Apple's Photos app (Edit > Adjustments) exposes two related but distinct sliders that both work by manipulating micro contrast — the darkness/brightness relationship at edges — rather than adding literal resolution or detail.
Darkens the dark side and brightens the light side of every edge where darkness meets brightness. This tricks the eye into perceiving the image as sharper without adding real detail — the presenter is explicit about using 'appearance' rather than 'actual sharpness.' Because the effect is fine-grained, it should be judged zoomed in (pinch-to-zoom); it isn't visible at full-screen size. Recommended starting point: ~40 (out of 100), not pushed to the max.
Works like Sharpness but at the scale of the whole image — it adds contrast in the areas surrounding lines throughout the photo, not just at individual edge pixels. Because it's a global effect, it should be judged zoomed out at full-screen size. Recommended starting point: ~50.
Sharpness = local/edge-level micro-contrast, evaluated zoomed in. Definition = global/whole-image micro-contrast, evaluated zoomed out. Both are illusions of detail via contrast manipulation, not literal added information — contrast with Noise Reduction via Light-Information Density (iPhone Photos App), which removes/reduces real image information (grain) rather than tricking perception. See also Mono Filter + Auto Tool as iPhone Photos App Editing Baseline for the recommended editing order and Vignette as Subliminal (Should-Be-Invisible) Eye Guidance for the companion Vignette slider used in the same pass.