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.
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.
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.