Camera AF and Lens Aperture

[Edward1064]

The post below prompts a question I have had for a while.  I use a 60D and Canon 100-400 mm zoom to photograph birds, and get some really good results.  While editing, I notice where the camera has focused, and the results (photo sharpness) appear to wander a little at times.  Usually I am shooting at 400 mm, where this lens has a maximum aperture of f/5.6.  I wonder if a faster lens, say one at f/4.0, would give better AF results due to its shallower DOF.

I'd appreciate other thoughts on this.

Thanks!

Edward

[Edward1064]

John, thanks for your input. I will look up the article.

[Edward1064]

John, ebiggs1,

This paragraph from John's reference is worth noting, andaddresses my original question.

The Effect of Lens Aperture
No matter what the sensor type, however, it will usually be more accurate with a wider aperture lens. Remember, during autofocus the camera automatically opens the lens to its widest aperture, only closing it down to the aperture you’ve chosen just before the shutter curtain opens. Phase detection autofocus is more accurate when the light beams are entering from a wider angle. In the schematic below beams from an f/2.8 lens (blue) would enter at a wider angle than those of an f/4 lens (red), which are still wider than an f/5.6 lens (yellow). By f/8, only the most accurate sensors (usually only the center point on the more expensive bodies) can function at all, but even then focus may be slow and inaccurate. This is the reason our f/5.6 lenses stop autofocusing when we try to add a teleconverter, which changes them to f/8 or f/11 lenses.

Edward

[ebiggs1]

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!