Archived posting to the Leica Users Group, 2000/08/23
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]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 >-----Original Message----- >From: Dan Honemann [mailto:ddh@home.com] >Sent: Wednesday, August 23, 2000 1:59 PM >To: leica-users@mejac.palo-alto.ca.us >Subject: [Leica] white dot and metering > > >Ok, here's a dumb question (so those of you who have low >tolerance for dumb >questions can skip to the next msg now). The M6 TTL meters >off of a white >dot on the shutter curtain. Why isn't the dot 18% grey? Does >the meter >test light reflected off both the white dot and some of the >black curtain so >that the composite represents grey? > >I'm assuming the meter works by comparing the reflected light >through the >lens to the light reflected off the white dot. > >And why do we use 18% grey cards anyway when "scientific studies now >indicate that an average scene actually reflects 13% (not 18%) >of the light >that falls on it" (Peter Burian, _National Geographic Photography Field >Guide_, p.137)? > >Curious Dan >