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Why does Kodak say to compensate their graycard?!

Michael Slater , Nov 02, 1999; 12:38 a.m.

Just bought a Kodak Gray Card. Skimmed through the instructions and read the following:

"Meter readings of the gray card cshould be adjusted as follows- 1) For subjects of normal reflectance increase the indicated exposure by 1/2 stop. 2) For light subjects use the indicated exposure; for very light subjects decrease exposure by 1/2 stop 3) If the subject is dark to very dark increase the indicated exposure by 1 to 1.5 stops"

HUH?

I thought the whole point of using this gray card to set my meter with was that it was exactly medium gray -- just what the camera expected. Why would I compensate the metering?

mslater@pacific.net.sg

Responses


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John Hicks -- , Nov 02, 1999; 12:47 a.m.

Because "middle grey" is 13%, while the card reflects 18%.

Michael Slater , Nov 02, 1999; 01:22 a.m.

Why don't they make the card 18%, like the meter expects, then? Seems dumb.

Also, why is the correction factor a function of the actual subject's brightness? Shouldn't it be that you just shift to compensate your meter for 15% reflectance and leave it at that? That would be some compensation level 'n' and would have nothing to do with the actual subject's reflectance.

Now I feel like I wasted my time buying this card.

Ellis Vener , Nov 02, 1999; 01:42 a.m.

Michael why don't you try running some tests? What Kodak is telling you is to:

a.) place the meter in the scene with the lighting you'll be using.

b.) meter the gray card with a reflectance (preferably a spotmeter.)

c.) using that reading as a base, adjust your camera settings for the subject matter. Try this and see if it works. You are thinking the gray card is a shortcut to perfect exposures, but it isn't any such thing. It is a tool that you need to learn to work with. the other point of using a gray card is to give your printer an industry accepted standard neutral point to aim for when printing. It is probably not as important as it used to be, but it is still a standard.

As for why doesn't Kodak adjust it, consider this: it is very rare that two brands of meters will give identical readings. Part of the problem is that different meter makers have different ideas of what is middle gray, just as different film manufacturers have different ideas of what the best color pallette is. Why don't they all calibrate their meters to the Kodak standard? Well two reasons: a.) Market research of consumers as to what average users consider the best exposure. By consumers I include dedicated amateurs and professional photographers as well as those people who shoot one roll every fifteen months. B.) The gray card and it's 18% reflectivity standard derives from the printing / reproduction industries not the purely photographic endeavor.

Scott Eaton , Nov 02, 1999; 08:50 a.m.

Michael is right, it is dumb. However everybody in this thread is right, but for the wrong reasons.

Kodaks' advice is right, but a bit misleading and it has nothing to do with metering standards or grey card characteristics. It has to do with the limited dynamic range of film.

If I meter a scene with a grey card and the subject is extreme white you'll want to decrease exposure a bit because of the limitations of photographic film rendering pure white. Same with a very dark object. In that instance you'll want to increase exposure a bit. Notice Kodak's advice indicates corrections only for extremes.

The grey card is actually right, it's just that film can't record all the brightness range in most scenes and what you are doing is "fudging the extremes of the curve" to make sure your subject comes out right.

Here's the scenario: I got outside on a bright sunny day with fresh snow fall along my my black dog and grey dog. If I use my grey card to meter chances are my grey dog will look perfect, but the snow wil wash out and the black dog will lack detail. If I decrease exposure to compensate for the snow the grey dog still looks good, but the black dog turns into a silouette. If I increase exposure for the black dog the grey dog still looks good but the snow bleaches out.

The positive side is that the grey card gives you a near perfect frame of reference to make decisions from.

Daniel Taylor , Nov 02, 1999; 08:54 a.m.

my gray card doesn't suggest any compensation for mid-tone averaged scenes. it suggests biasing the exposure reading to maintain details in the extremes. it's a dull world, that finds itself painted 18% gray.

Terry Carraway , Nov 02, 1999; 08:59 a.m.

From discussions at other sites, these instructions were in Kodak gray cards until the mid 70s, then they went away for some reason.

The reason for an 18% gray card is, like someone else pointed out, that it is a standard for middle gray in the printing industry.

Meters are set for average scene brightness, which works out to something like 12.5% gray.

So there is a correction for metering off an 18% gray card. With color print film, no big deal there is enough latitude. With slide film, it is something to think about.

Daniel Taylor , Nov 02, 1999; 11:07 a.m.

the last comment makes little sense, and is in fact potentially confusing/wrong unless qualified as pertaining to an evaluative metering system only. a spot or incident meter is calibrated against a standard, which must be known and referenced to be of value.

evaluative/matrix/weighted metering can reflect an arbitrary assessment of average scenic values. statistically, I would think this is a bit lighter than 18% gray (maybe not). the important thing is to characterize your lightmeter through simple experimentation and compensate accordingly.

John Hicks -- , Nov 02, 1999; 11:15 a.m.

Roger Hicks did some checking and found that when the instruction sheet for the Kodak grey card was revised back in '70s, someone inadvertently left off that part about compensating. Now it's back.

Bob Atkins , Nov 02, 1999; 11:29 a.m.

Scott's right of course. A grey card gives you the right exposure for a mid toned subject. If you have a black cat on black velvet and you want to keep any detail in the image, you have to overexpose a bit since film (especially slide film) doesn't have the dynamic range to record much detail at 2 to 2.5 stops under (or over) exposure. Similarly if you are shooting a showshoe hare on snow and you want detail in the highlights, you have to underexpose slightly from the grey card reading or you'll blow out the highlights.

It's similar to the "sunny f16" rule, which is more like "sunny f19" for white subjects (anyone who's shot snowy egrets in sunlight may even prefer the "sunny f22" rule) and "sunny f13" or "sunny f11" for dark subjects, when you want to retain detail.


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