Nikon D5100 Aperture problem?

I have a Nikon D5100 with a Nikkor 50mm 1.8 AF lens (which is a manual focus lens on this camera). When I try to adjust the aperture ring on the lens, the camera tells me to put it back to 22, when I want it to be at 1.8 or 2.8. I don't understand why it won't let me change the aperture?

The aperture ring settings are for some older film cameras that lack electronic control of the aperture. With your camera you want the aperture ring set to f22 to enable electronic control. Then set your aperture using the camera command dial (in A mode), or command dial plus exposure compensation button (M mode).

If you want the camera to control the exposure, you must put the aperture to f/22 and leave it there. This is the standard procedure for virtually every Nikon made in the last 25 years.

More or less, this is the "lock" mode for the lens, and even though the aperture ring indicates f/22, the aperture is fully adjustable by the controls on your camera from f/1.8 to f/22.

If you move the aperture ring off f/22, the exposure mechanism is disabled in the camera and you can then manually adjust the aperture with the ring.

This can be useful if you are using a bellows, close up converters, or a lens reversing ring. But since many people don't use these things these days, Nikon has been converting their lenses to the "G" model, which does not have an aperture ring.

DSLR's only allow the use of the aperture ring in Manual mode. Sometimes, even that wouldn't work. With dSLR's, the right way to adjust the aperture size is by turning the wheel. Read the manual on how to do it under different modes. You may have to press something else before that.