On 13/11/2013 5:14 PM, Darren Addy wrote:
On Wed, Nov 13, 2013 at 5:04 PM, Matthew Hunt <[email protected]> wrote:
Only the effect on metering, not on light reflecting onto sensor
during long exposures.
You know, I 've been trying to picture how that even happens. I can
understand how light from the viewfinder might affect the exposure.
After all the mirror is still down while metering, right? So if your
eye isn't at the viewfinder, it is possible that it could be bouncing
around in there and affect the meter.
But this problem? The mirror is UP, during the exposure. So that
implies that the mirror does not fully cover the screen on the bottom
of the pentaprism. Light, coming from the viewfinder (crack between
the mirror and the screen???) must reflect off the back lens element
(or goes through the lens reflects off the dark filter and comes back
at the sensor?). That's the only thing that I can picture. If someone
has an alternative explanation, I'm all ears.
The light seal between mirror and screen is never perfect, and of course
on long exposures we tend to not be covering the viewfinder with our
eye, so more light gets in through the finder anyway. I have also seen
camera where the mirror sags a bit on long exposures and allows even
more light to pass. Once stray light is in the mirror box, some is going
to find it's way to the sensor.
bill
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