Wikipedia
Before the advent of photometers which accurately measure the brightness of astronomical objects, the apparent magnitude of an object was obtained by taking a picture of it with a camera. These images, made on orthochromatic photoemulsive film or plates, were more sensitive to the blue end of the visual spectrum than the human eye or modern photometers. As a result, bluer stars have a lower (i.e. brighter) photographic magnitude than their modern visual magnitude, because they appear brighter on the photograph than they do to modern photometers. Conversely, redder stars have a higher (i.e. fainter) photographic magnitude than visual magnitude, because they appear dimmer. For example, the red supergiant star KW Sagittarii has a photographic magnitude of 11.0 to 13.2 but a visual magnitude of about 8.5 to 11. It is also common for star charts to list a blue magnitude (B) such as with S Doradus and WZ Sagittae.
The symbol for apparent photographic magnitude is m and the symbol for absolute photographic magnitude is M.
The photographic magnitude scale is now considered obsolete.