What are these megapixel ratings based on? Who does the testing?
You bring up a very good question. The term "Megapixel lens" is basically meaningless marketing hype but we are all stuck with it.
The real term we should be using when it comes to evaluating lens optics is essentially "resolving power". LP/MM or "Line pairs per millimeter" is the generally accepted quantitative measure for the resolution of a lens but that doesn't necessarily translate to CMOS sensors quite the way it does with film because there is a relationship with the size of the pixels in each sensor. With the way modern CMOS sensors function, it gets a bit more complicated than that as it involves the concept of MTF (
Modulation Transfer Function) which is an important criteria for evaluating lens performance but one that can be variable. There is literally
no standardization for this in the world of lenses we are concerned with here. Much of the confusion comes from the fact that with MTF there are multiple kinds of MTF and it is a complicated concept.
So, I have no clue who does "the testing", if anyone. (I'm speaking of the people who are marketing and selling lenses.) I'm sure such testing is indeed done in manufacture, but as far as I am aware lenses are not designated in any
quantitative measurement of "Megapixels" by lens producers when they sell them to OEM suppliers because there is no such thing as a "Megapixel lens". So, some lenses are better corrected for optical flaws than others and some are sharper with more resolving power than others but there is no agreed upon objective methodology for comparing lenses unless you test each lens individually side by side. And so any designation of the number of "Megapixels" of a given lens by a manufacturer is somewhat arbitrary to the point that a 3MP lens from one manufacturer may not match a 3MP lens from another. At least that's my current understanding.
There used to be an excellent online sort of PowerPoint video presentation that explains all this in great detail but I can't seem to find it at the moment, assuming it even still exists.
For now, I would recommend perusing the following White Papers published by
Schneider Kreuznach Optics, one of the world's premier lens manufacturers to gain a better understanding.
The Seductive Siren Call Of The “Megapixel Lens” (And Why You Shouldn’t Listen to It)
Do You Know What You Are Really Buying? - A clarion call for lens-tolerance standards (and better understanding those already in place)
