The system has been expanded to accommodate the stars revealed by telescopes, so that the dimmest objects we can detect now are about magnitude 30 or so. A star 5 magnitudes brighter than another is producing 100 times as much energy. To compare intrinsic brightnesses the artifice of imagining all stars to be the same distance from earth is used. M can be found if m and the distance are known. More generally, if any two of the three quantities m , M and distance are known, the third may be found from them.
Measured brightness depends on the technique used.
The simplest example is the difference between photographic and visual magnitudes. Common photographic film is less sensitive to red light than it is to the rest of the visible spectrum. So a star which emits most of its light in the red part of the spectrum will appear dim in photos but brighter, compared to neighboring stars, when seen directly with the eye. V is the same as M B - M V .