I was wondering why some lenses extend when when focusing and some extend when zooming and some don't extend at all. I have a fantastic lens by canon 70-200mm f/2.8 that does not extend at all. I love that.
I am looking into another lens purchase and there are both extenders for opposite reason. Also one extends at 24mm (lowest mm) (which is confusing) while the other extends at 55mm (highest mm)(makes sense)
The link has the two lens I'm talking about:
http://www.dpreview.com/products/compare/side-by-side?products=canon_17-55_2p8_is&products=canon_24-70_2p8&sortDir=ascending

Many of today's lenses were designed long before digital DSLRs were even thought about. The 24-70 f2.8L is one of them. Canon decided there wasn't a need to change to full internal focusing on the lens because of R&D costs as well as passing the additional cost to the customer could reduce its popularity.

Rare earth glass, internal focusing and zooming, constant minimum f stop at all focal lengths and IS/VS are all items that increase the prices on lenses. Sometimes the size of the lens prevents certain alternatives to be viable.

The EF-S 17-55 f2.8 is designed to be used on crop cameras only. The "L" designation, Luxury, only goes on EF lenses which are designed for full frame cameras as well as crop cameras.
They are also more expensive because of better build and better features than the normal EF lenses.

As I was saying: I would have liked to keep the 5D but the money wasn't at hand to purchase the new camera.

People find features on the newer cameras that they could put to good use. If I won the Lotto, ROFL,I would
consider a 1DMark IV but not until after I rent one and try it out for a week. I may find that it is too big for my hands (I have small hands for a man}. If so I would stick with the 40D.

You have a heavy lens with the 70-200mm f2.8L. I found that it doesn't balance well on an XXD camera at all. I found it cumbersome and tiring caring around all day. After having one for a year I checked and I had only taken four photos with it using an f stop wider than f4. I now have the f4L IS version and it works well on the 40D and the 5D with no balancing problems even when using a 1.4x TE. It also weighs less than the 24-70 f2.8L.

The XXD, 7D and the 5Ds are all smaller than the 1D and 1Ds series. The Rebel series are not built as well as the above which are a lot more durable.

All of the cameras from the 20D to the new Dx have all of the controls for taking photos in the same place. Only the menues are different.