If 18% Grey Is A Midtone ....

[John Brownlow]

Tim, the answer to this is simply that the ANSI standard for light meters results in them giving a reading which is equivalent to a reflective reading off a 12-13% gray card. The standard photographer's gray card is 18% for reasons that are mostly historical and come from the world of printing, although Ansel Adams is also supposed to have badgered Kodak to make their cards 18% gray, for reasons that are not clear.

There is more here:

www.richardhess.com/photo/18no.htm

and here:

www.bythom.com/graycards.htm

[Clay Turtle]

Hard to believe that something so simple as a grey card could be so misunderstood by so may!
When I bought my first grey card, it came in a pack of three. White, 18% grey & black. the first assignment the photghraphy instructor gave the class was to go out & shoot the cards.

You go out set the three cards up & take a reading off them (individually) if you photgraph them at the assigned values and process the film & print what you have is three photos of an 18% grey card. 18 % grey is the norm because like the f16 rule, it produces a normal density negative. I don't particularly use the Zone but I do read high & low values of the scene to know how many stops of light (range) are in the scene.

Being old school, you expose for the shadow & print for the light so to speak, knowing the range helps to decide how much I am going to over expose (negative film). Chrome has a half stop above & half below latitude so ya go into it knowing you have an extremely limited range.

[Jeff Conrad]

I think Hess misinterpreted ANSI PH3.49-1971; there's nothing wrong with
his logic or his math, but the incident light constant of 30 applied to a
hemispherical rather than flat reflector. PH3.49 showed several different
values, and did not do a very good job explaining the reasons for doing so.
The current standard, ISO 2720, does a better job differentiating between
flat and hemispherical receptors, although units now are lux for
illuminance and cd/m^2 for luminance. Illuminance properly is measured
with a flat receptor, and most meters have a C equal to 250. An analysis
similar to Hess's shows that when the reflective K is 12.5 (Sekonic), the
reflectance that would give an equivalent reading is about 15.7%; for K =
14 (Minolta, Pentax) the equivalent reflectance is 18%. I cover this in
detail in the article at

http://www.largeformatphotography.info/articles/conrad-meter-cal.pdf,
so I so I won't repeat it here.

In practice, I'm not so sure this is how it works; incident readings that I
get on several different meters comparing the flat and hemispherical
receptors are much closer than the claimed values for C would predict.
I've not been able to get an answer on this from Minolta or Sekonic.

Of course, a typical outdoor scene includes areas of direct light as well
as shaded areas, and it usually is three dimensional rather than flat. I
discussed this in a private exchange with Ctein, and he essentially
suggested a slightly different interpretation of "reflectance": the ratio
of the light reflected by a typical scene to the light incident upon that
scene. In that case, the "reflectance" would include the effects of shaded
areas as well as actual reflectance differences, and that "reflectance"
might work out to around 13%. I believe that he and I agreed in concept if
not quite in terminology.

All of this, of course, ignores differences in meter spectral sensitivity,
which we beat to death a couple of months ago.

I actually don't find it hard to believe that so many find this confusing,
especially when even the manufacturers' technical reps don't seem to
understand it.

[Clay Turtle]

Thanks Jeff for the pointer toward the pdf. I saved it to file so I can go over at my leisure.
I have problems communicating effectively so I will use an example of a shot I took & had 30x40 printed. It is convientent for me as it hangs on the wll behind me.

First I would point out that the print processor didn't like the idea of printing it that large to begin with . . . a 4x5 color negative with ASA 400 film. She was right, it is grainey but I like the effect besides the birds in the middle ground posed the need to keep shutter speed high enough to stop their actions (didn't want blurry blobs of white in the mid-ground); the cattle feeding in the heat of the day weren't in consideration. Bright but cloudy day, sun high overhead casting deep shadows under the trees (mid-ground with cattle) & the tree line extending to the lake & the far tree line across the lake extending the backgound into the sky line. Fore-ground grasses field of gold & red extending a couple of hundred yards to a lone tree in fall colour at the edge of the middle ground . . . incident light meter reading? Of what the bright fore-ground, I am not going to run a couple of hundred yards down to the tree line to measure the shadows or the bark on the trees. Taking meter reading a scene is a good thing to do, having highlight & shadow content helped to set the exposure but I sat & waited till the shadow of a cloud passing cast a shadow in the fore-ground giving the grass a cool green shade yet the fluffy clouds extended a soft blue cast downward to merge with the blue waters of the lake. Knowing what you are metering & setting the correct exposure are just as important as knowing the characteristics of the film you are using. They are like the normal negative the starting point for a good photograph.

[Clay Turtle]

Then why is Oak tree bark in semi-shade 4 stops darker and white puffy clouds on a bright partly cloudy day only 2 stops brighter?

Just for added info .... light green grass (not the dark blue green bermuda stuff but that bright spring green) in full but hazy sun reads a full stop brighter than my grey card.

18% grey isn't a shade of grey, it is a value of reflectance which involves the both intensity & frequency of the light source.