Depth of Field

The Nikon 105mm lens, creates a narrow depth of field, generally depth of field is inversely proportional to focal length of a lens. For any given focal length, the depth of field varies directly with the aperture selected. That is, the wider the aperture (e.g. f2.8) the shallower the depth of field. If you use f22 you can maximize depth of field (the width of the focus area in sharp focus). Most lenses are considered to be at its sharpest at about 2 or 3 stops down from widest aperture, that is at f5.6 or f8, so you sacrifice sharpness if you maximize depth of field. Also pleasing bokeh (blurring of the background) is also effected as you widen aperture say to f-stop number f2.8 in a fast lens.

In this image of Hermione's eye you can see the band of sharp focus, the depth of Field (DOF)

D300, ISO 200, f 3.3 1/500 sec.

I have darkened the image so you can see this more pronounced. Interesting heh?