# Fast lenses and AI Servo



## Kit Lens Jockey (Jan 28, 2017)

Is it just me, or does AI Servo just simply not work very well with fast lenses wide open? I was just taking photos of an outdoor event at night, so I used my Sigma 50mm at set at 1.4. Since I've recently gotten the 5D4, I decided I'd give AI servo a try since this involved people moving.

Man, did the camera ever blow it. Often with one shot focus at 1.4, sometimes shots will just be a little off because The subject moved in the instant between when it gets focus and when the photo is taken. But with AI servo, it's just awful. I mean, it's not even close. I've got so many photos that are just downright blurry. It's not just like front focusing or back focusing a little, it's like the camera was just completely off in the weeds, focus-wise, when it decided to take the photo. I have all the shutter release settings set to focus priority, but still it's awful. Is AI servo just not suited to the precise focus needed with a fast lens?


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## Luds34 (Jan 28, 2017)

I saw the word "subject" in your post, which makes me assume your shooting people at what would be considered normal distances to fill the frame to a certain degree. Aka, unless your at or near infinity focus, 50mm at f/1.4 is going to be a pretty darn narrow DOF. Throw in "at night" focusing and the lower light will challenge focus. And while I'm not an expert on the Sigma 50mm Art lens, a number of people have had some pretty inconsistent results shooting in one-shot let alone any sort of focus tracking. At one point I owned Sigma's old 50mm EX lens and while I could get a perfect shot at f/1.4 that was on a static subject. Ai-servo, tracking was much more hit and miss and pushed me to just spray and pray a bit more.


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## Larsskv (Jan 28, 2017)

I've had good results using AI Servo shooting slow moving subjects with the 85LII, even on a 6D (using the center point)

I believe the lens is a major reason for you problems. That said, precise focusing with large apertures is always a challenge when shooting moving subjects.


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## Alex_M (Jan 28, 2017)

You can get in-focus shots with Sigma 50 1.4 Art in low light and AI servo mode wide open but only by chance and if you are very lucky. Good lens though.


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## Viggo (Jan 28, 2017)

95% of my shots are wide open with fast lenses and no dof, but Ai Servo is no less accurate than One Shot. 

However, the five Art lenses I had were all like what you describe; it's like it just chooses a random distance to focus on and you just better hope you're lucky enough to have your subject within the focused area, which was about 1 out of 10.

I track fast children and other insane objects and I get pretty much 8/10 in focus over all, most of the time it's a miss is soccer practice in iso 25600 light at f2 1/1000s and most of the misses are just me and not the gear.


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## YuengLinger (Jan 28, 2017)

Viggo said:


> 95% of my shots are wide open with fast lenses and no dof, but Ai Servo is no less accurate than One Shot.
> 
> However, the five Art lenses I had were all like what you describe; it's like it just chooses a random distance to focus on and you just better hope you're lucky enough to have your subject within the focused area, which was about 1 out of 10.
> 
> I track fast children and other insane objects and I get pretty much 8/10 in focus over all, most of the time it's a miss is soccer practice in iso 25600 light at f2 1/1000s and most of the misses are just me and not the gear.



And you are no longer using the 50mm Art, iirc. :


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## sanj (Jan 28, 2017)

It is you. I use servo focus for action photography all the time. 

It comes down to technique:
1. Learn focus point selection.
2. Learn various focus point set ups for action
3. Lean back focus. 

There is a learning curve here.


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## neuroanatomist (Jan 28, 2017)

Kit Lens Jockey said:


> Is it just me, or does AI Servo just simply not work very well with fast lenses wide open? I was just taking photos of an outdoor event at night, so I used my Sigma



...and there you go. Sigma and AF performance go together like salt and mustard...like Fred Astaire and Ginger from Gilligan's Island...like Batman and The Green Arrow...like, well you get the idea. 

I have mostly keepers even with the 'slow AF' 85L II when tracking moving subjects.


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## DominoDude (Jan 28, 2017)

Sigmas are actually one of the very few lens makers where it could be beneficial to leave the lens cap on. I wish my camera had the menu option beneath "Release shutter without card" to be "Don't release shutter with attached Sigma lens".

Even in the best light and with plenty of contrast the focusing hit rate tend to be utterly low. Best chances are with stationary subjects at a predetermined distance and manual focusing. To me it's way too erratic to be worth using anything at all from Sigma.


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## Kit Lens Jockey (Jan 28, 2017)

Hmm, so perhaps this may be the fault of the Sigma lens? Sometimes it really does feel like the lens and camera just aren't even in sync with each other, especially in AI servo. I mean, setting it to focus priority, I would expect things to be almost all dead on focus, but I'll take a photo in AI servo, look at it, and I'm just like "What!? How did the camera think that was in focus??" I'm not talking about just a little back focused or front focused, I mean I can't even see a single part of the photo that's properly in focus.

Really too bad. When that lens is in focus, it's fantastic. But I have been curious as to how the Canon 50 1.2 would perform for a while. What turned me off was its second rate sharpness and higher price compared to the Sigma. but if it just plain works better with the camera, maybe I will have to entertain the idea of getting the Canon lens.


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## Larsskv (Jan 28, 2017)

Kit Lens Jockey said:


> Really too bad. When that lens is in focus, it's fantastic. But I have been curious as to how the Canon 50 1.2 would perform for a while. What turned me off was its second rate sharpness and higher price compared to the Sigma. but if it just plain works better with the camera, maybe I will have to entertain the idea of getting the Canon lens.



Sorry to point it out, but the 50L is known for problems in terms of focusing. Read Roger Cicalas description of this here: https://www.lensrentals.com/rent/canon-50mm-f1.2l

I've had the 50L and won't say it is bad at focusing, but it's the least good L lens in terms of focusing that I have used.


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## YuengLinger (Jan 28, 2017)

Now that the subject has been broached...Canon has a gaping hole in its prime lens lineup--50mm!!!

If I want to shoot at 50mm wide open, 24-70mm 2.8 II. Sigh...

If they could knock 35mm out of the park with 1.4L II, maybe, someday, possibly... :-\


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## [email protected] (Jan 28, 2017)

I think this is a bit unfair to Sigma.

Ai servo at night is tough, and requires you give it every advantage, such as helping guide the focus proximity with back button focus and good point selection. If you want it to be quicker and accurate, and shoot at night.

You also cannot tolerate a lens that isn't locked in with microfocus adjustment. Especially with a lens whose correction changes by calculated subject distance, like the Sigmas.

My 50 Art does about as well as my 35l II did when it was zeroed in. I shoot mostly at 1.4 to 2 and mostly in poor light at moving subjects. As difficult as this is, I don't have any illusions that it would be easier with a Canon lens. At 50mm their lineup is pretty poor value and performance.

If you do not wish to zero in a Sigma Art lens, that is understandable. It takes more time than I always estimate. It is a hassle. But if you are going to shoot low light movement at 50 in afservo (and you need it sharp), I think the only other option I can think of is the new Tamron 45 f1.8. 

When the light makes focusing impossible, I go manual focus, at which I'm terrible. If you were to do that, the Zeiss options are available and also leave the Canon 50s a bit red-faced, although not much more than the Sigma does, at a ludicrous price, and without any autofocus capacity.


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## Viggo (Jan 28, 2017)

YuengLinger said:


> Viggo said:
> 
> 
> > 95% of my shots are wide open with fast lenses and no dof, but Ai Servo is no less accurate than One Shot.
> ...



Haha, nope, and never ever again will I use any Art lens 8)


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