Direct flash is probably the most commonly used form of flash photography because many cameras have a built-in flash unit which can be used only in this manner. The flash fires when a light sensor determines that a picture will otherwise be too dark, and illuminates a nearby subject to more or less the correct extent. Such flash units are usually small and powered by the the camera's batteries. Most consequently have limited power and range of operation. More sophisticated cameras have accessory shoes into which detachable flashguns can be fitted. These units usually have their own internal battery power and consequently exhibit much greater power and range of operation.
Principal problems associated with direct flash are that the light produced by the flash unit is harsh and originates close to the camera and hence the optical axis of the lens. It therefore produces hard shadows and may make a human subject appear too brightly lit against a black background. The small difference between the optical axes of the flash unit and the camera's lens may also lead to an unsightly rim of hard shadow around one side of a subject.