18% Gray Card

[jcoffin]

<p>Henry: Sad to say (since it's widely quoted) but Thom Hogan's article about meters not seeing 18% grey is <strong>complete nonsense</strong>, almost from beginning to end.</p>

<p>Just for an obvious example, consider this bit: "No manufacturer I've talked to knows anything about a K factor, though, and they all speak specifically about the ANSI standard as their criteria for building and testing meters."<br>

<br />Now, here's the reality: the real standard is not from ANSI, it's from the ISO. To be precise, it's ISO 2720-1974(E). Anybody who doesn't know about the K factor (in intimate detail) has absolute <strong>no clue</strong> about the standard. In fact, it wouldn't be too far off to say that the K factor is the primary subject of the standard (along with the "C factor" -- the K factor applies to reflected light meters and the C factor is the corresponding factor for incident light meters).</p>

<p>The first discussion of the factor is in section 3.5: "The calibration constant K (for reflected light meters) and C (for incident light meters) may be assigned a value within the limits given in 6.3."<br>

In 6.3, we find that if you're using ASA-style speeds, the permissible range for the K factor is 10.6 to 13.4. If you're using DIN-style speeds, the permissible range is 13.3 to 16.9.</p>

<p>When you're using ASA-style speeds, the equation for K is given as: K = LtS/A². <br>

Where:<br>

L = luminance in cd/m2<br>

t = exposure time in seconds<br>

S = arithmetic film speed<br>

A = f-number</p>

<p>It also specifies that: "The constants K and C shall be chosen by statistical analysis of the results of a large number of tests carried out to determine the acceptability to a number of observers, of a number of a number of photographs, for which the exposure was known, obtained under various conditions of subject matter and over a range of luminances."</p>

<p>As examination of that equation shows, however, meter calibration isn't really based on reflectivity at all -- it's based on emission from a known, diffuse light source. Based on the range of luminance of sunlight, and the permitted range of K values, either the 12% grey claimed by Thom <strong>or </strong>18% grey could be used as the standard. We have to, however, pick a value of K close to one end of the permitted range <strong>and</strong> pick a luminance value quite a ways away from the average to justify 12% as the standard. By contrast, using an average value for the luminance of sunlight, 18% grey does correspond to a permitted value of K. On the other hand, if we use an average value for the luminance of sunlight, and the middle of the range of permitted values of K, that corresponds to a grey card with a reflectance of about 16%.</p>

<p>Bottom line: it wouldn't be too far off to summarize Thom's article as: "Neither Thom Hogan nor anybody he's talked to has any clue about this subject."</p>

[Alan Marcus]

<p>A tip of the hat to Jerry Coffin from Alan Marcus<br>

Well spoken!</p>

[leighb]

<blockquote>

<p>"Neither Thom Hogan nor anybody he's talked to has any clue about this subject."</p>

</blockquote>

<p>+2 for that. ;-)</p>

<p>Clueless is as clueless does.</p>

<p>- Leigh</p>

[eric friedemann]

Speaking of what clueless does, is there anyone actually uses a gray card for metering?

[leighb]

<p>I have, on occasions when I forgot my incident meter.</p>

<p>Now I use a Sekonic L558 which is both incident and spot, so I no longer use the gray card.</p>

<p>- Leigh</p>

 

[eric friedemann]

Exactly, for shooting medium format film and non-TTL studio strobes, I use a pair of Minolta incident flash meters. On my

D700 digital bodies, I use the current iteration of Matrix metering which, as far as I can tell, predicates exposure on the

prevention of clipped highlights- not on searching for a middle gray, whatever that is.

[andrew_ranville]

<p>Digging up an old thread but I found all the data that various members mentioned quite fascinating. Maybe they could help with this question?</p>

<p>I'd like to print a gray card lithographically. I will need to do this as a screen of black (percentage of K). The question is... what percentage of black do I set my file at before it goes to press? All things being equal, please assume I'm printing on a pure white sheet of matte paper. There will be no coating or finishing on the paper that would affect the specular qualities.</p>

<p>Thanks!</p>