I am using d200 nikon with 28-70mm lens. when i take fotos the F number, which i think denotes aperture?

Question

changes, is their manual way to control it.

Answer 1

Yes. In manual mode, as you rotate the command dial, the F stops will either increase or decrease, + or -. This lets more or less pass light pass through the lens.

Answer 2

Yes.  Set the control dial to either M or A.

In M (manual mode) you're controlling both aperture and shutter speed.  In A you set the Aperture and the camera will automatically choose the shutter speed.

Sounds like you need to spend a little time with the owner's manual.

Answer 3

The others are correct but I will add that unless your lens is a constant aperture lens like an f2.8 the aperture will change as you zoom in or out. For example many lenses will be an F4-5.6. This means that at 28mm your lens would have a max aperture of F4 but at 70mm your lens would have a max aperture of F5.6.

Answer 4

Put your camera on aperture priority and the aperture will stay the same, but your shutter speed will vary according to the light, or if you zoom your lens in or out.

You can set it in manual mode, and select your aperture and shutter speed yourself.  They won't change unless you change them.  Be sure you understand about light first.

If you have your manual, refer to it for how to change modes.