don't bother, save up and get a longer lens. The extender is meant for
prime lenses and L class zoom lenses.

The 28-135 IS is not really sharp enough at 135mm to warrant a
teleconverter; you might get a tiny bit more reach with a 1.4x, but not
much.  I have never had any luck getting sharp images with my 28-135 and
a TC with my 20D.  With a 10D, it would be a litle bit better, since the
resolution of the sensor is smaller.  For a camera like the 1D, with
widely spaced pixels, a TC would be useful, too.

Forget it.  Save your money and buy a 70-200 f/4.0L.  Best L zoom
Canon makes for the price.

The comments of others are true, though something of a counsel of
perfection, right for them. The right answer for you depends rather on
what you want to do with the pictures, and how high your quality
standards are.

If you want to make 12x16 prints for exhibition, you will be sadly
disappointed.

If you want to make 5x7 prints from a commercial processor to hand
around, or images to show on the web at modest size, you may well be
quite happy.

If you feel the latter is your position, then do not buy one of the
Canon converters. They are designed to work only with some of Canon's
more expensive high grade lenses. They have a marked protrusion to the
front. and may physically clash with the rear element of the other
lenses.

Third party TCs are variable in quality, from poor to quite reasonable.
The Kenco pro range is well spoken of, though I have no personal
experience; they are also markedly cheaper than the Canon ones. Search
the archive for details.

Also, beware of Sigma TCs, as the older ones just flat won't work with
your XT. I had a Sigma 1.4x from the 90s which just won't work with my
10D. The reasons for this are well aired here basically Sigma are too
cheap to pay Canon for a licence to use the EF technology so they try to
reverse engineer it. I usually works OK on bodies current at the time
the lens is designed, but often fails on later ones. They will rechip,
once, for a short period, but usually you find you are stuck with
something they say is too old (well that's my experience anyway).

As others have mentioned, forget it - even if you are, as David
suggests, only interested in 5x7 prints or web images.

What hasn't been mentioned is that you will lose one stop for every 1.4x
extension.  So your 28-135 f/3.5-5.6 will become a 40-190 f/5-8 with a
1.4 extender and a 56-270 f/7-11 with a 2x extender.

Even if this seems like a reasonable compromise, but you have to
remember that the autofocus on your Rebel will stop working at f/5.6 and
higher.  So these extenders will also make your lens a manual focus one
and I don't think the focus screen on the Rebel is adequate for accurate
manual focus.

By the time you have paid for your camera to be modified to fit a decent
manual focus screen *and* bought the extender, you could probably get
some really good glass instead.

It just isn't an economic proposition under any circumstances.

Canon 70-300/4-5.6 IS
Canon 200/2.8

http://digitcamera.tripod.com/#slr