An object's apparent photographic magnitude depends on its intrinsic luminosity, its distance and any extinction of light by interstellar matter existing along the line of sight to the observer.
Photographic observations have now been superseded by electronic photometry such as CCD charge-coupled device cameras that convert the incoming light into an electric current by the photoelectric effect.
By the late 19th Century, an improved measure of the apparent magnitude of astronomical objects was obtained by photography, often attached as a dedicated plate camera at the prime focus of the telescope.
These photographs were created by exposing the film over a short or long period of time, whose total exposure length accumulates photons and reveals fainter stars or astronomical objects invisible to the human eye.
Early black and white photographic plates used silver halide emulsions that were more sensitive to the blue end of the visual spectrum.
This differs from other expressions of apparent visual magnitude[2] observed by the human eye or obtained by photography:[1] that usually appear in older astronomical texts and catalogues.