Ive read reviews that certain lenses have better close up performance than others. Ive also come across the term focus shift as well. Can anyone explain why a lens can be better close up than others and what focus shift means?

Thanks!

-- Karl Yik (karl.yik@dk.com), May 10, 2002

Answers

Lenses can be optimized for a particular distance or range of distances; some have a broader range than others. Typically faster lenses have a narrower range of optimum distances, since they're more difficult to optimise in the first place.

Focus shift is an apparent change in the plane of sharpest focus when a lens is stopped down. This can happen when a lens is not completely corrected for spherical aberation and the cone of light that is supposed to come to a point at the film plane doesn't quite come to a point because some of the light rays from the edges come to a focus at a slightly different plane, either before or after the true plane of focus. At the true plane of focus, these edge rays will appear much more out-of-focus than at a plane in between the true focus plane and the edge rays' plane; this intermediate plane is where focus appears best at maximum aperture. As the lens is stopped down, the influence of the egde rays is gradually eliminated and the plane of best focus moves back to the plane of true focus.

-- Douglas Herr (telyt@earthlink.net), May 10, 2002.