The flash guide number is a characteristic value for the performance of flash units. This dimensionless parameter indicates the maximum amount of light emitted when flashing with an LED flash as used in digital cameras, mobile phones and smartphones.
The flash guide number is specified for a light sensitivity of ISO 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 type of camera. With digital cameras, such as those 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.