at f2.8 and subject 20 yards away (60') - DOF is only 1.29 feet; 0.65 ahead. So, think a person can move more than 6 inches in a second? YEP

So, what works in one scenario doesn't work in another. Also why extrapolating can be dangerous.
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Old Oct 6, 2006, 9:31 AM   #40
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You have to consider that even though your DOF may be XX feet, the sharpness will still fall off each side of the center of DOF. When considering that you may be cropping that shot later it will matter unless your not concerned with optimal sharpness.


It really depends on what your doing. Surfing for example... the surfers and the magazines are all about sequences for layouts. Even if the page is not going to be a sequence, providing a tight sequence increases the chances that one of the shots in that sequence will be chosen.

On the MKII.. C.Fn-21. After first shot, more emphasis is given to focus over drive speed. Enabling will be give more priority to drive speed.
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