Roger over at LensRentals.com has written great article on optical quality assurance and what goes into making sure you get a lens that works as intended.
After we published a number of posts about copy-to-copy variation, people were quick to say that this company or that needs to ‘just' improve quality control. I totally agree but realize most people don’t have a clue what ‘improving quality control’ would really look like. I think they have some vague idea of hiring a guy named Joe to sit at the end of the assembly line, check all the lenses, and reject all the bad ones. Optical quality control for lenses is way, way more complicated than that.
Before we start, let’s get a couple of things out of the way.
- If you expect that the lens you buy has the exact same MTF that the manufacturer published, then don’t read this. You’ve got too much reeducation to do.
- If you ever say, “for that kind of money I expect perfect” please don’t read further. Perfect doesn’t exist. For $20,000 and up you can get pretty close. For $2,000 you should be able to get reasonably close.
On the other hand, a reasonably expensive lens should have reasonable quality control and minimal copy-to-copy variation. We know it can be done; some manufacturers are already doing it. Others totally suck. But when they suck, sticking Joe at the end of the assembly line won't really fix things. If you want to read this pretty long article, I'll explain why, and also explain some of what can be done. Read the full article