"Greg" wrote in
news::
"Al Denelsbeck" wrote in message
Curious, though. It would seem that hitting the midpoint in RGB would
accomplish the same thing, except the midpoint (127,127,127 or
thereabouts)
appears to be about 1/3 to 1/2 stop brighter (photographic terms). Any
idea
why this is? Why would the midpoint in LAB not match the midpoint in RGB?
128,128,128 is closer to the midpoint than that, isn’t it? Punching the numbers
into the CIE Color Calculator:
http://www.brucelindbloom.com/ColorCalculator.html
the luminance changes from 18.4 (L*=50) to 21.6 for sRGB 128,128,128. 21.6/18.4
is 1.17, which represents only 0.17 of a stop, which is not even 1/5th.
Well, two things to consider…
1) The amount I stated was only a guesstimate, mostly from staring at photos onscreen too much of the day. It’s also subject to my gamma ;-)…
2) There’s a difference between the luminance change of the actual monitor output, as measured by a light meter, and the onscreen representation of a photograph converted to 24-bit RGB. The monitor has an exceptionally narrow light range, and right at the moment, my pure black screen with no room lights is pegging about 5 seconds at f2.8, ISO 100, which is much higher than full moonlight on a grey card.
And even then, it’s highly subjective. Not only do the monitor specs come into play, but the exposure range of the film that is scanned. Slide films I use are 4-6 stops in latitude, print films about 9. And both of these appear "full gamut" in RGB space. So there’s a teensy bit of play 😉
Bruce’s site is producing some weird results for me, and I’m not sure what he’s working with yet…
– Al.
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