Sharpness vs Resolution vs Contrast vs Acutance vs...

[twinquartz]

Often, when we say "this lens is really sharp", is that what we really mean?
Who is skilled enough to give a proper definition of
Sharpness, Resolution, Contrast, Acutance - and how they interact?

[sichko]

The last three are easily defined. For me, "sharpness" is a subjective function of resolution, acutance and (maybe) contrast. I would struggle to write down a (mathematical) description.

It's interesting that the perception of sharpness, or at least the description of what is, and of what isn't, sharp appears to vary considerably.

[Excalibur]

Interesting question in what our brain (through our eyes) sees......I'm still using an old tube tv and many times the pictures look sharp and I think to myself "how can this be".......................................

[Orio]

Sharpness is a subjective, non-measurable description of human perception, which is based on the combination/interaction of two measurable objective optical properties:

- Resolvance (more correct term when referred to a lens) or Resolution: the quantity of information that a lens is able to read from a real subject. The higher the quantity of information that a lens can record from a given space, the higher it's Resolvance;

- Acutance or Micro-Contrast: the ability of a lens to make the difference between two small adjacent similar tones discernible. The neater the difference, the higher the lens' Acutance/Micro-Contrast.

[Ray Parkhurst]

The objective definition of Resolution is the point at which contrast drops to 10% (MTF10), which is subjectively where you can just "resolve" a feature. Sharpness is more subjective but the generally accepted objective measure is where the contrast drops to 50% (MTF50).