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Light meters are not calibrated for 18% grey.


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#1 Edgar Dubrovskiy

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Posted 11 November 2007 - 06:17 PM

"Meters Don't See 18% Gray"
from http://www.bythom.com/graycards.htm

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...g?msg_id=000eWN http://www.photo.net...g?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.richardhe.../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.
---------

So what do you think about it?

Edited by Edgar Edgar, 11 November 2007 - 06:21 PM.

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#2 K Borowski

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Posted 11 November 2007 - 08:39 PM

Well, it's hard for me to think anything with your having done such a thorough job in exploring the issue; good job! I had heard either 12.5 or 12 myself as a value in the past, but have never adequately explored the issue. I think it is actually 12% that is the correct number. My mind just slips to 125 because it's an ASA value, I suspect. Remember that there are three scales of measurement being used in photography: arithmetic, geometric, and logarythmic, and percentage is a very finicky thing anyway. If you add 7.5 percent to a number and then subtract 7.5 percent, you don't get your original number! So 18% reflectance may somehow correspond to 12 or 12.5 percent using another system determining the light that comes off of the card.

In any case, this is all academic, as many people alreadly compensate for this discrepancy by rating their films 1/3 of a stop or 2/3 of a stop higher than their ASAs. Now, for stills shooters, using reversal film with only 1/2 to 1 stop of latitude in either direction, this is much more of a dilemma. Again, though, many shooters compensate 1/3 stop OVER box speed with slide films for a more saturated look.

Unless you're into scientific analysis imaging, you're really grasping at straws here. Modern color neg stocks eat a half a stop for breakfast ;-) It does go to show that TESTING is important when critical work is to be done. I think we all know by now not to trust Kodak's particular variety of double-plus good Orwellian doublespeak.

~KB
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#3 Edgar Dubrovskiy

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Posted 12 November 2007 - 03:10 AM

This insn't my article, actually. Found it on website.
But big thanks for the answer!
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