problom with my new Extender EF 1.4x II

hi all
i have purchased new canon 100-400 with canon Extender EF 1.4x II
and i have the canon 350d / Digital Rebel XT

but the auto fucus does not work

can anybody help me please

I believe that your autofocus requires a f5.6 or faster to work properly.

Your 5.6 lens & 1.4 converter combined is f8

Right - the aperture of the 100-400 + 1.4x TC is f/6.3-f/8; only the 1D series will maintain AF at those apertures.

From the Canon website:

This tele extender can be used with fixed focal length lenses 135mm and longer (except the 135mm f/2.8 Softfocus lens), and the EF 70-200 f/2.8L, 70-200 f/2.8L IS, 70-200 f/4.0L, and 100-400 f/4.5-5.6L IS zoom lenses. Superb optically, it preserves the image quality of the lens it's mounted to and multiplies its focal length 1.4x. Effective aperture is reduced by one f-stop; autofocus is possible on any EOS camera when combined with a lens having an f/4 or faster maximum aperture. The new version II maintains the outstanding optics of the previous version, and adds enhanced weather-resistant construction, and improved anti-reflective surfaces in the barrel.

The 100-400 f/4.5-5.6 doesn't appear to meet the necessary criteria for maintaining autofocus capabilities.

I've always been told that AF capabilities using the TCs are totally lens dependent and have nothing to do with the camera bodies. I use the 2x II on my 300D with the 70-200 2.8 IS (2 stop reduction, effective max. aperature of 5.6) without any problems. Does it the body really make a difference?

The 3x0D and x0D/Dx0 series have an f/5.6 max AF aperture capability. This is documented in, for instance the 20D manual on p. 64; the 350D manual p. 70.

Do take a look at the Teleconverter sticky FAQ thread in the lens forum.

Thanks guys. I learn something new everyday on this board! Does the 5D require f/5.6 also?

From the FAQ:

Requirements for Auto Focus
Canon makes SLRs with TWO distinct Aperture requirements for Autofocus. (AF)

A: The 1 series can AF with apertures as small as f/8
Models include; EOS 1D, 1Ds, 1DMkII, and film cameras EOS 1 and EOS 3

B: All other EOS bodies require f/5.6 to AF
Models include D30, D60, 10D, 300D, and 20D
Film bodies = all of them except for the 1 and 3

Afraid so, Canon saw fit to limit AF to 5.6 on all bodies bar the 1 series pro bodies to prevent inacurate AF. Your 70-200 2.8 works with the 2x convertor as it doesn't exceed 5.6. The 100-400 is already at 5.6 so even with the 1.4 extender it is prevented from AF by the limiter in the 300d's electronics. I have the 100-400 and 1.4 extender also and found a work round on this forum which involves covering 3 of the pins on the lens that send data to the camera. In effect covering those pins stops the lens sending the max aperture data and allows AF. In practice it isn't very effective as the AF is very slow and hunts like crazy making manual focus a quicker option. Which is probably the reason Canon limited it to 5.6 in the first place. Hope this clears it up for you.

[QUOTE=BTBeilke]Thanks guys. I learn something new everyday on this board! Does the 5D require f/5.6 also?

YES

To test the min aperture any body will stop focussing at, stop it down in 1/3 increments press the dof preveiw button, 1/2 press shutter button and see if the distance scale moves.

D60/10d/20d all stop focussing at F5.6:o

The key here is "all other EOS bodies,.
This includes the 5d and 350D.

Thanks. That is what I assumed when I read it, but since it wasn't specifically listed, I just wasn't sure if the FAQ had been updated or not since the release of the 5D. With the 5D falling into the space between the 1D series and the 20D series, I thought perhaps this limitation had been upgraded on the 5D.

Well your right of course. it had not been uodated :0

I'm off to do that now