Handholding rule
From The Glossary of Digital Photography
A rule of thumb suggesting that the photographer can handhold a camera with acceptable sharpness at a shutter speed of 1/focal length of the lens. Thus a 400 mm lens needs a shutter speed of about 1/400 to be reasonably sharp. If you learn to hold the lens properly and time your breathing, you might pick up one f-stop (dropping it to 1/200). If you are not very good at holding the camera, you might lose an f-stop (needing 1/800). Use of image-stabilization cameras or lenses will also affect this calculation.
