Last update: 15 Sep 03

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-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.

What Should You Do?
If you use 35mm print film, just ignore the whole issue--the exposure latitude of the film you use easily handles any difference.

If you use 35mm slide film and you want to shoot "perfect" exposures, you'll need to shoot a gray card under controlled conditions and then examine the resulting slide with a densitometer. (It doesn't really matter what value of gray the card is, as long as you match the densitometer against that value.) But most of us don't get that elaborate: shoot an evenly lit scene with a person in it and have that person hold a gray card and Macbeth Color Checker card. Spot meter only off the gray card (and make sure that the card is angled about 30 degrees between the lens axis and the light source so that you're really seeing the reflected light). Shoot a series in third-stop increments on either side of the exposure. Examine the results and pick the one you like. If it happens to be the metered value, do nothing with your camera. If it's the one that's +0.3 stop overexposed, then set your exposure compensation dial on the camera to +0.3 stops and shoot normally (or use a slower ISO value for that film).

If you shoot digitally, shoot a gray card under even lighting at the metered value, and at third-stop increments (use only spot or center-weighted metering, and make sure the card is angled slightly towards the light, as noted above). Look at the histograms for each exposure (on the camera, not in Photoshop, which uses a different method of generating histograms). If you're using a 18% gray card, pick the exposure setting that generates a centered value and set that in your exposure compensation control.

In the field (slide and digital users): if you meter off a gray card, use the compensation value you ascertained. If you don't meter off a gray card, I'd shoot at both the camera's exposure setting and the compensation value just to play it safe.

In short, if you're worried enough about exposure to consider the difference between a 12% and 18% gray card, then you should do a careful calibration of your camera and be discplined in setting exposure the same way each time.