RE: [Leica] white dot and metering

["Dan Honemann" <ddh@home.com>]

Ah, thanks Paul and others, this clears it up.  I was wondering if I should
start metering off of white surfaces rather than grey.  The calibration
explains it.  Is it possible for someone like Sherry to adjust the
calibration if one finds a consistent under/over-exposure when shooting
transparencies?  Might be simpler to adjust how one meters, though, huh? :)

Thanks again,
Dan

> -----Original Message-----
> From: owner-leica-users@mejac.palo-alto.ca.us
> [mailto:owner-leica-users@mejac.palo-alto.ca.us]On Behalf Of Paul
> Chefurka
> Sent: Wednesday, August 23, 2000 2:33 PM
> To: 'leica-users@mejac.palo-alto.ca.us'
> Subject: RE: [Leica] white dot and metering
>
>
> The spot itself doesn't have to be 18% grey.  It would only need
> to be that
> value if it were to serve as a reference point for a variety of meters all
> calibrated to that standard.  As it provides the reflected light for just
> one meter, it makes sense for the spot to be white, as that reflects the
> most light for the meter's cell to use - especially in low light.
>
> Essentially, the spot can be any reflectivity at all, because the whole
> thing (spot+meter cell)is a closed system.  In that case the system as a
> whole can be calibrated.  It's only in open systems that you need the
> individual parts to be calibrated.  This is essentially what you're doing
> when you carry a grey card.  All handheld meters "expect" to see 18%,
> electrically speaking, but the M6 meter is designed to "expect" to see the
> white spot.  "White spot + adjusted meter cell output" gives the 18% grey
> reference.
>
> You could use a white card with a hand-held meter and get
> accurate results,
> as long as you adjusted the meter to compensate for the non-standard
> reflectivity of the card.  This would turn the setup into a closed system,
> however - you couldn't introduce a standard, unmodified meter
> into the setup
> (i.e. use it with the with the white card) and get the same results.
>
> As to why 18%?  Not a freakin' clue.
>
> Paul