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Honored Contributor
Posts: 5,493
Registered: ‎06-25-2014

Re: Camera AF and Lens Aperture


@ebiggs1 wrote:

 

As I said, I have had many, many lenses and access to dozens more at work and I don't ever recall the aperture being the reason a lens AF was slower. (Keeping you are not doing a f2 vs f8 of course.) There are just way too many other factors to calculate.  I can give you another example.  I have the EF 50mm f1.2L II and it is the slowest AF lens I have used in a very long time. So there you are?


The salient point about that last observation is that an f/1.2 lens, because of its minimal depth of field, requires more accurate autofocus than other lenses do. There's undoubtedly a tradeoff between accuracy and speed; and if you're using, say, an f/4 lens, you might consider speed as important as accuracy. But if you don't get accurate focus at f/1.2, what's the point of taking the picture?

 

Of course the camera/lens collaboration knows what aperture is going to be used to take the picture. So if you wanted to drive up the cost of an f/1.2 lens, and possibly of the body that uses it, you might incorporate a feature that allows a compromise on accuracy vs speed if the picture is going to use a narrow aperture.

Bob
Philadelphia, Pennsylvania USA
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Posts: 12,750
Registered: ‎12-07-2012

Re: Camera AF and Lens Aperture

Edward,

Roger Cicala is one of my very favorite of all time article writters and reviewers. I am sure he is correct in his findings.  I will have to re-read that article sometime but you were asking about f5.6 to f4.

If aperture is the difining factor, I don't think the faster ap is going to make any significant difference.  No difference that you can see or even measure.

 

Going from the EF100-400mm zoom to a EF 500mm f4 is quite a different story.  It is the lens itself that is going to smash the AF speed and AF acciracy of the zoom.  Not the greater aperture (from f5.6 to f4).

 

We keep mentioning f2.8 because it does have an impact on the AF accuracy at least if not the speed.  My examples are for small aperture increases, right?

The lens itself is going to be the limiting factor.  Again a better lens is going to focus faster and more accurately.  A better camera will focus faster and more accurately, too.

 

Or, perhaps I am not understanding what you are asking?  Remember paper and lab testing are one thing and real world use is another.  Is something faster in a lab setting and is it noticable outside in my hands?  Maybe yes and maybe no!

EOS 1DX and 1D Mk IV and several lenses!
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Posts: 12,750
Registered: ‎12-07-2012

Re: Camera AF and Lens Aperture

"The salient point about that last observation is that an f/1.2 lens, because of its minimal depth of field, requires more accurate autofocus than other lenses do."

 

So right, Bob from Boston,

In this case the very wide ap of f1.2 did not help.  

And it is a very expensive lens. BTW, its big brother, the EF 85mm f1.2 L, is equally slow to AF even with its f1.2 max ap.

 

I will still go with the lens itself makes more difference that the larger ap does.  As well as the camera it is bolted on.

EOS 1DX and 1D Mk IV and several lenses!
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