Variation Facts and Fallacies: Digital Photography Review
“The main points are fairly straightforward:
1) Every lens and every camera exhibits slight variations relative to its twins that are detectable, but rarely significant.
2) Variations that wouldn’t make the slightest difference in a print may seem quite different when the numbers are presented in a lens review. And, just because one copy of lens X is sharper than one copy of lens Y, doesn’t mean they all are, or that they all will be in your camera.
3) Occasionally, an acceptable lens mounted to an acceptable camera combine their variations in a way that makes them unacceptable together. The lens may be fine with a different camera, and the camera fine with a different copy of the lens.
4) Really bad, soft, out-of-acceptable range lenses do occur. They are fairly rare though and easy to detect.
5) Camera autofocus is more variable and less accurate than you think.”
“This Lens is Soft”…. « Canon Rumors
Quite enlightening, if you ever wondered why many lens reviewers say that they needed to try 4-5 copies of a lens before they got a proper one. Then again, also makes you slightly paranoid about whether your camera body+lens combo works properly. Summary:
“The mechanical parts that are assembled to form a lens, lens mount, and sensor are going to vary a bit with every lens and every camera.This variation will cause every copy of a lens, and every copy of a camera body, to have slightly different characteristics.A lens may be fine on one camera and not another. A camera may do fine with one lens and not another.Some lenses (and cameras) will be far enough out of spec to just suck, no matter what they are mounted to.It seems logical that ‘bad batches’ can occur because a shipment of one or more parts is defective and not caught during routine testing (or the manufacturer decides it’s cheaper to ‘ship and repair’ than to hold a shipment).When the manufacturer knows about a “bad batch”, they probably identify the problem and correct it for future lenses, but they aren’t going to announce it unless they absolutely have to – when something is so bad it’s affecting overall sales of that item. Roger’s Rule of Problem Announcements: Once its announced that 5% of lens X has a certain problem, 50% of the members of any online forum will announce their lens has the problem. Whether they own lens X or not.Of course future batches aren’t necessarily better, just different. Problem A may have been fixed, but the new supplier of part 32543 may make a bad batch, or the machine tools used to lathe the last set of part 2433 may have become more worn and less accurate.”