The blog — actually it more a newsletter — at LensRentals has a beautiful entry (“This lens is soft” and other myths), which debunks the myth often found on photography forums that people have to go through numerous copies before they find the perfect lens. So why is there so much complaining on soft/or front-focusing lenses? The key to the puzzle is the definition of ‘fine’. Most people assume that ‘fine’ means ‘perfectly calibrated’. In reality cameras are like any other manufactured item, calibration is within a given tolerance range. This leads to people choosing lenses and bodies which match in the same direction of production tolerance:The bad thing is many, many people who did this then hopped on their online camera forum and made blanket statements like “I had to try 3 copies before I found one that was calibrated right”. In reality what they should have said was “I had to try 3 copies before I found one that was calibrated right FOR MY CAMERA”. Those other two copies might well have been fine on someone else’s camera.
The solution to the problem is not to switch copies of lenses, but to use the “lens microcallibration” feature. Unfortunately, this feature of course only exists on more professional type bodies. I have it on my Canon 5D Mark II, but not on my Canon 400D.
Still it allows to understand — from someone with access to a lot of lenses — to explain the discrepancy between the number of complaints and the observation from a rental company finding only a few % of bad lenses.