Just to help clarify the dual exposures involved in flash photography
(flash & ambient) I should have added that, on the camera's side, the
flash exposure is controlled only by the aperture and never by the
shutter speed. If using a TTL capable camera/flash combination or the
auto mode on the flash then the camera of flash can control the flash
exposure by controlling the duration of the flash. However, in no case
will the camera's shutter speed enter into the equation unless it's
erroneously set faster than the sync speed and misses the flash altogether.
On the ambient exposure side both shutter speed and aperture count as
always. However, since the flash control resides at the aperture the
aperture will have been pre-determined to suit the flash and any
remaining control of the ambient exposure will have to be the province
of the shutter speed.
Chuck Norcutt
Chuck Norcutt wrote:
> Remember that, with flash photography, there are always two exposures
> going on. The first is the flash exposure which is extremely short and
> bright. The second is the ambient exposure which you will allow or
> disallow according to the manual shutter speed you have selected. If
> you choose a high shutter speed in a dark room there will essentially be
> no ambient exposure added to the flash exposure. If you choose a long
> exposure, say 1/8 second, you may add a signficant amount of ambient
> light which may be useful for adding illumination to the background at
> distances the flash can't reach or can't reach with the same intensity
> as the foreground. This dual exposure control is used in the reverse
> sense when doing fill flash. In fill flash, the ambient exposure is the
> principal exposure and the flash exposure power is controlled such that
> it adds only a fraction of the ambient light level in order to brighten
> the shadow areas a bit.
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