Perceived sharpness at contrast edges—distinct from resolving power. High-contrast transitions appear snappier; low-contrast edges softer, regardless of lens resolution.
On set, you quickly notice: two lenses with identical resolution curves appear to have completely different sharpness. This is due to acutance — how contrastingly the lens renders edges. A lens can be technically high-resolution, but the contrast transitions can be so soft that the image appears muddy. Another lens, with the same resolution, delivers crisp, almost aggressive edge definition. This is acutance, and it should be considered independently of MTF curves.
Practically, it works like this: when light transitions from black to white at an edge, a lens with high acutance creates a steep contrast gradient — the edge appears sharp. Low acutance leads to smoother transitions, more "bloom." This is particularly visible in fine details, textiles, hair, or facial contours. Vintage lenses or softer modern glass exhibit low acutance; classic cinematography lenses (Zeiss Master Prime, Cooke Anamorphic) intentionally deliver high acutance for visual bite. Modern digital sensors amplify this effect — on an Alexa or RED, high acutance immediately appears more present, but can also look dangerously technical.
In editing or during the DI, acutance can be influenced by contrast adjustments in a high-pass filter or via unsharp mask. You can make soft edges crisper again or vice versa. But: what's in the lens is the true, spatial information — post-production is a patch-up job. That's why the DoP chooses consciously. For documentary, intimate dramaturgy, you often need low acutance — Cooke Anamorphics without the Sharp variants, or older Panavision lenses. For action, thrillers, visual pop: high-contrast lenses with aggressive acutance. Some cinematographers combine: primes for close-ups (higher acutance), zooms for wide shots (intentionally softer).
This is related to micro-contrast (how the sensor renders local light-dark transitions) and the general image character, which depends on focal length, aperture, and sensor format. However, acutance should be viewed in isolation from pure resolution — it's the how, not the whether.