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# Thread: Hiding in Plain Sight

1. Here are some values of K and C through the years. Because there are numerous factors involved in these values, there can’t be a perfect one-to-one comparison.

Joint Army-Navy Specification – JAN-M-58
Meters, Photographic Exposure (Reflected-Light, Photoelectric Type)

Sept 15, 1944

K = 1.25

ASA – Z38.2.6 – 1948
General Purpose Photographic Exposure Meters (Photoelectric Type)

K = 1 – 1.35
C = 15 – 30

ASA – PH2.12-1957
General Purpose Photographic Exposure Meters (Photoelectric Type)

K = 1 – 1.35
C = 22 + - 5

ASA – PH2.12 – 1961
General Purpose Photographic Exposure Meters (Photoelectric Type)

K = 1.06
C = 20.83

ANSI PH3.49 – 1971
For General Purpose Photographic Exposure Meters (Photoelectric Type)

K = 1.16
C = 30 +- 5

ISO 2720 – 1974 (R1994)
ANSI/NAPM IT3.302-1994
Photography - General purpose photographic exposure meters (photoelectric type) - Guide to product specification

K1 = 10.6 – 13.4 (0.985 – 1.245)
K2 = 13.3 to 16.9 (1.236 – 1.571)

Cardioid type receptor (hemisphere)
C1a = 320 to 540 (30 – 50)
C2b = 400 – 680 (37 – 63)

Cosine type receptor (flat disk)
C1b = 240 – 400 (22.3 – 37)
C2b = 300 – 500 (28 – 45)

2. According to the 1962 paper An Interpretation of Current Exposure Meter Technology by Allen Stimson,

"The new value K = 3.333 is used when the meters are calibrated at 4700K. The former value K = 3.6, which has been used when the meters were calibrated at 2700K, is 10% greater because the average cell is about 10% less sensitive at this color temperature. Consequently, no radical change in calibration of reputable American made meters is anticipated. However, all meters which conform will be more uniform."

"The incident-light value Iv involves the constant C which can vary +-24% within the limits of the standard and at the option of the meter designer. Incident light, as defined in the Standard for Photographic Exposure, is measured at the subject position in a plane normal to the direction of the camera. However, illumination for visual purposes is ordinarily measured in a horizontal plane. For these reasons, photographic incident-light data may not agree with those of illuminating engineers. The tolerance on the value of the exposure constant K for reflected light meters is less than that on the value of C for incident light meters, because of the greater variation in the acceptance angles and directions of aiming the incident light receivers."

"The ratio of K to C is the average scene reflectance for which the meter is calibrated. the mean value is now R = 3.333 / 20.83 = 16%. This value of reflectance is indicated when the reflected light meter is aimed from the camera toward the subject, and the incident light meter is aimed from the subject toward the camera. However, it is close to the reflectance of the "gray card" used with reflected light meters. Kodak's "Neutral Gray Card" had a reflectance of 18%. The difference is due to the angle at which the card is held."

Today the "ideal" appears to be 3.64 / 30 = 12%.

Here's the technical data for a Sekonic meter.

K = 12.5/10.76 * pi = K = 3.65
C = 340/10.76 = 31.6

3. I'd seen the comment about the angle a gray card is held. Of course you can move the card around and get different readings. What angle were they suggesting to hold the card?

I would normally use a gray card for flat art copying. In the setup I have two light sources at 45-degrees and the camera positioned so the film was parallel to the art. Thinking about flatwork, gets me to thinking about the difference between flat art and three dimensional subjects.

On average, flat art would have a short brightness range; while three-dimensional subjects could have a wider range to consider for its average because three-dimensional subjects include areas in shade (darker) and areas under more light (specular highlights).

4. Originally Posted by Bill Burk
What angle were they suggesting to hold the card?
I believe the second quote from Interpretation of Current Exposure Meters addresses this. The subject is perpendicular on a plane normal to the direction of the camera with a solar elevation of around 41 degrees.

Notice that the earlier standards don't appear to make a distinction between a cosine disk and a cardioid hemisphere receptor? I find that interesting. The difference in C between the two is around .71 to .74. For the Sekonic meter example, Cardioid 340/10.76 * .74 = 23.38, Cosine 250/10.76 = 23.23.

5. Originally Posted by Stephen Benskin
... The difference is due to the angle at which the card is held ...
This then, still puzzles me. Everything seems to agree that the card faces the camera, parallel with the plane of the film. So I don't know what difference is due to the angle at which the card is held.

Is the sentence, in context, talking about the difference between how a lighting engineer measures illumination versus the photographer's measurement? Obviously, a lighting engineer can make no assumption about where the subject or viewer is. Lighting engineer is merely lighting a room.

6. I think he is saying that lighting engineers measure on a horizontal plane, ie ground, with the meter pointed up. I also think Stimpson is suggesting that it's really hard to hold the gray card perfectly perpendicular to the camera plane. We're only talking about the difference between 18% and 16%. With an 18% gray card, the card only needs to be tilted forward about 20 degrees to produce a 12% reflectance when the solar elevation is at 41 degrees (60 degrees total). For 16%, that would be around 5 degrees or so.

7. The technical specifications for a Minolta Meter III

K = 14
K = 14/10.76 = 1.3
K = 1.3*pi = 4.08

C = 330
C = 330/10.76 = 30.6

Hemisphere
14*pi / 330 = 13%

Cosine
14*pi / 250 = 17.6%

Again, the early standards don't distinguish the type of receptor for the value of C. Did they use ranges to incorporate both types instead of separating them? Is it valid to use the K/C equation for a cosine receptor? If it is, it apparently helps to support the idea of 18% reflectance for flat art.

No matter what the exposure meter "sees", it puts it all on the same spot on the film - 8/ISO. I believe one of the reasons why the question of the percentage of what the exposure meter reads has so little written about in the more technical publications is because of the variance of conditions and that most falls within tolerance.

8. Stephen, forgive me if this has already been done, but have you considered combining all this stuff (not only this thread but all the other related topics you've raised on here) into a publication or extended article of some kind? I mean a unified, structured consolidation of these sensitometry/densitometry ideas and concepts into a logical order of some sort? I for one would consider it a very interesting resource to have as a reference text. And imagine all the crap it would debunk! You could call it something like "Do You Know What You're Getting?: Theoretical and Practical Sensitometry for the Analog Photographer".

9. Originally Posted by Michael R 1974
"Do You Know What You're Getting?: Theoretical and Practical Sensitometry for the Analog Photographer".
This would be great! I enjoy the somewhat syncopated conversations, but hopefully the core concepts are reinforced and could be summarized.

10. I did a thing a little over a year ago taking exposure from the average illuminance through to exposure placement. It was in the thread "Is the K factor relevant to me or should I cancel it out?" starting at http://www.apug.org/forums/forum37/8...cel-out-3.html.

I did give the idea some thought when I was writing the four quadrant reproduction diagram program, but anything beyond something like the K factor thread sounds like too much work for such a niche subject. I'm not sure how many people are interested in this stuff anymore. Few people participate in these discussions, and half of those seem to make derogatory comments. There's also so much I don't understand. The saying that you may think you understand something until you have to explain it to another person is so true.

I do like debunking myths though.

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