the 18% Myth

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Paramvir Singh
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the 18% Myth

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From here

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!


Meters Don't See 18% Gray
You've learned an untruth. Welcome to the real world of 12% gray.

From Here

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):

  1. Does Nikon calibrate its meters to ANSI standards? (My previous conversations with Nikon engineers leads me to believe the answer is yes.)
  2. 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-fet ... _id=000eWN http://www.photo.net/bboard/q-and-a-fet ... _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 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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Re: the 18% Myth

Post by Paramvir Singh »

Proven wrong! It's not 18%, but 13%, grey card!

    This is from page 137 of National Geographic Photography Field Guide (great book! buy it!!) by Peter K. Burian and Robert Caputo (Washington DC: National Geographic Society, 1999):

    "Scientific studies now indicate that an average scene actually reflects 13 percent (not 18 percent) of the light that falls on it. For the sake of consistency, gray cards have continued to be 18 percent gray, as is the one in this book. When using any 18 percent gray card for substitute metering, increase exposure by a half stop (+0.5 compensation factor) for most subjects, as Kodak recommends. If the subject is very light--a snow-covered landscape, for example-- decrease exposure instead, by a half stop (-0.5 compensation factor) from the gray card reading. This will help maintain detail and texture."

    I suppose we can safely assume that camera makers are infomred of this 13% grey card thing and have calibrated their newer cameras' build-in meters accordingly, but retained the use of the familiar "18% grey card" term as the metering reference so as to avoid confusion. But what about older cameras' build-in meters? Any comments from experienced photographers who have encountered persistent metering errors on older cameras using the 18% grey card as their metering reference?

    -- Hoyin Lee, December 17, 1999; 01:38 A.M. Eastern
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