I'll try sending this again:
> I have a Tamron 1.4 converter, but I've only used it with f2.8 or faster
> lenses. Even then, focus is a bit slower and more unpredictable.
> Although you lose 1 stop in the converter, your EOS camera will not
> "know" this. There is a chip inside the converter, but it doesn't seem
> to tell the camera body anything other than the f-stop of the lens
> itself. My guess would be that you can get away with this combination if
> there is lots of light (full sunlight) and something with good contrast
> in the subject. Zebras should be an excellent subject, distant, hazy
> mountains will not.
>
> The software in the camera (and most other EOS) will disable autofocus if
> the maximum aperture of the lens is smaller than 5.6. The Tamron
> converter allows you to cheat this, but there's a reason Canon chose 5.6
> as the minimum f-stop. It's not directly related to how much light
> reaches the autofocus sensors. If you get the chance, try manually
> focussing a 2.8 lens and then a 5.6 one of the same focal length. The
> focus snaps in much more positively with the faster lens, making it
> easier for you to focus. If it's easier for you, it's easier for a
> machine as well.
>
> If you're shooting in an automatic mode (Av or Tv) the camera's light
> meter will correct for the 1-stop loss. If you're in manual (M) mode,
> and using the built-in meter, you will automatically correct for it.
> Only if you use an external light meter, or just use "sunny 16", will you
> need to remember that the effective aperture of the camera is really one
> stop less than it indicates.
>
> Geoff Doane
>
> Charles Soper wrote:
>
> > Hello,
> > I have an A2E and I just bought a 100-300mm f5.6L lens with a Tamron
> > 1.4x SF AF tele-converter. With the tele-converter on, the AF seems
> > quite confused. Sometimes it finds the correct focus and other times,
> > the noisy non-USM lens just goes back and forth from minimum to
> > maximum focus.
> > Also, the largest aperture without the converter is 5.6. When I put
> > the converter on the largest aperture is still 5.6 not 8.0.
> > Does anyone one have advice or experience with these issues? I plan
> > to go to Kenya in late December/January and I hope to get some good
> > photos.
> > Charles
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