A modulation tranfer function (MTF) is only ONe measure of quality of a lens. What it represents is the capability of a lens to preserve contrast between black and white.
Say you have a perfect object, where there is a perfect switch between black and white along an infinitely thin line. Now you look at that object through a lens, and the transition between black and white will not be perfect anymore (except if you find me a perfect lens... which does not exist). The resulting image of the black/white transition will be flawed. the MTF measurement measures the importance of the "flaw".
We say that MTF is a good approximation of sharpness. That's true to some extend. It's a good measurement of the capability of a lens to resolve a line, if you will.
However, it does not take into account the capability of a lens to focus colours correctly (which is called chromatic aberration, or CA, and will influence the sharpness of an image). It does not take into account coma, which can be simply explained as the capability of a lens to focus light on the same spot, independant of the area of the lens being illuminated. It does not take into account resistance to flare, which will lowerr contrast and perceived sharpness. It will not take into account colour rendering, which again lowers contrast and perceived sharpness.
In addition, many elements define the quality of a lens, apart from pure sharpness.
So, MTF is useful, but does not tell the whole story.
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