AF with teleconverters?

[Mongoose]

Ok, I got the magnification thing figured out, I think, and with a converter I should be able to beat my Point-and-Shoot. But then I started reading reviews and realized that AF will not work with every combination of camera / lens / teleconverter.

So my question is: is there a handy reference somewhere where you can find out at a glance if a particular set up will AF, or do I have to comb painstakingly through hundreds of reviews to figure it out?

Thanks,

-
Elise

[Mongoose]

Never mind, I found your article on teleconverters. Again, very useful info, thank you so much.

-
Elise

[Bob Atkins]

Jut for anyone else reading this, Canon's crop sensor DSLRs and the EOS 5D series will AF when the maximum aperture of the lens in use is f5.6 or faster.

You lose one stop with a 1.4x TC and 2 stops with a 2x TC which means that you can AF with an f4 lens and a 1.4x (f5.6) or with an f2.8 lens and a 2x (also f5.6).

The 1D series DSLRs will AF with a lens of f8 or faster (but only with the center AF sensor). That means you can get AF with a 1.4x and an f5.6 lens (final aperture f8) or with an f4 lens and a 2x TC (also f8).

There are some tricks to "fool" the camer by masking pins and some 3rd party TCs don't change the aperture which the lens reports to the camera, so they will at least try to AF at smaller apertures, but quite often AF under such conditions will not be reliable.

[bmpress]

I guess this implies that you can not focus with a polarizer on an f4.0 lens?

[KeithB]

No, this has nothing to do with *exposure*, this has to do with *depth of field*.  If the depth of field gets too wide, the AF system has trouble focusing correctly, probably because the desired test point becomes to mushy for the control loop.  Assuming the polarizer does not drop the exposure below the EV threshold, you should be OK.

[bmpress]

Excellent point...thanks.