Photographic magnitude
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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 photoemulsive film, 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 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 photographic magnitude than visual magnitude, because they appear dimmer.
The photographic magnitude scale is now considered obsolete.
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