A measurement that is an improvement on the older Full-Width Half Maximum measurement, invented by Larry Weber and Steve Brady. The half-flux diameter is the diameter in pixels that contains half the energy in a star image. In other words, if you add up the ADU values (less the background) inside the diameter, and outside the diameter, you will get the same number. This measurement gives a very similar answer to FWHM, but it is much more robust in the presence of seeing and noise, and can handle images that are so out-of-focus that they look like "donuts". The Half-Flux Diameter also varies linearly with focus position, except very close to best focus. This makes reliable autofocus possible.