September 21, 2014, 12:39:33 PM

Author Topic: AI Servo AF Versus One-Shot AF For Stationary Subjects  (Read 6772 times)

sagittariansrock

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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #15 on: March 25, 2014, 03:28:42 PM »
Thanks for sharing this very useful piece of information.

I got befuddled by the "AI servo not locking focus" a few days ago when I was focusing and recomposing, and kept taking one fuzzy image after another. It felt like witchcraft, that the image would go all blurry on me just as I recomposed to take the picture (I was using BBF but foolishly kept my finger on the BB) until I realized that I was on AI servo.
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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #15 on: March 25, 2014, 03:28:42 PM »

ScubaX

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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #16 on: March 25, 2014, 04:17:27 PM »
This was very helpful, especially the low light improvement with One-Shot center focus.

There is a video on B and H's Youtube channel on the Canon Autofocus System that I found very informative.

http://youtu.be/iAx86nblZ2g
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Click

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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #17 on: March 25, 2014, 04:44:58 PM »
Great info. Thanks for sharing.

Brymills

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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #18 on: March 25, 2014, 05:03:21 PM »
As well as being a good answer technically, it's also a well presented answer.  I do like his style of explaining things in a clear and concise manner.

SPL

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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #19 on: March 25, 2014, 05:07:49 PM »
this was great!, Thanks!

ritholtz

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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #20 on: March 25, 2014, 05:13:19 PM »
I am still trying to understand why it is not guaranteed to get a sharp picture with AI server for stationary objects with shallow dof as long as light is enough. But there won't be any focus confirmation beep. Considering there is no AF lock with AI Servo, it is not possible to do focus and recompose with it right?. Do I need to always switch to one shot AF with focus and recompose.
Thanks 
« Last Edit: March 25, 2014, 05:18:31 PM by ritholtz »
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Mt Spokane Photography

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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #21 on: March 25, 2014, 05:18:39 PM »
I read the reply, and thought to myself that someone finally received accurate information from a person who knew how it works.  Then, I saw it was Chuck Westfall.
 
You can select AI focus, which attempts to switch between the two modes by detecting movement of the subject.

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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #21 on: March 25, 2014, 05:18:39 PM »

candc

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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #22 on: March 25, 2014, 06:23:05 PM »
I am still trying to understand why it is not guaranteed to get a sharp picture with AI server for stationary objects with shallow dof as long as light is enough. But there won't be any focus confirmation beep. Considering there is no AF lock with AI Servo, it is not possible to do focus and recompose with it right?. Do I need to always switch to one shot AF with focus and recompose.
Thanks

If you want to focus and recompose using ai servo then you need to use back button focus. In order for it to work you need to go to the customize settings for the shutter button and deselect the "half press initiates autofocus" option.. That way you use your center point to focus using the back button then let go, recompose and push the shutter button.

I always set one of the custom modes this way along with the fast tracking AF settings you see posted here and on other websites. That way you don't have to fiddle around and make a  lot of setting changes to switch back and forth.

Sanaraken

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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #23 on: March 25, 2014, 07:23:45 PM »
Thanks for sharing.
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expatinasia

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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #24 on: March 25, 2014, 08:23:35 PM »
Thanks for sharing, Steve Todd.

Remember that the AF sensor in the camera has a low light threshold, typically EV -1 or -2 depending on the camera; this figure is quoted specifically for the center AF point with One-Shot AF. It's usually about 2 stops less than than with AI Servo AF, and even lower with off-center focusing points. Therefore, if maximum sensitivity for AF in low light is your priority, we strongly recommend One-Shot AF with the center focusing point.

I particularly liked this bit. I use off-centre focussing points a lot and did not realise that there was such a difference.
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sanj

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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #25 on: March 26, 2014, 12:57:38 AM »
I do not want to argue, but for me this seems bit irrelevant. If I am shooting anything with the slightest movement or if I am causing movement by hand holding, I need to be on servo.

If my camera is on tripod and the subject absolutely still, I would use live view to focus.

For my kind of photography: wildlife, street and such I can not be not on servo.

One shot and on top of that center focus point, do not know if that would work for me.

GaryJ

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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #26 on: March 26, 2014, 03:09:28 AM »
Thanks for asking…and posting the response!
  +1
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Badger

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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #27 on: March 26, 2014, 08:49:16 AM »
Learned something new today. Thanks, done for the day ;D
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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #27 on: March 26, 2014, 08:49:16 AM »

Jackson_Bill

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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #28 on: March 26, 2014, 09:26:10 AM »
I am still trying to understand why it is not guaranteed to get a sharp picture with AI server for stationary objects with shallow dof as long as light is enough. But there won't be any focus confirmation beep. Considering there is no AF lock with AI Servo, it is not possible to do focus and recompose with it right?. Do I need to always switch to one shot AF with focus and recompose.
Thanks

Why would you want to focus and recompose using AI servo, anyway? THe point of AI servo is to track moving objects. If you're going to focus and recompose, the subject has moved and you're OOF again.

Regarding your question on the lack of a sharp picture: From my experience with AI servo, which I use at least 75% of the time, that mode will "hunt", especially if you are hand-holding a long lens and there is a little motion. Combined with the need for more precise focusing when you have a shallow depth of field, the AI servo mode can end up with the focus somewhere other than you want it to be.


BLFPhoto

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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #29 on: March 26, 2014, 03:30:58 PM »
Correct....the reason Ai-Servo is not guaranteed sharp is that it never stops focusing as long as the AF on is engaged, whether at the shutter button or on the camera back.  If you're shooting motion...you're focused on maintaining your focus point on the subject.  If, on the other hand, you're shooting static, with Ai Servo, you're really trying to use it like One-Shot AF...and but while you held the AF-on engaged, you may have moved off the intended subject point slightly.  And there is the additional confounding factor that Ai Servo is apparently less light sensitive in practice than One-Shot.  More sensitivity to light means more sensitivity to the point of intended focus. 

Nevertheless, Ai Servo works exceedingly well across all of the newest Canon upper range, and is pretty darn good going back to the 7D, 1D Mk III and 1Ds III. 

Reading across many, many photography forums, it becomes plainly evident that many, if not most focusing problems are more related to operator error in misusing or mis-understanding the focus system of their camera and setting it up correctly for the type of shooting being done. 

Two things were clarified to me in the OP, the light sensitivity difference and the logical accuracy difference due to time to acquire focus between the two modes.  I certainly am a bit more informed by this thread. 
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Re: AI Servo AF Versus One-Shot AF For Stationary Subjects
« Reply #29 on: March 26, 2014, 03:30:58 PM »