Nikon Camera N90 - Manual (Page 189)

TO MAKE NIGHT BACKGROUND MORE VISIBLE-
When flash pictures are taken at high shutter speeds in dim
light, the background may come out dark. To improve
background exposure, use Slow Sync to extend the
automatically controlled shutter speed range down to 30 sec.
Without Slow Sync, the automatically controlled shutter speed
is controlled between 1/250 sec. and 1/60 sec. or 1/(focal
length) sec. When flash pictures are taken with this rather
narrow shutter speed range in dim light, the subject will appear
bright and well exposed, but the background may come out
very dark, almost black. Setting Slow Sync extends the
automatic controlled shutter speed range down to 30 sec.,
enabling background details to come out.
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