Setting powder does something optical rather than cosmetic in the ordinary sense. It changes the way light leaves the skin, and cameras register that change differently than eyes do.
Skin reflects light in two ways
Light hitting skin partly bounces off the surface as a direct reflection and partly enters the upper layers, scatters, and re-emerges diffusely.
The direct component produces the highlights read as shine, and it is concentrated wherever the surface is smooth and oriented toward the light source.
Powder works by placing fine particles on the surface that break up that direct reflection, distributing it across many small angles instead of one strong one.
Particle shape determines the result
Plate-like particles lie flat against the skin and produce a soft, even reduction in shine, which reads as a matte finish.
Spherical particles roll and pack differently, scattering light more uniformly and softening the appearance of texture without flattening the surface entirely.
Formulators combine shapes deliberately, which is why two powders of similar color can produce visibly different finishes on the same skin.
Cameras and eyes weigh reflections differently
A camera records a fixed exposure across the frame, so a specular highlight that the eye adapts to can clip to pure white in an image.
Direct on-camera light makes this worse, because the light source, the lens and the reflection all sit close to the same axis.
Powdering the areas that face the camera most directly, along the forehead, nose and chin, addresses the highlight geometry rather than the amount of oil present.
Flashback is a specific ingredient effect
Certain mineral filters used for sun protection reflect strongly in wavelengths a camera flash emits, producing a pale cast on skin that looked normal in the room.
The effect is not visible without flash, so it appears only in photographs and often surprises the wearer looking at the result afterward.
Products intended for photography usually avoid or limit those ingredients, which is why a flash-safe powder may offer no sun protection at all.
Application controls how it wears
Pressing powder into the skin with a dense tool deposits a thin, adherent layer, while sweeping it across the surface leaves a looser one that migrates.
Excess powder settles into fine lines and expression creases over the course of a day, which is what produces a cakey appearance rather than the formula itself.
Building in thin layers over several hours therefore holds up better than a single heavy application, and it is why touch-ups are done with a light hand.