Here's some new findings for all you 50mm F/1.8 owners. Despite the fact that we are told to lock the lens up to the minimal aperture of F/22 for only "P" or "S" mode, we should leave it locked even in "A" mode to avoid the "FEE" error appearing on the Nikon D90.
Secondly, the use of DOF Preview button found on the Nikon D90 body will now become vital in helping us preview how well exposed the final shot will appear literally. As you will notice, shutter speed begin to drop with us moving the aperture closer to F/22.
You tweak the aperture size using the Nikon D90's sub command dial (the one in front of the camera by default). Narrowing down the depth of field (with bigger aperture size) will give bokeh (a photographic term referring to the appearance of out-of-focus areas in an image produced by a camera lens using a shallow depth of field) effects on both the foreground and background of the subject.
There! We have taken our first step into the realm of Aperture Priority shooting.