On May 16, 2006, at 6:43 AM, Lucas Rijnders wrote:
Op Tue, 16 May 2006 15:02:43 +0200 schreef Steve Jolly
<[EMAIL PROTECTED]>:
Joseph Tainter wrote:
Brian, the duration between the pre-flash and the flash is very
short -- microseconds. If you are looking through the viewfinder
you won't notice it.
Isn't that because the mirror's up?
No. The whole point of the preflash is to use the camera's regular
metering system. That means the mirror needs to be down, as the
metering system is located in the prism.
The reason you won't notice it in the viewfinder is that only the
preflash happens when the mirror is down.
The preflash and ambient exposure happen with mirror down. The actual
exposure is made some couple of milliseconds later as the mirror
needs time to swing up and the aperture needs to stop down, the
shutter needs to be opened. Given the camera's 1/180 second maximum
flash sync speed and allowing for a 1/250 second 'shutter wide open'
time, and that shutter release happens after the aperture is fully
stopped down and the mirror is fully open, I would estimate the
minimum time between pre-flash and actual exposure is on the order of
1-3 milliseconds.
1-3 milliseconds is enough time for a facial expression to change.
Some people's eye-blink reflex is in that range, not all, so the
problem exists but is not consistent for all people.
Godfrey
