Archive 2011 · AF with fast lens

dmcharg
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p.1 #1 · p.1 #1 · AF with fast lens

Hi,

Looking for some advice on how to improve my technique for capturing images with 50D & 24-70 when shooting at F2.8. With stationary subjects at F2.8 its wonderfully sharp so i have no complaints. What i do find however is its much harder to get the focus right when i am taking pics of kids that are moving around and its not like they are sprinting around. I always keep an eye on the shutter speed and try to keep this as high as possible given the lighting. I also try to use the center focus point.

Any advice much appreciated.

David.

May 07, 2011 at 03:17 AM

J.D.
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p.1 #2 · p.1 #2 · AF with fast lens

Are you using AI Servo mode?

May 07, 2011 at 03:23 AM

dmcharg
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p.1 #3 · p.1 #3 · AF with fast lens

No i have never found it very effective. Maybe i need to try again.

May 07, 2011 at 03:25 AM

J.D.
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p.1 #4 · p.1 #4 · AF with fast lens

Yeah, make sure it's AI Servo and not AI Focus.

What body are you using and at what shutter speeds?

*EDIT* Sorry just saw 50D. That should do alright for AF. Are you using back button* focus? Makes things easier. You just mash the * button down with your thumb and push the shutter button when you think you've got what you want.

*This needs to be set through the custom function menu.

May 07, 2011 at 03:41 AM

Alan321
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p.1 #5 · p.1 #5 · AF with fast lens

AI Servo is pretty much essential for moving subjects, and even more so with lens/aperture combinations that offer little depth of field. One Shot is purely for stationary subjects. AI Focus, if your camera has it, starts out in One Shot and changes to AI Servo when it feels like it but you have little or no control over it.

Make sure that your chosen focus sensor is on target before you begin using AF.

You might get better results by allowing AF expansion unless you are very good at maintaining accurate aim while the subject moves.

If you drift off target or start AF while the chosen sensor is not on target then the AF system may track the wrong thing (wrong as in not what you wanted), and the only solution is to release the AF, re-aim, and then start the AF again.

Wrong focus will never be good, but even the correct focus can look bad if the shutter speed is too slow. For 70mm focal length most people need a shutter speed faster than 1/125 [ that's 1/70 for a full frame camera or 1/(70x1.6) = 1/140 for a crop sensor camera like the 50D ]. So aim for 1/160. And if the subject is moving then aim for 1/500 or faster. Be aware that some parts of the subject (hands, arms, turning head, etc.) can be moving much faster than the body of the subject, especially for kids who are playing.

Don't be afraid of cranking up the ISO to get the shutter speed up. Higher ISO will increase image noise but that is far better than blurry photos. Noise can be fixed with software but motion blur cannot.

And a final point; at f/2.8 there may not be much depth of field. It's quite possible that the camera has focused correctly but much of the subject is outside the thin DOF and will look blurry regardless of shutter speed.

- Alan

May 07, 2011 at 05:20 AM

dmcharg
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p.1 #6 · p.1 #6 · AF with fast lens

Thanks for the advice. Alan thanks for detailed response. Will experiment other focus modes and see how i get on.

May 07, 2011 at 12:09 PM

dontbsme_2
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p.1 #7 · p.1 #7 · AF with fast lens

I Had good results with the 50d, if i could keep my center focus point on my target using Ai Servo.....

John

May 07, 2011 at 04:54 PM

Alan321
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p.1 #8 · p.1 #8 · AF with fast lens

On some of the non-1-series cameras the central AF sensor is bigger than the others so that it can work better - either in less light or by working with a subject that is initially more out of focus than the other sensors can cope with. The downside is that if it covers a bigger "area" (actually a '+' shape or an 'x' shape) then it is harder for you to restrict focus to a small subject. It becomes easier for the camera to latch onto something else in the background or foreground, or another part of the subject at the wrong distance, instead of what you wanted it to focus on. If you have this problem then a non-central focus sensor (preferably a cross-type rather than a linear one) is possibly more useful. Unfortunately, the little box indicator shown in the viewfinder is almost useless for determining the size and shape of the focus sensor.

Another possibility is that the AF is just defective. My 40D would latch onto things that were nowhere near the single AF sensor that I was using, as if other sensors were in play when they should not have been. A number of people had similar issues with their 40D AF having a mind of its own but it was not a widely reported problem and so maybe we just unlucky. Nevertheless, no amount of user technique could overcome the AF problem.

- Alan

May 08, 2011 at 02:55 AM

ftemoto
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p.1 #9 · p.1 #9 · AF with fast lens

dmcharg wrote:
No i have never found it very effective. Maybe i need to try again.

You're not going to get in focus shots of moving objects without using AI Servo, especially with a narrower DOF at 2.8. AI Servo exists for those shots.

May 08, 2011 at 05:50 PM


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