Question about Canon DSLR bodies w/150-500mm or 150-600mm

Started Jan 23, 2014 | Discussions thread
AndyMulhearn Contributing Member • Posts: 594
Re: Question about Canon DSLR bodies w/150-500mm or 150-600mm

Flying Fish wrote:

A couple of things. First, none of the crop-sensor bodies from Canon (SL1, T5i, 60 and 70D, 7D) will autofocus well (or at all) if the lens's maximum aperture is less than f/5.6. F/6.3 is less, so you might have some AF problems when you're zoomed out to where the aperture goes to f/6.3.

As I understand it,  lenses like the Sigma 150-500 lie to the camera and tell it that the aperture doesn't go below f5.6, even at full zoom, so AF still works even though the max aperture is less than f5.6.

Reading between the lines this suggests that it's a limitation in the camera, not the amount of light that means AF doesn't work. i.e. the 7D can still focus at less than 5.6 it's just that Canon don't allow it to. Probably because they don't want people complaining on forums when they stick a TC on an f5.6 lens and AF is slow or inconsistent.

This doesn't apply to gear like some of the newer 1 series and the 5D mk III I think which will focus down to f8...

 AndyMulhearn's gear list:AndyMulhearn's gear list
Fujifilm X-E1 Canon EOS 7D Mark II Fujifilm X-Pro2 Canon EF 400mm f/5.6L USM Canon EF-S 17-85mm f/4-5.6 IS USM +7 more
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