Is the K factor relevant to me or should I cancel it out?
Sometimes I feel like I'm driving in the snow with all this system calibration. You turn right when you want to go left. Turn too far and if you're lucky you hit the hillside and wait for some helpful citizen to come along with a tow line to pull your radiator off. (OK, I was the helpful citizen. How was I supposed to know the brace behind the Fiero's bumper held the radiator. The driver was really nice considering).
I've heard that instead of being calibrated to 18% gray, most meter manufacturers include a "K-factor" which is approximately a 1/3 stop deviation away from 18% gray.
In the specifications for my meter, the Calibration Constant is 12.5 for reflected light. I have the opportunity to change this to any number I want.
Does the K factor improve the accuracy of my meter? Does it compensate for flare? Should I try to determine my own K factor?
IMO -- You'll be lucky if your meter is within 1/3 stop calibration one way or the other. If you are shooting B&W, just use the meter and judge from the negs where you need to go.
At least with LF landscape, a bad day of photography can be a good day of exercise.
K-factors vary from manufacturer to manufacturer. Some use '12.5' other use '14'. You'll find more info about this on wikepedia and the rest of the web. Nevertheless, I agree with Vaughn, for all practical purposes, it's better to consider your camera, meter and exposure method as one variable. As long as you keep that consistent, your results will be sufficiently consistent to make consistently good negatives. It won't hurt to assume that your meter is calibrated to 18% reflection, even if it isn't (and most likely it won't). Everything else will make your head spin in a hurry. I know, I've been there! Important is that your film receives enough exposure for the shadows to have detail and that development is keeping your highlights under control.
If you are, however, interested in more technical detail, check with another APUG member, Steve Benskin. I consider him to be the expert on this subject.
Last edited by RalphLambrecht; 12-22-2010 at 03:27 AM. Click to view previous post history.
If you are trying to match some standard meter or standard reading then by all means apply a "K" factor so you are calibrated that way. Otherwise creating your own working film speeds can wrap up all corrective factors in one step.
Forget about it.
Light meters of different designs perform differently when tested under 'ISO reference conditions' - where 'calibration' is established - and when they are used to take pictures in the real world.
The 'K factor' is the fudge factor that makes up for this difference.
The real test of a meter used to be "Does it give a properly exposed Kodachrome slide?" where a being off a 1/3 of a stop is noticeable. Kodachrome was the standard because processing was rigorously controlled and Kodachrome doesn't have a wide dynamic range. If a meter didn't produce a proper Kodachrome slide it got returned to the store and exchanged for one that did.
When shooting black and white this is all irrelevant. The dynamic range of the film is huge and the processing control is close to non-existent unless you are in the movie industry.
The value of going through the folderol of establishing a personal E.I. is that it is good training in metering and processing. The end result is always the same: "Expose Tri-X at 320." The speed dichotomy arises because the 'personal EI' methodology isn't the same as the ISO methodology. Set your meter at whatever it is that gives you the negative you want and be happy.
Last edited by Nicholas Lindan; 12-22-2010 at 07:10 AM. Click to view previous post history.
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I expect the variance in light meter calibration is less than the variance in the way you use it (angle held, etc.) so I wouldn't worry about it.
Thanks all, lots of great thoughts. Like I got to get that package off to Dwayne's pretty quick.
I've got a good grasp on process control thanks to my background in film-based prepress. I can pretty much call my CI for each sheet I process.
I'd like to be able to spot on a shadow and if I step down 4 zones, have it land on 0.1 (though I would normallly step down 3 zones for an extra stop of detail).
I just don't want it to turn into a fudge system where I lose another third stop for K factor and another full stop for safety factor and another stop for flare. I just want to nail down the facts and place my exposure on the film where I want it to be.
Then if I am off by one whole stop I won't care because I'll be covered.
K is more than just a calibration constant, itís an integral part of exposure and film speed too. Itís one side of a Trinity of constants - K, q, and P. This is a question of theory and not pragmatic usage. How can you know if your exposure is correct if you canít define what correct is, or what film speed has to do with exposure, or the relationship between film speed and exposure? Everything is interrelated. And K explains everything.
The best source is Connelly, D, Calibration Levels of Films and Exposure Devices, Journal of Photographic Sciences, Vol 16, 1968. I have a pdf copy of it and will send it to anyone who wants it.
Iím still trying to work out how to approach the topic here. It can be rather complex and itís easy to get caught up in the minutia and lose the big picture, but sorry, the minutia is necessary.
Sorry, flare won't allow for any certainty.
Originally Posted by Bill Burk
I recently had an opportunity to view the effect of light meter calibration constants when I was calibrating my homemade digital lightmeter. In this case, since it was an incident light meter, the constant is referred to as C and the typical values are 200-300 since the units of illuminance are lux rather than units of luminance, as is the case with reflected light meters and K. Anyway, changing the constant from 150 to 300 shifted the whole EV vs. illuminance curve up and down slightly. In other words, arriving at a custom film speed would do the same thing as changing the constant. So in your case I wouldn't adjust the constant. In my case, I will probably adjust the constant to whatever it needs to be to put the meter right on EV 15 (E.I. 100) in sunlight. Seems as good a reference point as any.