Question about narrowest aperture for auto focus... Please help!

Started Apr 11, 2010 | Discussions thread
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subhrashis
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Question about narrowest aperture for auto focus... Please help!
Apr 11, 2010

This is something which I can't understand....

It is said, that at least an aperture of f/5.6 is needed for autofocus to function. this is why nikon doesn't allow it's teleconverters to mount on slower lenses like the 70-300 ed vr.

Now, I use my D5000 with the Sigma 18-200 DC OS HSM f/3.5-f/6.3. So here, at the tele end, autofocus shouldn't function as aperture is being stopped down beyond 5.6 to 6.3. Yet it works, and even on the peripheral focus points...

So, my question is what is happening here? Is the AF working at it's limit? or is the lower aperture value that's relevant? If so, does that mean i can use a 1.4x tc with my lens?

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