I'm always curious about how we can say that the light is "perceived as being only half as bright". Any idea how these tests are conducted?
Originally Posted by RalphLambrecht
If I was asked to look at two stimuli, I dont' think I could say, "ahh yes, that looks exactly twice as bright".
Same thing with decibals, a 3dB increase is perceived as "twice as loud", but how are these types of qualitative measurements reliably made?
Between 90% reflection and 45% of reflection there is 1 stop (1EV of difference). If "middle grey" corresponded to 50% of reflection, that would mean that "middle grey" would be only 1 EV below 100% reflection, i.e. the brightest reflection you can have in nature.
So a card reflecting 18% of the light falling over it is around 2.3 or 2.5 Exposure values below a very bright white reflecting 96% of the light falling over it. The very bright white is the shoulder of your slide, and 2.3 or 2.5 below it is your middle grey.
9% is an EV below middle grey, and 4.5% another one, and there you are, with a very dark object reflecting only around 3% of the light falling on it you reach, more or less, the "foot" of your slide, somewhere between 2.5 and 3 EVs below middle grey.
And any case, if what above is in contrast with what Ralph writes, then disregard it
Diapositivo, it's a very good explanation, but it still makes 18% seem like an arbitrary choice. What am I missing? Ralph?
Vaughn, That's funny I use the same analogy. I say, if you put all the tones in this room in a blender, you'd get middle gray.
Originally Posted by Vaughn
18% gray (or 12.5%) are NOT related to "middle" gray of a silver print. (The 18% gray card is an exposure tool, not a printing tool).
We know the log D range of reflected values in a silver print is roughly 2.0 log D (glossy paper approximation).
The half-way point would be 1.0 log D.
The conversion from log D to percent reflectance is:
log D = Log10 (1/percent reflectance)
For an 18% gray card, the log D would be 0.74, which is darker than the middle gray in a silver print on glossy paper.
Last edited by ic-racer; 04-07-2011 at 12:15 PM. Click to view previous post history.
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The 18% gray card reflects 18% of the light that falls on it. This 18% reflectance is the midway point in regards to increments of exposure between white nd black. The 50% refers to a difference in stops, not the actual amount of reflectance. Ten exposure zones between Zone0 and ZoneX. 5 stops from either extreme lands you on Zone V, smack dab in the middle, 50% percent of that range from either end. 18% gray.
That's the best layman's terms I can come up with.
Last edited by Christopher Walrath; 04-07-2011 at 12:23 PM. Click to view previous post history.
Let's see the numbers. What reflectance are we calling zone VIII for instance?
Originally Posted by Christopher Walrath
You called me out. You caught me away from my library so mind this won't be exact but it will be really close.
Mind you a freehand curve on a hand mand eleven point x and y graph but this is real close to the numbers. Never committed the exact numbers to memory but this will put you there. The % numbers regard percentage of light reflected at that zone. And zones 0 and X are probaly a couple of points from the extreme values here so it might be SLIGHTLY skewed, like I said, close not perfect. But very close IIRC.
But now I'm having trouble understanding why it goes from 100% to 68%, and then 50%. I guess it's like f/ stops, and how every other number is double/half.
Errr... umm... numbers... as Barbie said, "Math is hard".
I'm not understanding this statement.
Originally Posted by ic-racer
As I understand it, one of the best reasons for using a reference card is to be able to "place" a subject in relation to a standard.
In the case of a Kodak gray card, if we have a reference shot including the gray card, that "18%" gray shade becomes directly translatable from scene to paper for all the related shots.
If the Kodak card can be considered a "middle grey" subject when it's in the scene, then there is a connection to the print.
Mark Barendt, Ignacio, CO
"The mind that opens to a new idea never returns to its original size." Albert Einstein