Circle of Confusion and Sharpness
A bright point is never perfectly projected onto the sensor. The laws of optics transform it into a small disc: the circle of confusion. As long as this disc is small enough (approximately 0.02 mm), the eye perceives a sharp point.
A high-resolution sensor requires an optical system capable of projecting very small circles of confusion to fully utilize its potential. That's why a quality lens is essential with modern sensors.
Diffraction: The Physical Limit
The smaller the aperture (f/number), the more light diffracts. Beyond f/16, sharpness noticeably decreases. The "sweet spot" is generally between f/5.6 and f/11.
Underwater Application
In macro photography, f/8 to f/11 offers the best compromise between sharpness and depth of field. For wide-angle: f/5.6 to f/8 to maximize available natural light. The viewport on the underwater housing adds an optical interface: flat viewports induce aberrations, while dome viewports correct them for wide angles.
Optical Aberrations
Chromatic: Purple/green fringes in areas of high contrast. Geometric: Barrel distortion (wide-angle) or pincushion distortion (telephoto). Vignetting: Darkening of the corners. Underwater, the choice between a flat viewport and a dome viewport is crucial to minimize these effects.