Photometry consist on the accurate measurement of the light flux from celestial bodies. In order to achieve this, a device that count the photons that arrive to a determined area during a certain amount of time is used. This device can be a photomultiplier, as well as a more recent CCD. To obtain the same results for the same celestial object measured in different equipment, calibrations and normalizations must be done. For that, Photometric filters (as the UBVRI system) and calibration stars as those in the Landolt list or M67 stars must be used. By comparing the measurements obtained in the calibration stars, we can obtain the correction factors for the equipment used. If the calibration stars are far from the object in study one must take into account the atmospheric extinction. As important as the measurement accuracy is the accuracy of the measurement uncertainness (error) calculation, a tough job sometimes.
To know more, I advise the reading of the book from William Romanishin " An Introduction to Astronomical Photometry Using CCDs ", perhaps the best introduction to the thematic, and gently offered.