How can I adjust my camera aperture to my lenses minimum?

I have this Canon rebel xt (350d) with the 35-50mm lenses which is suppose to give me 1:3.5-5.0. but no matter how hard I try I can't set to an aperture less than 4.5. I usually work in the AV mode which is aperture dependent. why is that?

Dr Sam's got the right idea, but let me see if I can make it more clear. When your lens is zoomed all the way out (in it's widest angle position), that's when you'll have available the f/3.5 aperture. As you zoom all the way in (to the max telephoto position), that's where you'll have the f/5.0 as your widest aperture. That's why you'll see the lens ratings as a range.. 1:3.5-5.0 in yours.

Using the Av mode is fine. Once you understand the relationship of apertures and shutter speeds together, then you can get whatever effect that you want with any of the manual or semi-manual modes. Whether you want a shallow depth of field, deep depth of field, frozen motion, or blurry motion in your photo, you can do it in any of the modes.

If you are at 35 mm, the maximum aperture (which is called so because it is the largest diameter, even though it is the smallest number) is f/3.5. As you zoom out a bit, that will change in a progression until you reach 50 mm, where the maximum will be f/5.0. I don't know if this is a linear progression or not in your lens, but if you are not at 35 mm, you will not be able to get all the way to f/3.5.

In the Aperture Priority (Av) mode you should be able to close your aperture all the way down and the camera will choose the appropriate shutter speed.

When I am shooting and want to control my depth of field, I use the Aperture Priority mode. If stopping action is my priority, the I use Shutter Priority. For unusual or special exposures, I use Manual mode. Notice I never let the camera decide the shutter speed/aperture combination for me? They may as well keep the Program Mode at the factory for the way I shoot.

P would be Programed, Tv would be Shutter Priority and M for manual. All of this is explained in detail in your cameras manual