# Can somebody help me understand K factor

Discussion in 'Exposure Discussion' started by Rich Ullsmith, Feb 3, 2011.

1. ### Rich UllsmithMember

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Actually, I am interested in it so far as it pertains to my G2.

Reading up on my camera today, I learned (per "service manual") that the K factor for the Contax G2 metering system is 1.3.

I researched "K factor" and ran into all sorts of the type of mathematics I spent my youth avoiding.

From my reading, I reached the unsteady conclusion that the meter in my Contax registers 1/3 stop brighter than the scene really is. (Assuming an 18% gray, 45 degree light, etc.)

Is my understanding of this correct? Should I have the exposure compensation dial at -1/3, or do I have this backwards? Please advise. Thanks for the help up front, and I will check back in the a.m. Rich.

2. ### Stephen BenskinMember

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I've been working on an explanation of K in the B&W Film, Paper, Chemistry forum. The thread is "Is the K factor relevant to me or should I cancel it out?" and is on page 3 at the moment. I'm almost finished with it, so it should cover what you want.

One thing you will find is that the assumptions of 18% and 45 degrees aren't accurate. K=1.30 is only 1/6th stop difference from K=1.16 which is considered normal for average conditions.

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3. ### wiltwSubscriber

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In the ISO standard for light meter calibration, there is the K factor for reflected light meters and the C factor for incident light meters. K is the reflected-light meter calibration constant

From Wikipedia...
"Calibration constantsDetermination of calibration constants has been largely subjective; ISO 2720:1974 states that

"The constants K and C shall be chosen by statistical analysis of the results of a large number of tests carried out to determine the acceptability to a large number of observers, of a number of photographs, for which the exposure was known, obtained under various conditions of subject manner and over a range of luminances.

"In practice, the variation of the calibration constants among manufacturers is considerably less than this statement might imply, and values have changed little since the early 1970s.

"ISO 2720:1974 recommends a range for K of 10.6 to 13.4 with luminance in cd/m². Two values for K are in common use: 12.5 (Canon, Nikon, and Sekonic[1]) and 14 (Kenko[2] and Pentax); the difference between the two values is approximately 1/6 EV."

Not sure why the K values are an order of magnitude different between what you mentioned vs. the K factor in the ISO standard, but since yours expressed no units, that might be the source of the discrepancy.

4. ### Rich UllsmithMember

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What I'm hearing here (and reading elsewhere) is this factor is not an issue with what is essentially a high-end point and shoot camera. I have not had problems with transparencies or negatives with it, but I was wondering about an easy improvement.

I have put my Sekonic spot and G2 90mm on the same gray card at the same time, and indeed they are not the same.

5. ### Stephen BenskinMember

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It is. 1.16 cd/ft^2 = 12.5 cd/m^2 1.30 cd/ft2 = 14.0 cd/m2

I like to use cd/ft2 because it is also the multiplication factor for K in the equation A^2 / T = L*S / K.

256 * 1.16 = 297
256 * 1.30 = 333

256 comes from f/16 or 16^2 which is what the luminance theoretically would be for statistically average conditions if there were zero light loss in the lens' optical system and if the meter had a perfect spectral response.

6. ### Stephen BenskinMember

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K isn't important if you are just shooting. However, if you are interested in the concept of exposure, K is an integral part.

7. ### Stephen BenskinMember

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I've attached a few of the pages from "Is the K factor relevant to me" thread.

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8. ### eddymMember

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Make some shots, bracketing exposures, and find out.

So which gives the best exposure?

Use that.