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Micro contrast is basically the ability of the lens-sensor combination to render small differences in light levels. It is easiest to see in B&W for obvious reasons, and generally is connected to lens element count.
Like everything in photography, there are always tradeoffs. Small lenses with low element counts and designs like the Cooke triplet have great contrast but suffer from more chromatic aberration and distortions.
Modern lenses, on the other hand, have much more complex designs with lens costings, aspherical elements and a gazillion number of elements and groups, resulting in images with zero distortions and aberrations, but with less micro contrast or "flatter".
That's why old manual lenses tend to be so small compared to FF modern counterparts.
TL;DR: go to a flea market and buy old 1970's/1980's pancakes.