Light meter calibration

dkonigs · Sep 3, 2021

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.

BrianShaw · Sep 3, 2021

Wouldn’t it just be easier to get a degree in metrology? That kind of worry would make sweat off of photography for the remainder of eternity.

BrianShaw · Sep 3, 2021

… but don’t let me discourage you. Best wishes figuring this out!

wiltw · Sep 3, 2021

The best way is to take the meter to a technician who has a calibrated light source, so that the meter can be adjusted to a KNOWN standard.

Then you can validate your hypothesis about your various alternate methods vs. the standard-adjusted meter, to verify which one(s) makes sense.

gone · Sep 3, 2021

My experience w/ this is that if I have 10 different meters, I may end up with 10 different readings. Some sort of reverse standardization. I calibrate any handheld meter by comparing it to a Nikon N70 or N8008s. Those have meters that are always dead accurate. The N70 only cost $4.95, but w/ shipping and the lithium batteries you may as well buy the N8008s for $30 or so. It uses cheap AA batteries, and is a better camera to shoot if you wear glasses and have any Nikon lenses or third party lenses that can work w/ adapters.

ic-racer · Sep 3, 2021

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

That is nice to do the math as you are doing because the usual conversion tables give round numbers.

dkonigs · Sep 3, 2021

wiltw said:
The best way is to take the meter to a technician who has a calibrated light source, so that the meter can be adjusted to a KNOWN standard.

Then you can validate your hypothesis about your various alternate methods vs. the standard-adjusted meter, to verify which one(s) makes sense.

A calibrated light source could be used as part of this method, in place of the sun. Its not really an alternative against which to verify.

As far as standard-adjusted meters, that's exactly what my various lux meters are.

The goal is basically to figure out how to translate that "standard calibrated reading" into what camera light meters are supposed to show under the same light conditions.
(And to not depend on some lab in another city that's run by some old person on the verge of retirement whose methods aren't published.)

I'd also love to have a standard calibrated light source. Its just not always clear where to find one, or which ones are most useful for this application.

wiltw · Sep 3, 2021

dkonigs said:
A calibrated light source could be used as part of this method, in place of the sun. Its not really an alternative against which to verify.

As far as standard-adjusted meters, that's exactly what my various lux meters are.

The goal is basically to figure out how to translate that "standard calibrated reading" into what camera light meters are supposed to show under the same light conditions.
(And to not depend on some lab in another city that's run by some old person on the verge of retirement whose methods aren't published.)

I'd also love to have a standard calibrated light source. Its just not always clear where to find one, or which ones are most useful for this application.

Oh, I misunderstood the purpose My mind went numb in seeing the formulae

There is this article in Wikipedia https://en.wikipedia.org/wiki/Light_meter
in which it mentions:

"With a hemispherical receptor, ISO 2720:1974 recommends a range for C of 320 to 540 with illuminance in lux; in practice, values typically are between 320 (Minolta) and 340 (Sekonic). The relative responses of flat and hemispherical receptors depend upon the number and type of light sources; when each receptor is pointed at a small light source, a hemispherical receptor with C = 330 will indicate an exposure approximately 0.40 step greater than that indicated by a flat receptor with C = 250. "

How do your findings seem to jive with that?

dkonigs · Sep 3, 2021

momus said:
My experience w/ this is that if I have 10 different meters, I may end up with 10 different readings. Some sort of reverse standardization. I calibrate any handheld meter by comparing it to a Nikon N70 or N8008s. Those have meters that are always dead accurate. The N70 only cost $4.95, but w/ shipping and the lithium batteries you may as well buy the N8008s for $30 or so. It uses cheap AA batteries, and is a better camera to shoot if you wear glasses and have any Nikon lenses or third party lenses that can work w/ adapters.

The methods I outlined above could be used to convert the readings from an existing meter (like the one in a Nikon SLR) into equivalent readings for all the other meter types. You'd just have to convert f/stop and shutter speed back into EV, and reverse some of the calculations.

The real problem is twofold:

You still need a standardized and consistent test setup (keeping in mind that the camera has a reflected-light meter)
Given the same source of light, reflected and incident meters don't necessarily give the same result. If they are supposed to match, then there's some debate over what sort of grey card they're supposed to match on. (and its not always 18%)

dkonigs · Sep 3, 2021

ic-racer said:
That is nice to do the math as you are doing because the usual conversion tables give round numbers.

I've come across some of these conversion tables in my research. They're pretty good for converting EV to Lux, but don't seem to be quite as good for the reverse. (especially since EV is a logarithmic scale and Lux is a linear scale.) I'd much rather have a direct calculation. It may seem complex if you write out all the steps, but once you bake it into a spreadsheet or program (on a computer or programmable calculator), its really easy to do.

Maris · Sep 3, 2021

I use film to calibrate photographic light-meters.

The standard scene outside my house is shot in a series of stepped exposures around late morning on a sunny day where the light won't change for a while.

The film is developed immediately and the best negative, and my exposure notes, tells me the correct camera settings.

Then I go back into the same scene (the light hasn't changed), do my usual metering routine, and adjust the meter's film speed dial until the meter reads out the camera settings I already know are correct.

Calibration done.

The same approach can calibrate a light-meter when there is a developer change, film change, or filter change. I often find the adjusted light-meter's film speed dial is set to about half the ISO box speed.

None of the above is useful for calibrating meters for Luminance in units of Candelas per square meter,or Illuminance in units of Lux but those radiometric parameters tend not to have direct applications in simply taking and making photographs.

Bill Burk · Sep 3, 2021

Don’t you want to use a backlit opal screen illuminated by a stabilized tungsten light source filtered to daylight?

ic-racer · Sep 3, 2021

Here are a few fascinating calibration devices.

ic-racer · Sep 3, 2021

Maris said:
I use film to calibrate photographic light-meters....

Gossen's advice too.
https://gossen-photo.de/en/calibration-of-exposure-meter/

ic-racer · Sep 3, 2021

Here is a diagram of a calibrating light source with specs that one could use to duplicate something similar:

Stephen Benskin · Sep 3, 2021

Is Quality Light-Metric still around?

dkonigs · Sep 3, 2021

Stephen Benskin said:
Is Quality Light-Metric still around?

AFAIK, they actually closed earlier this year. And that's really the problem... I absolutely do not want to depend on a place like that, and their "magic" process for calibrating these things.
First, I want to know what these places are actually doing, and not just trust them and their magic process that I'm not supposed to know or care about details of.
Second, I want a way of getting equivalent results myself, using tools that I'm actually able to have in my possession.

I believe in actually having traceable standards whenever possible, and calibrating things to those standards. Even if you convince me "just make sure it gets the results you like on film", I'd rather that be the second step. First, make everything match up given the same situation. Then, and only then, determine any further adjustments beyond that standard baseline for the results I want. That way those further adjustments can be totally independent of whatever device I happen to be using on a given day.

Stephen Benskin · Sep 3, 2021

dkonigs,

Color temperature plays a rather big factor in meter calibration. The chart is from the attachment below, Stimson, Allen, An Interpretation of Current Exposure Meter Technology, Photographic Science and Engineering, Vol 6, No. 1, Jan-Feb 1962. It had to be split-up to upload.

wiltw · Sep 3, 2021

Stephen Benskin said:
dkonigs,

Color temperature plays a rather big factor in meter calibration. The chart is from the attachment below, Stimson, Allen, An Interpretation of Current Exposure Meter Technology, Photographic Science and Engineering, Vol 6, No. 1, Jan-Feb 1962. It had to be split-up to upload.

View attachment 284294

And that chart shows 2850K 'color temperature of source used for calibration', and not concidentally virtually matches the chart posted in post 15

ic-racer · Sep 3, 2021

Once your light meter is calibrated it will give a perfect exposure of the calibration light source

Stephen Benskin · Sep 3, 2021

ic-racer said:
Once your light meter is calibrated it will give a perfect exposure of the calibration light source

I was just working out a post along the similar idea of realistic precision. After all, it's an exposure meter and not a light meter.

The spectral sensitivity of the photo cell as illustrated in the above chart should in itself be a strong argument against expecting impossible levels of precision. Then there are the exposure constants K and q. A hand held meter is separate from the camera's optical system and therefore has to assume a general set of conditions. A camera's optical conditions change. The hand held exposure meter's assumptions don't. Through the lens metering doesn't have this problem.

BrianShaw · Sep 3, 2021

If the negative looks good, the meter is accurate enough.

Sirius Glass · Sep 3, 2021

BrianShaw said:
Wouldn’t it just be easier to get a degree in metrology? That kind of worry would make sweat off of photography for the remainder of eternity.

wiltw · Sep 4, 2021

Kodachromeguy said:
Where is this technician to be found in USA (or Europe, Japan, etc.)? Tell us who to contact.

At one time, virtually any camera repair shop had the means of calibrating camera meters or handheld meters. If that is no longer true, photography is in a very sad state of affairs

PerTulip · Sep 4, 2021

Kodachromeguy said:
Where is this technician to be found in USA (or Europe, Japan, etc.)? Tell us who to contact.

I just sent my Sekonic to Sekonic for calibration.

Light meter calibration

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