58TECHNICAL INFORMATION/PHOTOGRAPHY TECHNIQUES continuedFLASH PHOTOGRAPHYAn electronic flash unit is convenient not only for night anddimlight shooting but also as a supplementary light to fill in theshadows in daylight. Daylight fill-in flash is especially effectivewhen shooting outdoor subjects which are backlit or in motion.With a Nikon dedicated flash, such as the SB-15 SB-16B orSB-18, the FA offers fully automatic through-the-lens (TTL)control of the flash exposure. This means that while the shutteris open, a special silicon photodiode (SPD), located at thebottom of the mirror box, reads the light reflected directly off thefilm and determines the timing to cut off the flash output,ensuring correct exposure.
