Ilford FP

FP is a cubic-grain black-and-white film from Ilford Photo with a long history. It originated as Fine grain Panchromatic roll film in 1935. Like its faster partner product, HP film, it has gone through a number of versions since then, with the latest being FP4 plus.
The film is known for being versatile, with usable results even when underexposed two stops or overexposed six stops. This is due to the fact that FP4 features a double layer emulsion. It combines a layer of low sensitivity fine grain particles and a high sensitivity larger grained layer. This kind of film was already known in the 1920s and 1930s. Being perfect for the amateur's 6x9 roll film cameras these old types lacked sharpness in the 24 x 36 mm format due to the overall thickness of both layers. But the FP4 of 1968 combined an amateur-friendly double layer emulsion with the sharpness of a typical thin layer emulsion. This was possible because both layers together were only 7.5 μm thick.