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Old 11th of April 2012 (Wed)   #1
tedyun
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Default Problems with AI Servo on 550D

I'm having problems using the AI Servo AF mode on my Rebel T2i. I am mainly taking pictures of my family, particularly my kids while they are playing. The lenses I am using are 50 f1.4, 17-55 f2.8 and 70-200 f2.8 IS mk I

I had never really gotten AI Servo to work, but recently, I sat down and am making an effort to try to understand this feature. I can't seem to get it to track a subject.

I have my daughter stand a distance away. Using manual (center point selected) or auto AF point selection, AI Servo selected, I lock onto her (using standard or back-button AF), then have her walk slowly toward or away from me. As she does this, I understand AI Servo should adjust the focus. However, as she walks, she moves out of focus. I even have her stop, but the focus never adjusts.

Should the the lens USM be operating as AI Servo is functioning? The focus ring does not adjust as it does when it initially locks onto the subject.

Thanks in advance for any advice!
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Old 11th of April 2012 (Wed)   #2
Lowner
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Default Re: Problems with AI Servo on 550D

You do have the shutter release half pressed to activate the AI Servo? If you are keeping it pressed, the only other user issue is that you must keep the focus point locked onto the same place on the subject.

If you are doing both, then just maybe you need to get the camera examined.
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Old 11th of April 2012 (Wed)   #3
tedyun
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Default Re: Problems with AI Servo on 550D

Thanks Richard. I actually found my User Error by reading an excerpt from the Canon EOS Rebel T2i For Dummies book.

I even went to my local electronics store to test out another T2i, only to find out I was having the same problem.

It turns out that AI Servo does not work when Live View is enabled. I must look through the viewfinder! When Live View is enabled, all AF modes behave as one shot.

I did not know that there was a difference between the VF and LV.




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Originally Posted by Lowner View Post
You do have the shutter release half pressed to activate the AI Servo? If you are keeping it pressed, the only other user issue is that you must keep the focus point locked onto the same place on the subject.

If you are doing both, then just maybe you need to get the camera examined.
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Old 11th of April 2012 (Wed)   #4
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Default Re: Problems with AI Servo on 550D

At least you have the answer now
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Old 11th of April 2012 (Wed)   #5
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Default Re: Problems with AI Servo on 550D

Not only that, but I must get used to using the viewfinder again. As a recent immigrant from PnS land, I find it a bit awkward.

The last time I used a viewfinder, cameras had film!

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At least you have the answer now
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Old 12th of April 2012 (Thu)   #6
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Default Re: Problems with AI Servo on 550D

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Originally Posted by tedyun View Post
Not only that, but I must get used to using the viewfinder again. As a recent immigrant from PnS land, I find it a bit awkward.

The last time I used a viewfinder, cameras had film!
Very few of us regularly compose using the LCD. Besides anything else, locking focus on the LCD is so sloooow.
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Old 12th of April 2012 (Thu)   #7
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Default Re: Problems with AI Servo on 550D

u found 550D using AI SERVO with USM lens get good result?
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Old 12th of April 2012 (Thu)   #8
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Default Re: Problems with AI Servo on 550D

I have found liveview useful when using a tripod, as it locks the mirror up on my 5D2. My grateful thanks to Elie for the information. Otherwise I don't use it.
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Old 12th of April 2012 (Thu)   #9
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Default Re: Problems with AI Servo on 550D

I am finding this out! It's like a totally different camera, using the VF. I had no idea that there was such a big difference in AF speeds. Oh, the pictures I've missed because the Live View AF was so slow...

The difference in AF speeds was the main thing that struck me when I switched over from my Sony F828...Now I know the "secret."

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Originally Posted by mwsilver View Post
Very few of us regularly compose using the LCD. Besides anything else, locking focus on the LCD is so sloooow.
I'm getting better results using AI Servo with USM, in VF vs LV. I am not sure if it is up to pro standards, but it is working as I've always wanted it to. I am just testing it out right now, and will report back once I try using it in real situations.

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u found 550D using AI SERVO with USM lens get good result?
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Old 12th of April 2012 (Thu)   #10
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Default Re: Problems with AI Servo on 550D

Live View uses a completely different method of focusing. It's called Contrast Detect and the imaging sensor itself, which is exposed in Live View, is doing the focusing. It's the most accurate focus method on your camera... but slow, slow, slow! It's actually the same type of focusing method many point n shoot cameras use... and why people are so amazed how fast SLRs focus can be and one of the key reasons they upgrade to them.

The normal focusing method, as seen through the viewfinder (with Live View off), is called Phase Detection and uses the array of AF points you see indicated in the viewfinder.

In the VF focusing mode, you have a choice of One Shot, AI Servo, or AI Focus. One Shot is used for stationary subjects. AI Servo is used to track moving subjects.

AI Focus isn't really a focus mode at all... it's more like automation on top of automation. The camera is supposed to decide whether or not the subject is moving, then switch to the other AF mode that's appropriate. In the past I've experimented with it and found it has a delay, so I stopped using it and just switch back and forth between One Shot and AI Servo myself.

One Shot is more accurate than AI Servo. It focuses, locks on the subject, then stops and gives Focus Confirmation. You can use it with multiple AF points or a single, manually selected point. The latter is the most accurate if possible. If using a single point, the center AF point is the most accurate and quickest in all Canon cameras (some have other higher performance points elsewhere).

Because it locsk on and then stops, One Shot is usable with a "focus and recompose" method, if using a single AF point.

AI Servo is continuously updated focus, constantly adjusting to follow a moving subject. It never locks, so doesn't give Focus Confirmation. Same as One Shot, it's most accurate using a single AF point, so long as you are careful to keep that point right on the place you want the camera to focus. You have to maintain half press on the shutter release button continuously for AI Servo to keep tracking. It is not practical to use for a "focus and recompose" method of shooting.

Subjects coming directly toward the camera or going directly away from it are the hardest for it to maintain AI Servo focus on. A subject moving past you, side to side or at somewhat of a diagonal, is easier to track.

You also might want to check out Back Button Focusing if you will be shooting in AI Servo a lot. This is a popular sports shooters technique, but really is usable with any type of moving subject and makes it possible for AI Servo to be used with a "focus and recompose" method, too. Basically, separating AF from the shutter release button... reassigning it to a button on the back of the camera that's operated with your thumb... puts the photographer more in full control of what focus is doing.

Yes, lenses can make a noticeable difference too.... In general, USM lenses (also HSM in Sigma or USD in Tamron) are faster, more accurate and quieter focusing. In general, lenses with larger apertures that offer up more light to the AF sensors also can be faster. There are exceptions, though... for example, by design the 85/1.2L is slower focusing, to emphasize accuracy over speed. Accuracy is more critical with super shallow depth of field rendered by very large aperture lens such as that.

Poor quality filters can have a negative effect on auto focus. Converserly, a lens hood might help at times, keeping oblique light from striking the lens.

Last edited by amfoto1 : 12th of April 2012 (Thu) at 10:28.
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Old 12th of April 2012 (Thu)   #11
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Default Re: Problems with AI Servo on 550D

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Originally Posted by amfoto1 View Post
* * * *You have to maintain half press on the shutter release button continuously for AI Servo to keep tracking. It is not practical to use for a "focus and recompose" method of shooting.

* * * *
I'd suggest one addition to Alan's excellent summary - using back button focusing AI Servo can easily be used for focus-recompose by acquiring focus on a stationary subject, then releasing the AF button so that focus is "frozen" and will not change if the camera is moved to recompose.
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