Describe Stopping down?

Ok, yes I'm a newb and should take a class.
I would like stopping down explained as far as what it does for an image in certain light situations...and or where or how do you do this on my camera..lol told ya I was a newb.

A stop is one doubling or halving of shutter speed or f stop (1/2 the amount of light). Stopping down usually refers to decreasing the aperture by a single stop resulting in 1/2 the amount of light entering the camera. ( eg. f/5.6 to f/8 )

Stopping down makes the aperature smaller (larger numerically). For example your sigma 70-200 2.8 has a maximum aperature of 2.8, which is on the large side. Since the aperature is large more light can enter the lens allowing the shutter be open for a shorter amount of time (fast shutter speeds stop action) the other thing to consider is depth of field. Stopping down reducing the amount of light you are getting but increases depth of field. Also, most lenses tend to get sharper as you stop them down.

On your XT you can set your camera into aperature priority mode by moving the dial to Av, this way you select the aperature and the camera selects the shutter speed. Then you can use the black dial behind the shutter button to change your aperature. Take some time to experiment with it and get an idea of the effects it has.

Thank you very much!

There is one more point here that has to be added.

Ask a beginner, "What is the purpose of aperture setting?" The beginner will describe it as something that has to be balanced against the shutter speed and ISO in order to get a good exposure. That much is true, but there is more to it.

Ask an expert, "What is the purpose of aperture setting?" The expert will describe it as a way to control the depth of field. Beginners always get confused by the term "depth of field," so think of it as "depth of focus." With a large aperture (low f-number) you can control the focus so that the subject is perfect and everything in the background is blurry. For some subjects, that is desireable. However, as you stop down the lens aperture (changing it from f/4 to f/5.6 to f/8), you will be getting more and more background into focus, and for some subjects, that also is desireable. The good thing about using good camera lenses is that it gives you the flexibility of controlling these things. With a slow lens (poor maximum aperture) you won't have so much control. It is permanently stopped down and can't get any more wide open.

---Bob Gross---