AF on a Canon 7D at f8

Chr1stof

Chr1stof

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I was just doing some reading on the internet and came across a few discussions about the 7d not having AF at f8.
I did a search here on TP but all the search terms were too short so I couldnt find anything.
Could someone explain what this is all about? I cant belive/understand AF would not work for some reason.
Thanks.
 
Think it should read when the lens is used with a Canon Tele Converter, the limit on non series 1 bodies is f/5.6
 
Ahh, didnt know that. I also noticed a 400mmL /1.4TC for sale on here, the ad reads that AF doesnt work except on series 1 bodies in that combo. Is that at any F stop or would it work at only f5.6 on say a 7d or 550d?
 
The AF on the 7D needs a maximum aperture of f/5.6 in order to function. so works fine with the 400mm f/5.6. When you add a 1.4x converter you lose 1 stop of light, so the effective maximum aperture of this lens becomes f/8 and therefore AF will not work. This applies to all Canon DSLR bodies with the exception of the 1 series cameras, where AF will function with a maximum lens aperture of f/8...but I think only with the central AF point.
 
Except on the 1-series bodies, Canon switches out the AF at higher than f/5.6 becuase it doesn't work well. Phase-detect AF works by comparing images from both sides of the lens and when the diameter of the aperture gets physically too small it can't differentiate properly. No Canon lens has a lowest f/number of higher than f/5.6.

You can fool the system and make it try to AF by taping the pins on an extender/telecon, but that doesn't make the aperture any bigger as seen by the AF sensors. Some third party lenses that run to f/6.3 at the long end work by doing this, and you can usually get a way with that albeit not optimally.

For some reason, some cameras are better with the pins-taping trick than others. I had a 40D which wouldn't play at all, whereas my old 350D would, even if it focused very slowly. This was with a 100-400L and 1.4x telecon, effectively at f/8. Others have had the same experience. I've not tried a 7D with this but word has it that it also seems to be one of the Canons that doesn't respond well.

Contrast-detect AF on live view works at any aperture, but needs more light than phase-detect.
 
Just for info, I have a 550d. I saw an article somewhere else and thought it was specific to the 7d.
 
Chr1stof said:
Ahh, didnt know that. I also noticed a 400mmL /1.4TC for sale on here, the ad reads that AF doesnt work except on series 1 bodies in that combo. Is that at any F stop or would it work at only f5.6 on say a 7d or 550d?
Click to expand...

There's a couple of bits of information that you may find useful to understand this.

1. AF is always done with a lens wide open.

2. The limitation on the f-stop required for AF to work is dependant on the maximum aperture of the lens, not the aperture used for the exposure.

So, if you have an f5.6 lens then AF will work, even if you shoot at f22. If you stick a 1.4x TC on this lens then the aperture changes to f8 and AF will no longer work. If you tape the pins on the TC then the aperture is still f8, but the camera thinks it is f5.6 - so it will try to AF. Most of the time it will fail. Some people call this 'working'.
 
As ever, thanks to all who responded to another one of my noob questions :)
On my old Olympus 35mm (which is obviously manual focus) the image in the viewfinder was split making MF easier. I can imagine MF on a 400mm lens is quite tricky, even with live view.

Cheers!
 
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