Canon 2x Extender III

[rjackjames]

I recently bought the Canon 2x Extender III for my canon 500mm lens, so I can do birding. When I added this to my 500mm and to attach it to 7D body. I lose Autofocus, is there anyway to regain auto-focus or i should return the product. What shall I do?

 

[tirediron]

Moving to Canon Lenses Forum

 

[tirediron]

Yes there is. Add more light. You're losing AF because the lens is now effectively a 1000mm f11 lens. There's simply not enough light getting through it under normal conditions for AF to work. I would suggest either focusing manually if you really need all 1000mm, or better yet, return it for a 1.4x TC.

 

[Light Guru]

Yes there is. Add more light. You're losing AF because the lens is now effectively a 1000mm f11 lens. There's simply not enough light getting through it under normal conditions for AF to work. I would suggest either focusing manually if you really need all 1000mm, or better yet, return it for a 1.4x TC.

Considering the crop faster of the 7D its more like a 1600.

 

[tirediron]

Yes there is. Add more light. You're losing AF because the lens is now effectively a 1000mm f11 lens. There's simply not enough light getting through it under normal conditions for AF to work. I would suggest either focusing manually if you really need all 1000mm, or better yet, return it for a 1.4x TC.

Considering the crop faster of the 7D its more like a 1600.

Almost... kinda, sorta... The focal length of the lens is the focal length of the lens. Period. The effective field of view of the lens when used on an APS-C body is equivalent to that of a ~1600mm lens.

 

[Overread]

The 500mm is an f4 lens by default. With a 2*TC added you lose two effective stops of light from the setup, taking you to a 1000mm f8 lens. Now on Canon camera bodies auto focus will not engage after the camera detects that the setup attached has a maximum aperture smaller than f5.6*. 1D line camera bodies (with the exception of the new 1Dx and I think also the 1DMIV) will AF up to f8 and then the auto focus cuts off.
With a Canon DSLR with live view you can retain auto focus up to f8 as well if you use live view itself (the auto focus here is a different set of sensors and setup to that used by the camera normally). Note that Live view af is slower and not generally as good as the cameras regular AF setup.

You can also consider taping the pins on the teleconverter, I forget which ones you put tape over, but the idea is that by taping over the metal pins on the lens mount the camera body becomes unable to detect the attached teleconverter. As such the AF sensors remain active and because only some of the pins at taped over the camera retains its normal control over the lens (ergo the AF motors still work).

It should be noted that the AF cutoff is generally done because after the cutoff the accuracy and speed of the AF drops significantly and the setup is more prone to hunting and losing focus. I would give it a try though as in good light you might well get serviceable AF performance on a high end lens like the 500mm. You'll have to google to find out which pins to tape, but the info should be pretty easy to find.


*a couple of 3rd party lenses which are f6.3 or similar at the long end trick the camera by not reporting the aperture correctly to the AF sensor section

 

[rjackjames]

Thank guys for the replies. I would tape the pins and see if that works if not I will return it. Since I have no use for it. I did lots of research before I bought it n now I am disappointed.

 

[mjhoward]

The 7D AF Module is only sensitive to f/5.6 apertures. Pro bodies will generally have at least one focus point that is sensitive to f/8, which is what you would need with that combination. If you get the 1.4x Extender, your AF will still work on the 7D because the effective aperture would be f/5.6. I tried taping the pins down (last 3 on the left when looking at them) for use with a 7D for use with the 800mm 5.6 with a 2x and it didn't work so I would be surprised if it works for you.

 

[TCampbell]

Most cameras really can't auto-focus beyond f/5.6... they may struggle a bit at 6.3, 7.1, and if you can get to 8 you'd be very lucky.

This isn't just the auto-focus system... grab a 35mm film camera with a split prism focusing spot, stop down the aperture blades to f/8 and try to use the split prism. You'll find that when you can see through half of the split prism the other half will be completely black.

You can try to focus manually. You might also be able to switch to live-view and let it do contrast detection AF (not ideal to leave the camera in live view all the time.)

2x focal reducers really call for f/2.8 or faster lenses.

 

[rjackjames]

I have tested the 2x with the 70-200mm, not bad, just slight autofocus issues. Also I tried the 1.4x with my prime 500mm, had slight focusing issues trying to capture, birds in flight. My next test is not to use the extender and see the difference in tracking.