The following are bits of threads that address the reflectivity of the common Kodak Gray Card:

From Photo Net:

Sorry for the tone of my post above, but I find it difficult to believe that an experienced amateur or professional photographer, especially someone with "30+ years of experience" would cling to the misconception that was expressed above. Experienced photographers know that that the gray card will not produce middle gray densities in the film. In fact, this is pretty common knowledge.

We know that no matter what we meter off of, as long it is of uniform reflectance and the meter has no spectral response problems (very rare), it will produce densities associated with what the meter is calibrated for, not the reflectance of the subject (in this case an 18% gray card). If the meter were calibrated for 12%, for example, it would produce 12% densities in the film no matter what you metered off of. In other words, it's what the meter is calibrated for that is important, not what the reflectance of the subject standard is.


So, what are reflected light meters calibrated for? With a little help from William G. Hyzer, P.E., and Phil Davis, and apologies for the quotes and length of this post, here's the deal. General purpose reflected light exposure meters are calibrated for 10% reflectance. This is the geometric mean of the range 1% to 100%. However, many reflected light meters have a built-in "K" factor of 1.16 over-exposure. 10 x 1.16 = 12%. So, most reflected light meters are calibrated, in reality, to about 12%.


"The origin of the mystical 18% reflectance is to be found in the psychophyiscal visual scale which was experimentally deterimined about 60 years ago by the Munsell Color Company." (Hyzer)


"It seems that Ansel Adams confused this visual midpoint...with the photographic (geometric) midpoint of 10% and was abetted in his error by film standardisers who, prior to about 1961, decreed that there should be a large saafety factor in film-speed ratings. This was to ensure that photographers got adequate negative shadow detail at a time when photoelectric exposure meters were not too common..."


"Adams invented the Zone System in 1941 when, because of this large safety factor, black and white film speeds were half what they are today." This "half speed" is what gave sufficient latitude for the use of a Zone V of 18%. When in about 1961, b&w film speeds magically doubled overnight, it was not because of any change in their chemistry, but because the film-speed Standards had been rewritten to reduce the safety factor ot one-third of a stop."


"The reason for the 18% reflectance gray card standard is unclear, but may have originated in the graphic arts, where the luminance range of typical copy subjects is approximately 1.5 in log terms. For subjects of this range, the gray card is, in fact, a middle gray, because the 18% reflectance..." (Phil Davis - Beyond the Zone System. By the way, the checkerboard problem is discussed in this book. To produce acheckerboard target of 18% reflectance, you would have to cover about four fifths of its area with black and only about one fifth in white. He explains why.)


He continues..."The normal range of an ordinary photographic subject, however, is generally considered to be 7 stops, or 2.1 in log terms. The middle gray of this range is 1.05, which translates to a reflectance of ABOUT 9%. IN OTHER WORDS, THE STANDARD GRAY CARD IS A FULL STOP TOO LIGHT IN VALUE TO REPRESENT MIDDLE GRAY FOR NORMAL PHOTOGRAPHY."


The bottom line is that if the photogrpher's equipment is calibrated properly, the correct E.I. for the film is used, and everything else in the process is correct, a reflected light reading taken off of a gray card, with the vast majority of reflected light meters, will cause and underexposure of approximately one stop.


Gray cards are, however, still valuable as a known standard.


Again, sorry for this long response, but I had to respond to a post that was so incredibly cock-sure of itself, lecturing everyone, and ironically, was so wrong himself. Things are not always either black or white. They are usually shades of gray.

-- Tom Johnston , June 07, 1999; 06:20 P.M. Eastern

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Well Tom, after quoting 3 paragraphs from a book Ive never heard of, can you explain why gray cards are still used? If, assuming what you said earlier is your last response, the meter will underexpose a full stop, why are they so widely use? Why does one of my teachers, who was the editor for National Geographic, teach us to use gray cards? Why do the another 3 of the professors in my program, who combined have over 75 years of AP experience, still teach us this? I certainly have never been exposed to what you are saying. Do I have 30 years of experience? NO... Do I know everything? NO... But everything I have learned and read contradicts what you are saying.

-- Neal Vaughan , June 07, 1999; 06:32 P.M. Eastern

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Neil:

I'm really very sorry that you didn't know this. But that's really your problem (no offence intended). This is a commonly known fact. In fact, this subject comes up at least once or twice each year in the photographic press. I'm also sorry if you haven't heard of any of the sources I mentioned. But you should be able to find a copy of Beyond The Zone System easy enough (and I'm surprised you haven't heard of it). Light meter manufacturers admit this as well. In fact, I discussed this with Gossen designers for an article of mine that was published in the now defunct Camera & Darkroom magazine. They will tell you the same thing. For another source, see a previous post that mentions another discussion of this. Here's another source: The article EXPOSURE METERING MYTHS, DCCT, Jan/Feb 1992. And, TECH TALK, Photomethods, August 1991. Also, check this out in any book on advanced photography or photographic theory. Check out the source in the previous post. Do you think I (and others) are making all of this up.


Even the film manufacturer used to advise photographers of this fact.


The 18% gray card is still useful because it is a known reflectance. If you test you film speed using it you are, in effect, calibrating your meter to 18% gray. The fact is, reflected light meters are simply not calibrated to 18% gray. 18% gray cards are a graphic arts standard. Do some testing. You will find this out for yourself.


If you calibrate for the Zone System, you are taking all these factors into consideration.


No knowledgeable person would deny this. If you pressed your friend at National Geographic, I'm sure he will tell you the same thing.


Check it out for yourself. THIS IS NO SECRET. It's a known fact.


What more can I tell you? I'm sorry if this shakes up a cherished belief of yours.

-- Tom Johnston , June 08, 1999; 09:41 A.M. Eastern

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From the BY THOM website:

Meters Don't See 18% Gray

You've learned an untruth. Welcome to the real world of 12% gray.



A recent discussion on one of the digital camera sites caught my attention. Basically, someone had shot a picture of a gray card, and then remarked that his meter must be off, since the histogram that the camera generated showed the peak of values to the left of center.

This reminded me that we've all been brainwashed into believing something that isn't true: light meters don't measure 18% gray.

Let me elaborate. But first, let me state that the 18% myth is so ingrained in the photography world that virtually everyone just parrots the party line. This includes Nikon USA, who will tell you that their camera meters are calibrated to 18% gray (talk to the Nikon Japan camera engineers, and you get a different story, as they'll respond "yes" when you ask if the Nikon meters are calibrated to ANSI standards; and yes, I had the chance to ask them a few years ago when I was in Japan).

Light meters are calibrated at the factory using ANSI standards. The standard has always been for a luminance value that is roughly equivalent to the reflectance of 12% gray. (Notice I used the words "luminance" and "reflectance." Luminance refers to a certain amount of light energy that is measured directly, while reflectance refers to light as it is seen after bouncing off an object. There is a subtle, but important difference.)

It appears that the 18% gray value comes from the print world. On printed material, it's claimed that the half way point between black and white reflects 18% of the light. So a neutral gray (not whitish or blackish) is 18% gray. It very well may be that Kodak continues to market 18% gray cards because it is easy to produce and monitor this reflectance in production. (Or it may be that it's unclear what the companies producing meters are really doing. It doesn't help that the "technical" information from many of the companies involved in meter production contains contradictory information. For example, Sekonic's web page mentions 14% and claims Minolta uses a higher setting, while Minolta's English pages claim yet a different value.)

ANSI standards (which, unfortunately, are not publically published--you have to pay big bucks to have access to them), calibrate meters using luminance, not reflection. For an ANSI calibrated meter, the most commonly published information I've seen is that the luminance value used translates into a reflectance of 12%. I've also seen 12.5% and 13% (so where the heck does Sekonic's 14% come from?), but 12% seems to be correct--one half stop lighter than 18%, by the way. I haven't seen anyone claim that ANSI calibration translates into a reflectance of 18%.

So, there are two questions that need to be asked (and of engineers at Nikon that would know of what we speak, not the Nikon USA folk who read translated documentation and learned from the same Photography 101 books we did):

Does Nikon calibrate its meters to ANSI standards? (My previous conversations with Nikon engineers leads me to believe the answer is yes.)
Would you need a 12% gray card to get the correct exposure using an ANSI calibrated meter (i.e., is the luminance setting for ANSI really equivalent to 12% reflectance?)? (I believe the answer is again yes, but we can make do with 18% gray cards. Simply take a reading with the card angled between the lens axis and light source, then open up 1/2 stop.)
You'll note that some recent Kodak gray cards have had a somewhat cryptic message on them about using compensation to get correct results. There have been several threads on photo.net discussing this issue without resolution:

http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=000eWN http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=000gMS

But don't take the vacillation in photo.net posts to mean that that 12% isn't a fact. Former Shutterbug editor Bob Shell co-authored a book with Martin Silverman and Jim Zuckerman that goes into great detail about the issue (The Hand Exposure Meter Book).

One interesting speculation on the origin of the difference is here:

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

But everyone I talk to seems to point to Ansel Adams. Bob's book even quotes a Kodak veteran who says that Adams was so vehement about the issue, that he apparently spent a "whole day and most of a night" at Kodak arguing for 18% gray. Still, no one I talk to at Kodak can tell me why Adams wanted 18%.

The bottom line, however, is that whatever method of setting exposure works for you, use it. For a long time, I used to set exposure by metering white (highlights). Then Kodak and Fuji changed all the film stocks I was using, and I had to start over, so I used gray cards and compensated slightly.

This whole thread started with someone noting that a gray card exposure with a D1x produced a histogram with a peak to the left of center, by the way. The ANSI/18% issue may or may not be the cause. But I'd bet it is.

lance [lance@graphlink.com] writes: I noted your question about Adams' insistence on 18% gray card as opposed to a 12% or other standard. He actually answers that question in his Negative book on pages 33 and 42-43 (in my older editions).

Basically, the two issues are that 18% IS mid gray on "geometric" scale of black to white. I may have some problems with this, but whatever. The other issue is the "K" factor that is a supposed correction factor put into meters by their makers. This may be the reason for the shift from 18% to a lower number.

Thom's Response: Yes, I've seen that same reference in my very old editions of the books. 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.

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From Richard Hess' web site:

Richard L. Hess
Why the 18% Gray Card is a problem

The question that many people ask is: "If you meter an 18% grey card with a reflective meter, it will give you the same reading as an incident type. "

Well, yes and no. This has been the traditional line, but in fact the standard camera meter is calibrated more to something like 12% or 13%.

I anticipated a roughly 1/2 stop difference in exposure and got it when comparing a used Minolta Autometer IIIf and my Nikon F4s's and Nikon FA. If you factor this 13%-18% difference into account, then all four meters were with approximately 1/3 stop of each other (I say approximately because the FA's manual readout does not resolve that closely).

My 1987 copyright Kodak gray cards do not mention this, but I think the more current version suggests opening a 1/2 stop.

I wanted to understand certain things about this.

In digging into the "American National Standard for General-Purpose Photographic Exposure Meters (Photoelectric Type)" ANSI PH3.49-1971 I have come across a very plausible explanation for this.

As we know, an illuminance of one footcandle falling on a perfectly reflective surface produces a surface luminance of one footlambert. Since the reflectance is diffuse, the footcandle and footlambert are related by a factor of pi.

So knowing that and that incident light is measured in footcandles while surface luminance is measured in footlamberts we can examine the equations in the ANSI standard:

2^EV = A^2/T = BS/K = IS/C

EV equals exposure value T = effective exposure time in seconds A = actual f-number of lens diaphragm S = ANSI speed of film (using the ASA series, not the old DIN degrees) B = field luminance in footlamberts K = exposure constant (reflected light) I = incident light in footcandles (illuminance) C = exposure constant (incident light)

Now, the ANSI standard gives the value of C as 30 +/- 5. If you use a value of 30, and run through the calculations based on an ISO 100 EV value, the illuminance comes close to matching a stand-alone Minolta illuminance meter (different receptor geometries). Also, if you work backwards and forwards with these numbers, I also confirmed the Sunny-16 rule, although as one might expect in SoCal in the middle of summer that it was closer to Sunny 16.5.

Interestingly, using the value of 30 for C rather than the implied value of 27 in the Minolta book provides closer correlation to the Minolta illuminance meter that we have at work.

Then after some more math in the standard, the value of K is derived as 3.64.

After all is said and done, all we need to do is look at the ratio of K/C which is 3.64/30 or 12.3%. That is where the 12-13% comes from. It is NOT 18%. Using the 18% gray card as a metering reference will cause approximately 1/2 stop underexposure as the reflected light meter is assuming 12%.

Hope this helps!


You can e-mail Richard at richard@richardhess.com
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