Light meter calibration

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dkonigs

dkonigs

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So let me get this straight...
Its perfectly okay to send your light meter to some qualified expert to get it calibrated to a standard.
But if someone dares to think that maybe, just maybe, they should have a way to do this standardized calibration themselves... then the whole concept of calibration shall be entirely dismissed as not relevant, a waste of time, not the what you should care about, and a goal to be shamed and mocked.
 

BrianShaw

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I think that’s a bit extreme, wouldn’t you agree?
 

MattKing

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Depends on what you have available to you.
For most photographers, it isn't necessary to do this regularly.
So for reasons of practicality and economy it only makes sense to do this yourself If you have equipment and materials (or can buy them cheaply) that can be used for the calibration process.
If instead it requires equipment or materials that are not at hand and expensive, it makes sense to send the meters to someone who already has that equipment (and needs to make it pay).
Setting yourself up to do the calibration may also be something you enjoy - that is fine too, even if it doesn't make financial sense.
 
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dkonigs

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This is how far I got thinking about calibrating light meters… I bought the light


<a href="https://www.photrio.com/forum/threa...-related-purchase.117153/page-86#post-1858997">what waS your last photography related purchase?</a>
Yeah, most of those official calibrated light sources also fit the description of "stabilized power supply and a halogen bulb". You know what else does? My enlarger :smile: Unfortunately, my enlarger is nowhere near bright enough to use for this purpose (I tried), unless I bring the head low enough that readings are no longer consistent from lumisphere-style incident meters. (and even then, they're near the low end of the range these things are designed to measure.

Of course another part of this whole process is that I want to make sure my reflected light (e.g. spot) meters are also reading correctly. So I need to know, given a consistent light source, an incident reading, and a gray card, exactly what reading a spot meter should be giving me.

I was hoping for some comments on whether I read that document correctly, and whether I was on-track with that math I put together.
 

ic-racer

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I was hoping for some comments on whether I read that document correctly, and whether I was on-track with that math I put together.
You might also want to check out this paper, which I don't see referenced in Conrad's paper. I believe I got this paper reference from Steve B.
Screen Shot 2021-09-04 at 6.08.47 PM.png
 
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You might also want to check out this paper, which I don't see referenced in Conrad's paper. I believe I got this paper reference from Steve B.
View attachment 284355

Yup, that's another one of the critical papers. The one I posted earlier is important too. I put together a compilation of different papers and various other sources into a document a number of years ago. It's called Defining K. It covers the theory from Illuminance through to the print (I believe). It should still be on this site somewhere.

dkonigs, it's great that you're interested. I'm all for understanding theory. Get the standards and the key papers. Exposure meters are just a part of the photographic process and need to be understood in the right context and perspective.
 
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BobD

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If you're going to go to such lengths to calibrate your meter, shouldn't you also test and adjust your shutter and measure the true T -values of your lenses? Doing one without also doing the others would seem pointless.

Or, maybe you should just learn to relax. :smile:
 

mshchem

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I have 4 identical Minolta IV F meters. I check them against each other. So far no real differences. The Minolta meters have adjustment inside the battery chamber, I've never touched it. I use sunny 16 as well. I thought I was going nuts a couple weeks ago, I was getting sunny 11 with my meter. I then realized how darn much haze we had from all the fires hundreds of miles from eastern Iowa. One really bad day it looked like a partial solar eclipse.

And No it's not nuts, it's very reasonable to have a calibrated meter. Especially shooting reversal film.
 
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Found a pdf version of Connelly's Calibration Levels of Films and Exposure Devices small enough to upload.
 

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Scudder, James, Nelson, C.N., and Stimson, Allen, Re-evaluation of Factors Affecting Manual or Automatic Control of Camera Exposure is also an informative paper. Parts of it are reproduced in Appendix C of the General Purpose Photographic Exposure Meters standard ANSI PH3.49 - 1971.
 

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dkonigs

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I should probably mention that I actually do have a copy of ISO 2720:1974 (formerly known as ANSI PH3.49 - 1971), and it appears to describe the same calculations as the Conrad document. The only difference is that the ISO standard is written from the perspective of a known light source and bench configuration, while the Conrad document is written from the perspective of known illuminance and/or luminance values.
 
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I should probably mention that I actually do have a copy of ISO 2720:1974 (formerly known as ANSI PH3.49 - 1971), and it appears to describe the same calculations as the Conrad document. The only difference is that the ISO standard is written from the perspective of a known light source and bench configuration, while the Conrad document is written from the perspective of known illuminance and/or luminance values.

Great, then you are all set.
 

Chan Tran

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[QUOTE="RalphLambrecht, post: 2460147, mems
My check is EV14.7 ISO100. Which agrees with the Sunny 16 rule and yours does not.
 

Sirius Glass

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The professional light meter expert that I used was heavily used by the Hollywood film industry and he use more than one light source for light meter calibration. He has retired and I will have to find someone else in the future.
 

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Good article. Gossen advice on the Sunny 16 rule was to aim your meter North at a 45 deg angle above the horizon at a clear blue sky around Noon. Has always worked for me. I do this periodically to make sure my meter(s) aren't way out of whack. A couple of caveats:
Different meters do not "see" the scene same way. Nikon use to use a heavy center weighed metering pattern, so does my Leica M7 and MR-4 meter. My Gossen meter is more of an average reading of the scene.
Bottom line is you need to "calibrate" the meter you're using, camera or handheld, to get the final picture you want after developing, scanning, printing and what not. It's more productive to shoot film (or digital) to determine your proper meter ISO.
 
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So I've been thinking a lot lately about how to correctly calibrate my various light meters. The Internet is chock full of people sharing their rules-of-thumb and get-it-in-the-ballpark methods, pretty much none of which even vaguely resemble what a professional might do. The closest you ever get to that, is recommendations for specific labs that can do calibration. No one ever talks about how those labs do it.

That being said, I've recently decided to tackle the problem myself. As a reference, I've been working from the Exposure Metering (by Jeff Conrad) document.

As tools, I've been using the following:
  • An incident lux meter with proper NIST-traceable lab calibration (I have an Extech LT300 and a Minolta CL-200A, though they basically read the same under the same test conditions.)
  • A Sekonic Exposure Profile Target II, which contains a very large and high quality gray card. I've measured this on my densitometer to have a density of 0.75, which translates to a reflectance of 17.78% (a.k.a. "18% gray")
  • A big diffuse light source, which in my case is the sun under an open sky on an overcast day. (I'd love to use an artificial source for this, but I don't have anything suitable. The smaller the light source, the bigger the variations your geometric conditions introduce with lumisphere-style incident meters.)
  • A collection of incident and reflective light meters to compare
I've also decided to do this entire exercise in "EV at ISO 100", since its a standard absolute unit most light meters can be set to read in terms of. There's a lot less room for "rounding error" than something as coarse as standard f-stops or shutter speed stops.

The goal is, given a lux reading from a lux meter, to determine:
  • What EV an incident-light meter should give at the same spot
  • What EV a reflected-light meter should give from my gray card placed at that same spot
That all being said, here's the math I've come up with for incident-light meters:
EV100 = log2((Lux * 100) / C)
(where C=340 for Sekonic lumisphere-style incident meters)

For reflected-light (e.g. spot) meters, it gets a little bit more complicated:
Z = 10(-1 * D)
Ls = (Z * Lux) / Pi
EV100 = log2((Ls * 100) / K)
Where:
  • D = measured density of the gray card being used
  • Z = reflectance of the gray card (can skip the calculation and set directly, if you already know it and/or don't have a densitometer)
  • Lux = the same incident lux reading used in the previous calculation
  • K = a constant that's typically 12.5 for Sekonic/Canon/Nikon meters, and 14 for Minolta/Pentax meters

In my latest worked example, for which I've already put together a spreadsheet to simplify, I get the following results:
  • Measured lux: 43400
  • Sekonic lumisphere incident meters should measure: EV=13.6
  • Sekonic spot meters should measure: EV=14.3
  • Minolta/Pentax spot meters should measure: EV=14.1

Does this whole process make sense? Is there some critical detail I've completely overlooked? Just throwing this out there to see what others think of this whole attempt at sorting things out.

Based on the numbers you presented, the EVs are correct, but there are a few potential variables that can influence the results.

Two immediately come to mind. Overcast color temperature is very blue compared to the standard's. The part of K that is relate to the spectral response of the photo cell is based on the 4700K of the standard. It won't be as accurate under an overcast sky.

upload_2021-9-5_21-22-17.png

upload_2021-9-5_21-30-55.png

upload_2021-9-5_21-31-17.png


upload_2021-9-5_21-23-14.png


The other consideration is Reflectance is based off a Lambertian surface or perfect diffuser. Gray cards have some degree of spectral reflection. In addition, the angle of the gray card or incident meter to the angle of the light source is also a factor. You didn't mention how you used the lux meter to establish the illuminance value. How the values are measured with the different instruments can make a difference.

upload_2021-9-5_21-27-3.png


I noticed Conrad uses 90% Reflectance at the top of his luminance range. While this commonly occurs, the statistically average model uses 100% Reflectance and has a log luminance range of 2.20. Using these values creates what I have found to be a good instructive model.
 

Philippe-Georges

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OMG, I must have done it all wrong for 47 years!
I thought I could thrust my Spectra (the Combi-500 pro was for long time my standard), Gossen, Sekonic and Pentax who, within about 1/3 of a stop, all agree...
 
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OMG, I must have done it all wrong for 47 years!
I thought I could thrust my Spectra (the Combi-500 pro was for long time my standard), Gossen, Sekonic and Pentax who, within about 1/3 of a stop, all agree...

Usage vs testing / calibration. Two different things. I believe the intent of most of the posts on this thread, including mine, is to point out if the goal is to calibrate a meter in order to photograph, there are better and cheaper established methods than attempting to reinvent the wheel.
 
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pentaxuser

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[QUOTE="RalphLambrecht, post: 2460147, mems
My check is EV14.7 ISO100. Which agrees with the Sunny 16 rule and yours does not.
Can you say what difference(s) there will be between a meter that reads EV15 and one that reads EV14.7 and how those differences manifest itself themselves in terms of what effect on the negative that then translates into the prints i.e. will the two prints show noticeable differences?

Thanks

pentaxuser
 
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