A question on LiveJournal's canoncameras community about lens softness provided a pointer to a couple of interesting articles about the myths and facts about "soft" lenses. Soft Lenses are ones that are not completely in focus due to spherical abberation, an effect that is sometimes desirable to produce smoother skin tones, but not something one wants happening without conscious choice. For this reason Canon produce a EF 135mm lens with soft focus, which has an adjustable amount of soft focus that can be dialed in. And Leitz, Nikon and others have also produced lenses with controllable soft focus.

On a normal lens, intended to take sharp photos, soft focus is a flaw -- and often one that causes people to send the lens back. As the article points out manufacturing tolerances will cause cameras to be slightly off perfect assembly and adjustment, and lenses to be slighty off perfect assembly and adjustment. If a lens and a camera with compensating misadjustments (both within tolerance) are combined then the result could be a perfect combination; but if misadjustments accumulate then the result would be a very poor combination, which is what leads people to send the lenses back as flawed. The good news is that many of the fancier modern SLRs have a microadjustment feature that allows the camera to be set (and remember) the precise adjustments needed for each lens. This is particularly important because manufacturing issues are not always random (for instance due to tolerances in the machining tools used to make parts), and modern sensors are much flatter and have the detail to reveal these flaws (historically the film was never held so flat or precisely in position).

For certain lenses the required tolerances for perfect focus are in the order of 20-50 microns (hundredths of a millimetre), which is smaller than an practical manufacturing process can achieve at reasonable cost. So the way forward seems to be including features that can automatically compensate for the manufacturing flaws, similar to the way that the flaw in the Hubble Space Telescope (wikipedia; NASA worldbook) was corrected through additional processing. (While the Hubble Space Telescope's issues were a design issue, the magnitude of the error is similar.) Presumably eventually these features will be included in more prosumer cameras (they're currently only in expensive professional models), perhaps with auto-setup features like modern receivers.

The Lens Rentals News Postings also contain lots of other interesting articles, including: Philipe Halsman, The Portraitist, The Polymath Photographer (about Bill Atkinson creator of MacPaint and Hypercard), how to get sharp telephoto images, using teleconverters and a primer on fast lenses. It seems to be well worth following for insightful articles. Also worth looking at is this post on acutance and the role it plays in sharpness, some notes on Canon's Error 99 catch-all error code, and suggestions for budget SLR equipment (early 2009).