# AI Servo AF Versus One-Shot AF For Stationary Subjects



## Steve Todd (Mar 25, 2014)

Having read several posts discussing the pros and cons of using AI Servo AF versus One-Shot AF for stationary subjects, I thought I'd ask the one guy who could best address this issue for us. 

He was kind enought to allow me to post his reply to my questions here:

Hi, Steve: 

There are no differences in focusing speed, focusing accuracy, or focusing point selection algorithms between One-Shot AF and AI Servo AF with EOS DIgital SLR cameras, period. 

The basic difference between them is that One-Shot AF locks focus as soon as it is complete, whereas AI Servo AF continues to track focus as long as it is active. This is why One-Shot AF is recommended for stationary subjects, while AI Servo AF is recommended for most types of moving subjects, especially those that move towards or away from the camera as opposed to lateral movement across the frame. 

There are other differences between One-Shot AF and AI Servo AF that can affect some kinds of photography: 

1) AI Servo AF allows photographers to release the shutter at will, regardless of whether focusing has been completed or not. This is intentional, in order to allow the photographer to prioritize capturing the peak moment regardless of focusing status. The trade-off is the fact that there is no guarantee that the focus will be sharp on a stationary subject in AI Servo AF, especially during handheld photography at close range with shallow depth of field. Under these specific conditions (one more time for emphasis, I am saying Stationary Subject, handheld photography at close range with shallow depth of field), One-Shot AF is a more reliable focusing method because it locks focus while AI Servo does not. 

2) As light levels diminish, eventually AI Servo AF will cease to function before One-Shot AF does. This is because One-Shot AF allows a longer sampling period for AF measurement in low light than AI Servo does. (The AF measurement sampling period is analogous to a shutter speed for the AF sensor. The longer the sampling period, the greater the sensitivity.) 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. 

Going back to point 1, current professional EOS models like the 1D C, 1D X and 5D Mark III give photographers more control over shutter release priority in AI Servo AF than older models. You'll notice that there are menu settings in the AF menu section for 'AI Servo 1st Image Priority' and 'AI Servo 2nd Image Priority.' These settings let you control how long the camera waits before releasing the shutter in AI Servo, which is better than older cameras like the 1D Mark IV or 5D Mark II. But it still lets the camera shoot when it is out of focus in AI Servo AF if you insist. In other words, shutter release in AI Servo AF is always a matter of "when," it is never a matter of "if" the subject is in focus. 

The bottom line is simply this: AI Servo AF is *not* equivalent to One-Shot AF for stationary subjects in terms of shutter release priority, especially for handheld shots with shallow depth of field, and we never claimed that it was. That's why we offer both focusing modes. This doesn't mean that AI Servo *can't* get it right. It means that One-Shot AF is more reliable under these specific conditions.

Hope that helps.


Chuck Westfall
Advisor, Technical Information
ITCG Prof Client Relations Division
Canon U.S.A., Inc.
One Canon Park, Melville, NY 11747


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## bseitz234 (Mar 25, 2014)

Great explanation from Chuck, thanks for sharing! Since I switched to BBF I usually end up using servo even for stationary objects, so I'm glad to have this explanation of the limitations of each to know when to switch back to one shot. Specifically, I knew about locking focus vs. tracking, and shutter priority vs. focus priority, but did not know about the light sensitivity due to sampling time, which is very interesting. 

Again, thanks for sharing!


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## neuroanatomist (Mar 25, 2014)

Thanks for asking…and posting the response!


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## J.R. (Mar 25, 2014)

Thanks for sharing this.


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## ajfotofilmagem (Mar 25, 2014)

This is one of those things we imagine to know the answer. But it is very good to make sure we're doing the right way. : Thanks for sharing.


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## mackguyver (Mar 25, 2014)

Thanks and I'm happy to see that his reply matches what I've experienced, and disagrees with most of what I've read. It seems like there is a lot of misinformation out there on the two settings. Also, I noticed that he didn't mention AI Focus


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## Viggo (Mar 25, 2014)

mackguyver said:


> Thanks and I'm happy to see that his reply matches what I've experienced, and disagrees with most of what I've read. It seems like there is a lot of misinformation out there on the two settings. Also, I noticed that he didn't mention AI Focus



Everyone knows Ai Focus doesn't work ;D, and none of the 1-series have Ai Focus.


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## mackguyver (Mar 25, 2014)

Viggo said:


> mackguyver said:
> 
> 
> > Thanks and I'm happy to see that his reply matches what I've experienced, and disagrees with most of what I've read. It seems like there is a lot of misinformation out there on the two settings. Also, I noticed that he didn't mention AI Focus
> ...


I know and I was really disappointed to see that my new 1D X was missing the scene modes (Basic Zones), too! It's supposed to be a sports camera, right? Then why is there no Sports, let alone Portrait, Close Up or other modes. I'm beginning to think the "X" in 1D X stands for the missing features and don't even get me started on the missing Direct Print button - even my 5DIII has that  ;D


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## R1-7D (Mar 25, 2014)

Great information!


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## Albi86 (Mar 25, 2014)

The part about focusing performance in low light was really helpful.


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## TexPhoto (Mar 25, 2014)

Great info. I stay in AI Servo 99% of the time, because I prefer it for all subjects. The only time I would prefer one shot is to focus wit the subject in the middle, then recompose with the subject off center. I don't do this much.


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## saveyourmoment (Mar 25, 2014)

Hope I am right, but one big difference is, that in one shot I can use the beep, when the focus is locked and there is a green dot inside the viewfinder that shows me, that the focus is locked( it stays green)
These things doesn't work/show up using ai-servo. (On my 5d iii)


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## Stig (Mar 25, 2014)

Thanks for sharing!

reminded me that I got stuck a bit in one shot and I wasn't really aware of the "servo allows shutter release before AF has locked on" part


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## saveyourmoment (Mar 25, 2014)

And btw, when you remove af-on from the shutter and using backbutton focus ( af-on) ai-servo acts like one-shot... (see custom settings)... just release af-on button and the focus will be locked...


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## Jack Douglas (Mar 25, 2014)

I've used AI Focus with my 6D quite a lot and it definitely works. In fact, for that rather AF lame camera it struck me as being better than AI Servo but I won't argue that point. I don't really understand the difference between those two, though.

Now, I'm a beginner and have only reached about 15 000 on my shutter but I've posted a fair number of shots on CR that are respectable so there is some proof for my assertion. Still,it's just my 2 cents worth. 

Jack


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## sagittariansrock (Mar 25, 2014)

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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## ScubaX (Mar 25, 2014)

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 (Mar 25, 2014)

Great info. Thanks for sharing.


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## Brymills (Mar 25, 2014)

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.


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## SPL (Mar 25, 2014)

this was great!, Thanks!


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## ritholtz (Mar 25, 2014)

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


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## Mt Spokane Photography (Mar 25, 2014)

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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## candc (Mar 25, 2014)

ritholtz said:


> 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.


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## Sanaraken (Mar 25, 2014)

Thanks for sharing.


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## expatinasia (Mar 26, 2014)

Thanks for sharing, Steve Todd.



Steve Todd said:


> 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 (Mar 26, 2014)

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.


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## GaryJ (Mar 26, 2014)

neuroanatomist said:


> Thanks for asking…and posting the response!


 +1


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## Badger (Mar 26, 2014)

Learned something new today. Thanks, done for the day ;D


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## BLFPhoto (Mar 26, 2014)

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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## FEBS (Mar 26, 2014)

saveyourmoment said:


> And btw, when you remove af-on from the shutter and using backbutton focus ( af-on) ai-servo acts like one-shot... (see custom settings)... just release af-on button and the focus will be locked...



I agreement that the focus Will be locked, but as explained in the beginning, ai-servo gives no garanty that THE object is in focus as you can always take the picture, while one-shot will only allow you to take a photo when the object is in focus (Beep, green light)

Francois


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## Wardenmsp (May 17, 2014)

FWIW - I use BBF and have assigned the DOF preview button to switch between AI Servo and AI One Shot. Then I can use BBF AI Servo and, while holding the back button, momentarily switch to AI One Shot to lock focus by pressing the DOF button. This works on the 5D Mk III and 7D, I know.


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## BL (May 19, 2014)

Using AI servo for both stationary and moving subjects with the 1DX. It's rather liberating not having to think about toggling AF modes now.


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