The flash guide number is a characteristic value for the performance of flash units. This dimensionless characteristic value specifies the maximum amount of light emitted when flashing with an LED flash, as used in digital cameras, cell phones and smartphones.
The flash guide number is specified for a light sensitivity ofISO 100 and a focal length of 50 mm. From these values, the range of the flash can be calculated with an aperture to be set. The flash range depends on the camera type. In the case of digital cameras, as built into mobile devices, the BLZ values are between 8 and 20. Compact flash units have BLZ values between 20 and 50. Studio flash units have even higher values.