Originally posted by nuaabill In DA* 16-50mm Serial Number Database, people marks good or bad copy of it.
But how do you judge that?
Plus, I will buy a used Da16-50, the seller told me it shows a few mm of back focus at 16mm. Is it common or that means a bad copy?
Thanks~
A percentage of the lenses (not just 16-50) produced by a manufacturer (not only Pentax) can be faulty. The major fault of the 16-50 that causes the lens to be bad is decentered elements. This results in an uneven sharpness at the image's edges. The top left corner might be crisp, the top right corner soft, for example.
To test this, a perfectly flat detailed surface (newspaper taped to plywood, or Lowell's favourite brick wall) is used as a target. The camera is mounted on a tripod with the camera back exactly parallel to the subject. The 2 sec mirror lockup timer is used to expose (avoiding camera shake) and the resulting image examined carefully on your computer to compare all the corners to see that they are the same contrast and focus. The edges will not be as sharp as the centre of the image, very much so at 16mm and f/2.8.
The back focus/front focus issue has to do with the way the lens and the camera body interface when AF is used to focus the picture. The camera thinks the lens is perfectly focused, but the actual point of focus is a few mm in front of or behind where the camera focuses. The k20 and k7 have a system to adjust the focus for each or your lenses so that the AF is spot on. My k10 does not, but can be shipped with the lens to Mr. Pentax for adjustment. If the focus is grossly erroneous, it has been fixed under warranty.