what is sync speed
Maximum flash sync speed. The maximum flash sync speed is the fastest shutter speed at which the entire image sensor can be illuminated by flash pulse. Most modern digital SLR cameras have maximum flash sync speed of ~1/250 of a second. A few camera models (e.g. Nikon D70(s), D50) have maximum flash sync speed as high as 1/500s. This duration is the upper limit of the flash pulse duration.