Photometry is the measurement of the amount of light from an image. There are, broadly speaking, two types of photometry. Aperture photometry measures the amount of light inside a region (the aperture) of fixed size. This is the kind of photometry done by photoelectric photometers. PSF photometry fits the image of an object like a star to the point spread function (PSF) of the camera. It is essential to doing photometry in crowded fields.