40D Lenses

[Mav]

Tiberius thanks for clearing that up I understand it now.
The lens you recomended the Tamron 17-50mm... when you say it has a constant widest aperture of 2.8, that means I can still close it down to f22 or so and at any zoom range I can have it at 2.8?
Can you link me to it? Do they make it for the Canon 40D?

You can stop lenses down to f/22 or whatever their minimum possible aperture is, but you really don't want to. Most DSLRs are diffraction limited past about f/8 or f/11 at which point you're still gaining depth of field, but steadily getting blurry and soft photos because of diffraction. This is a common newbie mistake.

You can see this easily here: http://www.kenrockwell.com/nikon/50-comparison/f-stops.htm