Canon extenders. Why no AF.

Most non-1-Series Canon camera bodies need a minimum wide open aperture setting of f/5.6 for autofocus to work, only Canon 1-Series camera bodies will autofocus at f/8.

So to autofocus your lens on a non-1-Series camera the lens has to be at least f5.6 now when using an extender 1 or 2 two stops is added to the lens so an f4 lens would become an f5.6 lens which will work, but if your lens is an f5.6 to start with like the EF 400mm f/5.6 L USM Lens and you add the 1.4 extender which adds another stop to the lens it will be a 400 mm f8 lens and a non 1 series camera can't autofocus.
 
Top answer, concise and informative :clap:
 
I've never really understood this either, does this mean that Canon (or all?) bodies do not AF at any f-stop over f/5.6? even without an extender? So shooting landscapes, do you lose AF at f/11? (I'm aware that DOF is massive at f/11 and landscape width anyway, but...)
 
The body will focus with the lens wide open and it only stops down to the required aperture when you press the shutter.

Bob
 
The Canon ones will only work with certain L lenses from 135mm and above, not sure about the Sigma. If you want something that will work with all lenses I would suggest the Kenko 1.4x Pro it has good reviews and I believe that it will fit most lenses but is better used just with telephotos or telephoto zooms.
 
EdBray said:
The Canon ones will only work with certain L lenses from 135mm and above,
Click to expand...

Just to get my own back :lol:

WRONG

What you meant to say was they only officially work with lens over 135mm, as shown in an article in Octobers EOS magazine they also work with the 24mm tilt/shift lens

extend.jpg
 
EdBray said:
And wrong!

A 400mm f5.6 with a 1,4 ext will actually be a 560mm f8.
Click to expand...

whiteflyer said:
Just to get my own back :lol:

WRONG

What you meant to say was they only officially work with lens over 135mm, as shown in an article in Octobers EOS magazine they also work with the 24mm tilt/shift lens

extend.jpg
Click to expand...


:popcorn: 1 - 1 :popcorn:
 
Well, I'm just gonna chuck in the obvious continuation question for this thread, since nobody else has.
Non 1 series body's won't attempt af with a max app smaller than f/5.6............why ?????
I know its come up before, but nobody really cobbled together a credible sounding answer for us non Canon shooters, I mean they're not daft, there's got to be some logical/mechanical/electronical/designical reasoning behind it.

*ignores spell checker red line under designical*
 
Its got to be a quality control control - ie the results tail off so bad after that Canon don't want to put their name to the output after that.
 
You can force Canon lenses to focus @ small than f5.6 with extenders - you tape some pins and then the extender doesn't report it's existence & the lens autofocuses albeit with a poorer AF performance than 'naked'.

Also some Sigma zooms are f6.3 at the long end wide open & I'm pretty sure they all AF, although I've not tried them.

Phil
 
Yes those nasty Sigmas do it by lying to the body apparently... well, on a Canon mount anyway!
 
philthejuggler said:
You can force Canon lenses to focus @ small than f5.6 with extenders - you tape some pins and then the extender doesn't report it's existence & the lens autofocuses albeit with a poorer AF performance than 'naked'.
Click to expand...

This is a popular misunderstanding of how AF works. It doesn't depend so much on how bright the light is, but much more on the physical diameter of the aperture.

You can tape the pins and fool the camera into thinking the extender isn't there, but the aperture has been (optically) reduced in size so the AF still doesn't work. It will try, but it's hopeless.

Folks that buy one of the new super-zooms that go to f/6.3 at the long end will be disappointed to discover the camera hunts and shunts around in a futile quest to do something it physically cannot. Sometimes it will work, but it's really not reliable. Or fast. Check the lens tests on DPReview. They cite it as a major drawback of these lenses.

I have a Kenko Pro 1.4x extender and Canon 100-400mm L, which is 560mm f/8 at the long end. Even with the pins taped, it will not AF at all, even though my 40D body tries very hard!

Richard.
 
Brocks said:
So the next question is, Does a canon extender only work with a canon lens, or can I use a sigma converter with a canon lens on visa versa?
Click to expand...

I have just returned a sigma 2*, as it gave a yellow cast, so if you can check it instore, that would be a good idea
 
HoppyUK said:
This is a popular misunderstanding of how AF works. It doesn't depend so much on how bright the light is, but much more on the physical diameter of the aperture.

You can tape the pins and fool the camera into thinking the extender isn't there, but the aperture has been (optically) reduced in size so the AF still doesn't work. It will try, but it's hopeless.

Folks that buy one of the new super-zooms that go to f/6.3 at the long end will be disappointed to discover the camera hunts and shunts around in a futile quest to do something it physically cannot. Sometimes it will work, but it's really not reliable. Or fast. Check the lens tests on DPReview. They cite it as a major drawback of these lenses.

I have a Kenko Pro 1.4x extender and Canon 100-400mm L, which is 560mm f/8 at the long end. Even with the pins taped, it will not AF at all, even though my 40D body tries very hard!

Richard.
Click to expand...

I have to say I haven't tried it but there is no misunderstanding. In strong light it is reported to AF at under 5.6 with any body, just not very well.
 
philthejuggler said:
I have to say I haven't tried it but there is no misunderstanding. In strong light it is reported to AF at under 5.6 with any body, just not very well.
Click to expand...

Phil, I have tried it, and at f/8 even with taped pins, it will not. It tries, but the focus vacillates wildly from max to min and back again. It sometimes stops at what it presumes is correct focus, but it's miles out. And I mean hopelessly out. Even in bright light, with high contrast subjects, my 40D will not AF at f/8, end of story. And given the reason stated above, that is entirely to be expected.

Note also that Canon does not make any lenses at all with a maximum f/number darker than f/5.6. Not one. And neither does Nikon. In fact, I don't think any camera manufacturer produces a lens darker than f/5.6 at any focal length. It is only third party brands like Sigma and Tamron that do it, because they can blame the camera makers when their lenses don't focus ;) :lol:

Richard.
 
HoppyUK said:
Phil, I have tried it, and at f/8 even with taped pins, it will not. It tries, but the focus vacillates wildly from max to min and back again. It sometimes stops at what it presumes is correct focus, but it's miles out. And I mean hopelessly out. Even in bright light, with high contrast subjects, my 40D will not AF at f/8, end of story. And given the reason stated above, that is entirely to be expected.

Note also that Canon does not make any lenses at all with a maximum f/number darker than f/5.6. Not one. And neither does Nikon. In fact, I don't think any camera manufacturer produces a lens darker than f/5.6 at any focal length. It is only third party brands like Sigma and Tamron that do it, because they can blame the camera makers when their lenses don't focus ;) :lol:

Richard.
Click to expand...


Hmm - I did a quick search on POTN, my usual haunt, and there seems opposite opinions. One user categorically claims he's successfully had his 10D focussing with 100-400L + 1.4x (pins taped) while others point out it is aperture size that matters & putting things like ND filters & CPL's on a lens doesn't stop AF, but a 'naked' f8 lens does.

I guess I have to say I haven't tried it and don't intend to!
 
HoppyUK said:
Phil, I have tried it, and at f/8 even with taped pins, it will not. It tries, but the focus vacillates wildly from max to min and back again. It sometimes stops at what it presumes is correct focus, but it's miles out.
Click to expand...


Its attempting to af, "hunting", whether it gets a lock or not doesn't matter.
The point is that without tape a Canon lens max app f/5.6 on any non 1 serise body, won't even attempt to af.

Do we know if even when taped, it will af and get the occasional lock, or is the actual focus recognition system/whatever, disabled and it just goes through the mechanical motions of af.
 
Hi, I've just had a play and have found a canon lens-convertor combination that does auto-focus (just slower than normal), with a 40D/50D body!



Its my "old faithfull" 35-350mm with the 1.4X II convertor. Maximum aperture is 5.6@350mm, lens only, so stick the 1.4X on and it shouldn't autofocus - but mine does! Both my cameras do not recognise the convertor as being there in this configuration, pic info says 350mm @ f5.6 whether convertor is used or not! Note that shutter speed DOES change to reflect the light loss with the convertor.
 
Just a bit of further info on how the AF works......

Using lenses of f/2.8 or faster and the body will command the lens to focus to a point within 1/3 of the depth of field "centre point". If the lens is slower than f/2.8 then the body will simply aim for "within DOF".
The body does not check that the lens has achieved the target although the lens firmware does do a check to see if it has stopped in the commanded position.

The AF confirmation will light if the phase detection value, measured by the body, is within the set tolerance.

Now the 10D v 40D equation.....
It's no news that the 40D AF is better than the 10D.....we all know this and Canon agrees. One of the reasons is simply down to better software and along with this comes the ability to tighten the tolerances. The downside is that the 10D may well confirm AF, with its wider tolerance, in situations where the 40D didn't.

Bob
 
To clarify what happens with my Canon 40D, Kenko Pro 1.4x extender, and 100-400mm f/4.5-5.6 L lens. With the extender attached, that becomes 140-560mm f/6.3-8.

When used as intended, the camera will not even attempt to AF. It is effectively turned off as the combination is past the f/5.6 cut off.

If you tape over the 3 left-hand pins on the extender so that the camera doesn't know it's there, it will always try to AF, and the focus confirmation light will light if/when focus is achieved.

When the lens is set at 100mm (140mm), effective aperture f/6.3, the camera will achieve AF reasonably reliably with a 'good' target. In average use shooting a variety of subjects, the hit rate is about 1-in-3. If the combination worked like this all the way to 400mm (560mm) I would consider the 'taped pins' trick to be worthwhile; certainly not perfect, but with the option of manual override also, worth a try.

But when used at the long end of the zoom, and bearing in mind that I bought the extender exclusively for use at this focal length, things get dramatically worse. The lens racks back and forth like crazy, then stops miles out of focus. Try again, same result. And again, and again. Meanwhile, the IS system is oscillating like mad. Sometimes the lens will stop at accurate focus, but rarely. I would say about 1-in-ten attempts and that is with a 'good' contrasty target.

I categorise this level of performance as NOT autofocusing. It tries, but fails. Useless.
 
You must log in or register to reply here.
Back
Top