What is the dial for on my Tamron 28-300mm lens?

I have a Tamron 28-300mm zoom lens and i noticed the numbers (22 16 11 8 5.6 3.5) on a small dial below where it tells you what zoom you are at. I tried turning the dial when the lens is on my camera but my camera (Nikon D50) gives me an error message? So I'm wondering what the dial is for. The image in the following link shows the dial, it is where the numbers are at the very bottom.

That's the aperture ring, it changes the size of the iris inside the lens. Here is a wikipedia page on aperture, and as you can see about halfway down the page there's a comparison between using a small aperture and a large one.

http://en.wikipedia.org/...i/Aperture

It is the manual aperture ring.

On your D50, you will have to lock the aperture ring at its smallest aperture, probably f/22. Then you can control the lens aperture using that control on your camera

The dial is the flat circle like the one on top of your dSLR with the letters PSAM among other icons. The one on the lens is a ring (not my precious) but an aperture ring to adjust the size of the hole of the lens. It would be best to leave it at its smallest setting (the largest f/number, 22). Even in M mode, you can control aperture size with the wheel, the partly hidden circle that you turn with your thumb.