1.4x or 2x teleconverters and autofocus

When the specs for a teleconverter says that full autofocus function is possible

with f/__ or faster/brighter lens, what happens with lenses that are slower? Is

AF completely disabled as if the MF switch is thrown, or does it hunt forever?

Or can it function depending on the ambient lighting condition, say outdoors on

a bright day?

 

Thank you in advance for any help.

The professional 1D & 1DS cameras can autofocus with F8 and faster lenses. Though focusing may be limisted to center point at F8.

 

The consumer cameras such as 5D, and XXD (20D, 30D, 40D, xti, rebel series) all only autofocus up to F5.6

 

If you have a F4 lens with a 1.4X you will be fine with any camera for autofocus. The F4 lens with a 2X will only autofocus on the pro cameras.

 

If you have an F5.6 lens with a 1.4X you will only be able to autofocus on the pro cameras. An F5.6 lens with the 2X will not autofocus on any camera.

 

Your focus will slow down with any telectoverter. Even if you camera will autofocus with the 1.4X or 2X, the focus will slow down.

i have found that the 1.4x paired with a prime lens works the best, AF speed is barely affected and image quality is still good if not as good. I have used sigma, kenko and the canon 1.4x extender on the 300mm F4 IS L and 200mm F2.8L and both lenses perform very well. Surprisingly the 300mm + 1.4x though stopped down to 5.6 still performed quite well.

 

On Zoom lenses like the 70-200 2.8L IS i personally have noticed a slower AF speed with the 1.4x

 

I used the Canon 2x on my 300mm and i lost autofocus (completely disabled) because the f4 becomes a f8

Thank you both for the replies!

A Canon extender has additional contacts that signal its presence to the camera. When these are not taped over, the camera will refuse to AF if the effective aperture is too narrow for the camera body (f/8 for 1 series, f/5.6 for the rest), and otherwise it will slow down focus operation. With taped contacts or a third party converter the camera will attempt AF, but it may hunt or fail depending on subject contrast, effective aperture and lighting conditions (in decreasing order of importance).

using 300 2.8IS with 2x, the AF speed is very slow. F5.6 should have enough light for the camera to focus. I dont' know why it's that slow.

Anson, the slowing allows the AF to 'settle' at the correct position. It might be possible to make an AF drive for a 600/5.6 lens that worked quite fast, but it seems to be the case that an AF drive designed for optimal performance on a 300/2.8 needs considerable 'calming' to work properly on an Extender 2x. I just did some experiments with my 135/2L and Extender 1.4x and Extender 2x. With either extender on its own, on a 40D body, accurate focus was obtained unhesitatingly (as checked using Live View) although I confirmed my previous more casual impression that the 135/2L does not combine well with the Extender 2x in terms of IQ. With both Extenders stacked, the camera thinks the lens/extender combination is a 270/4 but in fact it is a 378/5.6. So not only does the camera think AF will work, it actually should because the combination is still within the AF design parameters at f/5.6. However, AF hunts a bit and then settles to a position that can be improved on by focusing manually using Live View. The obvious conclusion is that the AF would need slowing still more to work well.