What is this about autofocus not working at f8?

[Roger_Cavanagh]

Chesty,

I think the discussions you have read will concern the use of 1.4x and 2x teleconverters on long lenses. The TCs will reduce the effective maximum aperture of a lens by 1 and 2 stops respectively. So a 400/5.6 might become a 560/8. By design, the 10D (and D30/60 and no doubt other Canon cameras) will not attempt to use AF in this circumstance because AF performance is not very good.

It does not mean that using the 400mm lens at f/8 will not work.

Regards,

[Ken Dunham]

Roger is correct. The D30/60 and 10D will AF only to f/5.6. The 1D and 1Ds will AF to f/8. A lens will still work above these aperatures, but will need to be manually focused.

Some suggest taping over pins on the teleconverter to "trick" the camera AF into thinking there is no TC. The tape (obviously) doesn't change the fact that less light is entering the camera, so results tend to be mixed. Cameras with "better" AF (eg 1Ds) will likely focus better with a taped TC than one with a relatively poorer AF (eg D30).

[chesty]

Okay, I just tried my 10D with the Canon 2Xextender which I am assuming is what people mean when they are talking TC.  Anyway, with that on, both my 300f2.8 IS and my 70-200 non-IS worked at every f stop including 8 and rather quickly as well.

So now I am confused.  The only lense I had that didn't work was my 100-400 IS and the autofocus didn't work at all.

So what gives?  Cause I don't have a problem at all.

[alanrew]

The important point is that autofocus always happens at the widest (or max) aperture value, and this aperture value is the limiting factor.

This is how I've always assumed that autofocus works on SLRs generally.

So a pertinent question is: do any Canon lenses have a max aperture of f/8 or smaller (without a TC being attached)? I've looked at the Canon web site and I can't find anything smaller than f/5.6.

[francois]

an f4 and then used the converter either a 1.4X or a 2X then it would not auto focus becuase the "new" fstop would be 6 or or 8

f/4 with 1.4 gives you an f/5.6 and this will be OK with a D30, D60, 10D. With a 2x you need to have a 1Ds, 1D, 1V or 3.

Autofocusing an f/8 lens is slower than an f/2.8 lens ...
I've read the official reason is that Canon wants "fast" autofocus.

My experience is that autofocusing my 100-400 with 1.4x  (it's an f/8) and a EOS-1xx takes very long compared to the same lens without the extender (and it only works with the central focus sensor).

You may want to try the "tape" hack to see that phenomenon.

francois

[chesty]

I agree.  I don't really see it as a fault as some have put it asking Canon to fix it.

You got me on my math 4 with 1.4 is 5.6,  

[chesty]

I have read on a couple of sites such as outbackphoto that the 10D as well as the D60 autofocus doesn't work at f8.  I have tried all my Canon lenses at f8 (set in AV) and they all work.

What are they talking about?

[chesty]

That makes sense.   I remember that my 100-400 would not autofocus at all with the 2Xextender.  I'll have to try that with my other lenses.  

Thanks.

[chesty]

So, basically without the tc the lens/autofocus work normally.  Put the tc on and it would not auto focus at f8 whether chosen automatically or manually correct?

[francois]

Chesty,
Your 70-200f/2.8 and your 300f/2.8 both become f/5.6 with a 2x teleconverter and your 10D can autofocus at f/5.6 max. On the other side, your 100-400 f/4.5-f/5.6 becomes an f/11 with a 2x teleconverter (or an f/8 with a 1.4x). This is why it cannot autofocus.
The important point is that autofocus always happens at the widest (or max) aperture value, and this aperture value is the limiting factor.

francois

[francois]

I've looked at the Canon web site and I can't find anything smaller than f/5.6.

I'm not aware of EF mount lens slower than f/5.6. But you can find Sigma (like 50-500mm f/4-6.3 EX APO RF HSM) and other third party lenses at f/6.3 and above.

What's important in the ability to autofocus is the telemetric aperture (ie: lens AND teleconverter).

francois

[chesty]

Okay, so basically, if I were to use something that say had an f4 and then used the converter either a 1.4X or a 2X then it would not auto focus becuase the "new" fstop would be 6 or or 8.  

Question then is, why has Canon done this?  It seems like it would be done due to a hardware/software limitation.

[Jonathan Wienke]

Because autofocus performance drops off at lower light levels. If the max aperture of your lens (including teleconverter) is smaller than f/5.6, autofocus performance becomes marginal to the point that Canon's engineers decided to simply disable it. On the high-end cameras with the 45-point AF system, (1Ds, etc) they put the cutoff at f/8.