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

[ebiggs1]

Edward,

A couple thoughts.  I use just the center point for shooting birds  Turn all the other off.  It is common practice to put that point on the bird's eye.

As the aperture gets smaller (larger number) some cameras will slow down theie auto focus speed. I imagine this also effects the AF accuracy.  But it will not jump around if you use just the center point.

This photo below was shot with a 500mm lens that is even slower than your 100-400 is.  But it is center point only.

It was hand held, too.  Av set the lens to f8 fixed. SS was 1/2000 and ISO is 800.

[Edward1064]

Thanks for your comments. I notice that all of your lenses have a max aperture that is quite large. When the AF operates, it opens the lens to maximum aperture, which then could do a little better job of focusing, it seems to me.

Edward

[ebiggs1]

It might.  I don't know if I have seen tests where a, say a f1.2 opposed to a f4 for instance.  The actuall cut off for most cameras is f5.6.  So I am not sure how much a faster larger ap will speed it up.  it sounds like ti should though. Doesn't it?

I have had many, many lenses over the years.  Some are just slower to focus and they have the same f number.  Other factors figure in for sure.

[Edward1064]

Thank you for your experienced input. If you have not noticed any effect of f-number, then that says something. I WAS looking fir another reason to get a 400 mm or 500 mm f/4 though. 🙂

Edward

[jrhoffman75]

This is a good, but long, article. . http://www.lensrentals.com/blog/2010/07/how-autofocus-often-works

Wider apertures focus easier because the outer light rays are further apart.

The center focusing point in Canon cameras is the most responsive. In the top end cameras it is the one that will allow AF to f/8. It is also the one you want to focus with initially if you are using AI Servo.

The 60D may max out at f/5.6 at the center focus point.

[Edward1064]

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

[ebiggs1]

"I WAS looking fir another reason to get a 400 mm or 500 mm f/4 though. "

Oh, well, ah, yes, faster lenses focus light-years faster than slower lenses!  Go buy iy, today. 

But in actual use a couple things need to be qualified.  Nothing electronic hits a brick wall where it works and bam, it doesn't.

Almost always it is a curve.  So, I suspect as you do get closer to a better light transmission and contrast, the AF is going to lock quicker and be more accurate. Of course the oppisite would be true, also, as conditions worsen.

Even the fact you change your camera from horizontal to vertical can affect the AF speed and its accuracy.

Another problem with this is, lenses are different.  Even the same make and model form the same company.  The better, higher cost, lenses get better stuff, too.  Why would they not?  You expect better as you move to "L" level lenses!

However, if you are in good light, I don't think you are going to notice a f2.8 lens vs a f4 being much different.  If it is a f4 vs a f5.6, there might be a slight difference.  You are getting to that "curve" limiting thingy.  Conversely, I doubt you would notice a big difference between a f1.4 vs a f2.8 lens.

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?

[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

[Edward1064]

BTW, here is a recent, favorite image of a violet-green swallow here in the area (Cupertino, CA).  The camera and lens did a pretty good job here, one I am happy with.