Medium grey 17%, why not 50?

[holmburgers]

Regardless of what the average scene reflects or what lighmeters are calibrated to, it all has to do with how our eyes compare different brightness levels. The human response to reflection (lightness) is not linear. For example, a surface reflecting 18% of the light that falls onto it, is perceived as being only half as bright (50%) as the illumination itself. The response follows the following equation:

L =116*(R)^(1/3)-16

where L and R are the lightness and the reflection in % respectively.

For example, use R=0.18 (18%) and L will return 50%.

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?

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?

[Diapositivo]

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

[holmburgers]

Diapositivo, it's a very good explanation, but it still makes 18% seem like an arbitrary choice. What am I missing? Ralph?

[artonpaper]

I tell students that if one took all the shades of grey that are in the average scene and mixed them up like paint, the resulting shade of gray would be "middle gray" and have an 18% reflectance...and that is what our meters assume the scene to be.

Don't know if this is 100% true, but it seems to satisfy them...LOL!

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.

[ic-racer]

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.

[Christopher Walrath]

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.

[holmburgers]

The 18% gray card reflects 18% of the light that falls on it. This 18% reflectance is the midway point (this is where the 50% comes in, not in reflectance, but in regards to the difference in stops) in regards to increments of exposure between white and black. Ten exposure zones between Zone0 and ZoneX. 5 stops from either extreme lands you on Zone V, smack dab in the middle. 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?

[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.

Zone 0....0%
Zone I.....1.5%
Zone II....3%
Zone III...6%
Zone IV...10%
Zone V....18%
Zone VI...26%
Zone VII..36%
Zone VIII.50%
Zone IX...68%
Zone X....100%

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.

[holmburgers]

Thanks Chris,

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".

[markbarendt]

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).

I'm not understanding this statement.

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.