Roger Cicala of Lensrentals.com explains why there is no such thing as a perfect lens:
For those of you who like to avoid reality, let me be very clear. There is no such thing as zero tolerance. Not if you measure accurately enough. If you want the very best quality obtainable, then a nice f/2.8 zoom would be priced at $20,000 to $40,000. What a coincidence — that’s about the price range of cinema-quality zooms.
Like most people testing lenses, we used Imatest. But maybe a lens-test projector would be better. Nope. Well, the gold standard was MTF measured on an optical bench. So we (despite the vigorous protestations of those-who-manage-the-money) bought an optical bench. It showed a similar amount of copy-to-copy variation. (The optical bench might make it sound better, though. It’s amazing the number of people who think the difference between 0.65 and 0.70 on a zero to one scale is ‘about the same’, while they think the difference between 650 and 700 on a one to one thousand scale is hugely different.)
The laws of physics, manufacturing tolerances, and simple economics mean sample variation can never be entirely eliminated. Scream all you want about how you expect a perfect lens, but unless you’re willing to pony up $20,00 to $40,000 per zoom you aren’t even going to get into the neighborhood of near-perfection.
There is good news, though, and it makes a lot of sense. In earlier posts I’d shown that for most lenses, the sample variation is generally right about at the limits of what we could detect visually in a photograph. That makes perfect sense. Lens designers are pretty smart guys and manufacturers are going to keep a pretty tight grip on costs. The logical place to keep sample variation would be where you can hardly see it.
Read the entire write-up with explanations and charts here.