Focus Stacking
Combining several frames focused at different distances into one image with depth of field no single exposure could achieve.
Measured in
count
In depth
Software aligns the frames, compensates for the small magnification change between them, and selects the sharpest pixels from each. It is the standard answer to macro work, where even f/16 gives only millimetres of depth, and to landscapes with a very close foreground. It requires a static subject and, in practice, a tripod.
Examples in practice
Forty frames stepped through a beetle's body produce one image sharp from antenna to abdomen.