I came across a complaint that a certain lens produces too much contrast. The lens was described as rendering "cartoonish" contrast when shooting a portrait with diffused light from a window, where other lenses are said to give a soft transition of light. The claim is that the lens eliminates midtones. The argument continues that contrast can be increased in post-production, but, like adding salt to soup, you can't really go back if there's too much.
Now, as I understand it, this isn't the way it works; when a lens is said to produce good contrast, this is about microcontrast and is really more related to resolution than to overall tonal rendering. Poor lens designs can reduce overall contrast by allowing stray light (flare or veiling glare), but that's uniformly a bad thing — it's not a tunable with a sweet spot which can go too far. Contrast in a print (or global contrast in an image file) can certainly be over-done, but that's a totally different thing.
Photographer and writer Kirk Tuck, who generally knows what he's talking about, makes a similar complaint in this hands-on lens review:
So, in lens design, given lower resolutions from the last three generations of digital imagers, coupled with acutance robbing anti-aliasing filters, camera makers started creating lenses that added snap and sparkle back in at the expense of longer tonal ranging and high resolution rendering.
So, is there something to this complaint? Is this really a possible flaw in a lens? Can a lens "eliminate midtones"? Can lenses have too much global contrast — or, too much microcontrast after all?