Ken Durling wrote:
>
> But what about a 2.8 lens not AF-ing well when stopped down to f/8 or
> slower? That's what I don't get. Isn't that what people have been
> saying? Maybe I misunderstood. If so, your above explanation makes
> sense to me.
>
> Ken
Ken, when you add a TC, you not only multiply the focal length by the
factor of the TC, you also multiply the maximum aperture. So if you add
a 1.4 TC to a 100-300 f5.6, you now have a 140-420 f8 lens. F8 is
insufficient to get autofocus at maximum aperture. To auto focus, the
lens needs f5.6. A 2.8 would be ok, since it, multiplied by 1.4 would
be roughly f4. Even with a 2x TC it would be ok, at f5.6. This is only
a problem with older EOS bodies, and possibly lower on the pecking order
new ones (not sure about that) since the EOS 3 and 1v AF down to f8. Or
possibly f11 on the 1v, I'm not sure about that either. I'm just a
lowly A2 owner. <G>
I think you misunderstood. the question was about why the lens was
hunting for focus with a 1.4 TC when the lens was reporting the aperture
as 5.6, even though the true opening was f8. (Remember, we multiplied
the aperture by 1.4?)
Skip
--
Shadowcatcher Imagery
http://www.shadowcatcherimagery.com
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