Originally posted by Ira Sv mode means you select the sensitivity (ISO) of the sensor, the camera will adjust shutter speed and aperture for proper exposure. If you're in a dark room and choose 100, there won't be enough light, so you would have to choose 800, 1600, etc.
I suggest you do the following:
Turn auto ISO off. Set ISO to 400. Go outside. Set to Av mode (aperture priority mode). Set aperture to 5.6.
Camera will now ONLY adjust your shutter speed for proper exposure. Look at the shutter speed the camera picked for you as you aim at a scene or subject, and notice how it changes when you point to different things of varying darkness/lightness.
Now, set your aperture to 8.
See the difference?
Keeping your ISO FIXED to start (not auto) and playing in Av mode will better teach you how the various elements affect proper exposure. When you run into a shot you can't do at 400ISO, you'll know to increase its speed.
So I take back what I said above. I still hate auto ISO.
This is solid advice - if you are new to dSLRs, it's harder to learn when you are juggling three settings (shutter speed, aperture, and ISO) versus playing with just one at a time. If I may add to this:
A. Outdoors, set the ISO to 200, put the camera in Aperture-Priority (Av), and play. Take the same photo with different apertures and compare the results. You won't notice much of a difference if you just point the camera into your backyard, so try to position yourself 3-5 feet from a subject with and interesting background 10+ feet behind.
B. Indoors or in low light, set the ISO to 3200 and do the same thing. When you review your shots, whether in camera or on-screen, take note of the aperture and shutter speed of each shot.
Enjoy!