# 6D & 24-105 f/4; AF point works as well with f/4 lens as f/2.8?



## ScottyP (Dec 19, 2012)

Is there any loss of, well, anything, in the AF function of a 6D when using a lens with max aperture of f/4, vs. its performance with f/2.8 or wider/faster lens?

I have seen here and there that certain AF points (center usually) do things at f/2.8 or better that the point can't do with wider lenses?

Relevant at all with 6D and 24-105 f/4???

Thanks!


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## Botts (Dec 19, 2012)

There is, at least in my 6D.

With the 70-200mm f/4 IS, in a dark room about 1/8" at f/4 and 12800, my 6D couldn't lock focus. Simply switching to the 40mm f/2.8 it was able to lock focus, and accurately.

**This was using single point AF set to the center**


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## verysimplejason (Dec 19, 2012)

Botts said:


> There is, at least in my 6D.
> 
> With the 70-200mm f/4 IS, in a dark room about 1/8" at f/4 and 12800, my 6D couldn't lock focus. Simply switching to the 40mm f/2.8 it was able to lock focus, and accurately.
> 
> **This was using single point AF set to the center**



I think it's the lens but not necessarily the aperture. It could be the focal length. Can you try with another F4 lens and compare? Thanks.


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## Badger (Dec 19, 2012)

Botts said:


> There is, at least in my 6D.
> 
> With the 70-200mm f/4 IS, in a dark room about 1/8" at f/4 and 12800, my 6D couldn't lock focus. Simply switching to the 40mm f/2.8 it was able to lock focus, and accurately.
> 
> **This was using single point AF set to the center**


Mild curiosity as to if these would be the same results with a 5DIII. I have the 6D and also have the 70-200 f/4 but haven't tried using it in low lighting situations. I have used the 24-105 in dim lighting and have been happy with it, but I was't comparing it to anything or experimenting, I was just using it.


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## Dylan777 (Dec 19, 2012)

ScottyP said:


> Is there any loss of, well, anything, in the AF function of a 6D when using a lens with max aperture of f/4, vs. its performance with f/2.8 or wider/faster lens?
> 
> I have seen here and there that certain AF points (center usually) do things at f/2.8 or better that the point can't do with wider lenses?
> 
> ...



1 stop bigger = x2 amount of light. Besides, 6D has one single x-cross in the center. You now add a f4 lens, I understand 6D has FF sensor, but don't expect to get cleanerr images under low light if your shutter above 1/60.


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## neuroanatomist (Dec 19, 2012)

When you use an f/2.8 lens on a camera that has a high-precision f/2.8-sensitive AF point, you get the benefit of that high-precision AF point that you don't get with a lens slower than f/2.8 ). This is specified by Canon as 'accurate to within one depth of focus' for normal precision and 'accurate to within 1/3 the depth of focus' for high precision. AF systems aren't perfect - any given shot will be within some range of 'perfect' - using a lens that enables the high-precision point generally means that more shots, on average, will be closer to 'perfect'.


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## Nick Gombinsky (Dec 19, 2012)

Botts said:


> There is, at least in my 6D.
> 
> With the 70-200mm f/4 IS, in a dark room about 1/8" at f/4 and 12800, my 6D couldn't lock focus. Simply switching to the 40mm f/2.8 it was able to lock focus, and accurately.
> 
> **This was using single point AF set to the center**



The AF module is separate from the sensor, and works outside the shutter (its always exposed). Ergo, ISO and shutter speed have no influence on AF speed. The AF module depends on how much light is available, and how much light your lens can transmit wide open.

Anyways, the settings you provided does give us an idea of how dark the room was, which is VERY dark.


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## ScottyP (Dec 19, 2012)

neuroanatomist said:


> When you use an f/2.8 lens on a camera that has a high-precision f/2.8-sensitive AF point, you get the benefit of that high-precision AF point that you don't get with a lens slower than f/2.8 ). This is specified by Canon as 'accurate to within one depth of focus' for normal precision and 'accurate to within 1/3 the depth of focus' for high precision. AF systems aren't perfect - any given shot will be within some range of 'perfect' - using a lens that enables the high-precision point generally means that more shots, on average, will be closer to 'perfect'.



Yes. Thanks. That was exactly what I was referring to.

So... even if I was shooting that f/2.8 lens at a narrower aperture (f/4 or f/16) the "precision AF" would still work to improve shots on that lens? Or does the precision AF only kick in when you actually dial in an aperture f/2.8 or wider?


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## neuroanatomist (Dec 19, 2012)

ScottyP said:


> So... even if I was shooting that f/2.8 lens at a narrower aperture (f/4 or f/16) the "precision AF" would still work to improve shots on that lens? Or does the precision AF only kick in when you actually dial in an aperture f/2.8 or wider?



The aperture you select for a shot is irrelevant. Phase detect AF (i.e. viewfinder AF) is always performed with the lens at max aperture, then the lens is stopped down to your selected aperture right when the shot is taken.


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## MintMark (Dec 19, 2012)

I understand that the lens always focuses with the aperture wide open, so when you use a f/2.8 lens and shoot a picture at f/5.6 the aperture is open at f/2.8 for focusing and then closes to f/5.6 when the picture is taken.

The 6d centre point is cross type at f/5.6 and vertical line sensitive at f/2.8.

I think in the dark room there are several scenarios...

You can shoot with a f/5.6 lens and AF can lock on to features using the cross type centre point.
You can shoot with a f/4.0 lens and it uses the same centre point, but more light means it can lock onto less distinct features.
You can shoot with a f/2.8 lens and the centre point also has higher precision if there are vertical line features... you'll get more accurate focusing.
You can shoot with a f/2.0 lens and there is even more light, so it can lock onto even less distinct features.

I think there are two factors at work. The maximum aperture of the lens determines how much light is available to the AF system and more light helps it to lock on to harder targets. It can also allow some AF systems to achieve higher precision, as neuro explained.


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