JOHN GERLACH , Nov 13, 1999; 03:23 p.m.
I would definitely stop the 300mm lens down to F/16 when I need more depth of field for landscapes. If the scene demanded it, I would even go to F/22. You will not get as much unsharpness caused by diffraction with the 300mm lens at F/22 as you would with a shorter lens like a 24mm at F/22. Given both lenses set at F/22, the aperture (size of hole) is bigger with the 300mm lens than the 24mm lens. As holes get smaller, a higher ratio of light strikes the edge of the hole and bends somewhat causing the diffraction problem resulting in unsharpness. As always, it's a tradeoff. While lenses tend to be at their sharpess around F/8, the slight loss of sharpness that you get by stopping down for more depth of field is very slight and generally of no consequence in the real world of selling images. If there is nothing to be gained by stopping down, then I would indeed shoot around F/8. Otherwise, stop down to get that extra depth of field.