Light Meter Calibration

Doc W · Jun 22, 2017

Is there an absolute scientific standard with which light meters can be calibrated or does it really matter as long as one uses the light meter consistently?

I am currently testing a new film. I sandwich a sheet of 4x5 in the holder with a Stouffer test tablet and use a light table as a light source. When I metered it in the past, it was always EV 13. However, just for the hell of it, I used another meter and it gave me EV 14.

Does this matter or not? If I use the same light meter throughout the test and in the field, will it make any difference? I know that with thermometers it doesn't matter that they all agree as long as I pick one of them to be the standard with all the others plus or minus a little.

Kawaiithulhu · Jun 22, 2017

https://www.yelp.com/biz/quality-light-metric-hollywood
That company/service comes up a lot in light meter discussions. Haven't tried them myself, though.

tedr1 · Jun 22, 2017

I agree consistency is important, in many cases more important than absolute accuracy. Try to not mix up the two meters

Jeff Bradford · Jun 22, 2017

George at Quality Light Metric calibrated my Gossen Luna-Pro last year. If your meter is consistent, you can make it work for you. If you compare 10 light meters with each other, none of them will match exactly.

Jim Jones · Jun 22, 2017

Light table brightness likely varies with voltage. Long ago incandescent light bulbs were available with a calibrated light output to be used at specific voltage. We could use any incandescent lamp at a consistant voltage as a personal standard. A regulated power supply from an otherwise useless electronic device might be useful for this.

Bill Burk · Jun 22, 2017

For the kind of test you are doing, I'd argue that you don't need a light meter.
Your light table is probably not the kind of light source meters are calibrated to, and you are not likely to use the light table as your main light source for photography, so neither the absolute standard nor the practical reason for choosing a light source are being met by the test.

But it's a good test to find your development time - contrast relationships. Just trust that with fresh film if you use a standard developer you are likely getting the rated speed when you meet the ASA/ISO gradient specifications (approximately 0.62 CI). So you can mark that negative's curve's 0.10 point above base+fog as the amount of light at the rated speed point. Then you can switch things like developer and agitation methods and find the "relative" change in EI that you get with your experimental technique.

Now if you go out with your meter and shoot at exactly what the meter recommends... and take that density and compare it to the curve you created here... you will find out if your meter meets expectations.

Doc W · Jun 22, 2017

Bill: the light table is more or less in the right colour temperature range for daylight, which is why I am using it. As for testing, I am using Ralph Lambrecht's spreadsheet which does most of the work for me. I am one of those guys who has only the very weakest understanding of what you said in your second paragraph. Btw, the film I am testing is Bergger Pancro 400 which has the highest b+f I have ever seen (0.22)!

Jeff: If all light meters "work," as long as they are used consistently, why get one calibrated? That was really my question.

AgX · Jun 23, 2017

There are DIN and ISO standards for light meter calibration.

RalphLambrecht · Jun 23, 2017

Doc W said:
Is there an absolute scientific standard with which light meters can be calibrated or does it really matter as long as one uses the light meter consistently?

I am currently testing a new film. I sandwich a sheet of 4x5 in the holder with a Stouffer test tablet and use a light table as a light source. When I metered it in the past, it was always EV 13. However, just for the hell of it, I used another meter and it gave me EV 14.

Does this matter or not? If I use the same light meter throughout the test and in the field, will it make any difference? I know that with thermometers it doesn't matter that they all agree as long as I pick one of them to be the standard with all the others plus or minus a little.

if an incident eading on a clear, sunny day give you EV15,your lightmeter is well calibrated. Consistency is more important than Accuracy when it comes to lightmeters!

ChuckP · Jun 23, 2017

One thing a calibration would check is if the light meter response is linear. If it isn't your film speed testing is only good at the light intensity you checked it at.

Doc W · Jun 23, 2017

RalphLambrecht said:
if an incident eading on a clear, sunny day give you EV15,your lightmeter is well calibrated. Consistency is more important than Accuracy when it comes to lightmeters!

What about a spotmeter?

narsuitus · Jun 23, 2017

Doc W said:
Is there an absolute scientific standard with which light meters can be calibrated...

Probably, but I did not have access to it.

Decades ago, when I purchased my first hand held light meter, I calibrated it against the Sunny 16 guide. I took multiple readings on bright sunny summer days and discovered that my hand held meter was always one-stop off. As a result, for decades I have had to double the ASA/ISO on the meter to obtain readings that were consistently accurate.

Chan Tran · Jun 23, 2017

To calibrate the meter you need standard light source of varying intensity. You would need 2 type of light source for reflective and incident.

Chan Tran · Jun 23, 2017

Doc W said:
What about a spotmeter?

Measure a Kodak gray card under the same lighting. Careful to avoid specular reflection.

John Koehrer · Jun 23, 2017

The meter you have has standards that are a bit lax. I don't believe there's a standard
for K factor for all makers.
Re: consistency Once you've got your standard(for your developing method) As long as you don't
change anything, you're good.
Spot meter: The reading's are influenced by the subjects color and I think, reflectivity(shiny or matte).
Once you learn it it's good too.

I'm lazy & haven't used anything besides an incident meter for about 100 years

Jeff Bradford · Jun 23, 2017

Doc W said:
Jeff: If all light meters "work," as long as they are used consistently, why get one calibrated? That was really my question.

The divergence between the high-scale and low-scale was non-linear. The high-scale was low by one stop, and the low-scale was high by two stops. Now I have a nifty sticker that says I'm all calibrated.

Bill Burk · Jun 23, 2017

Doc W said:
Bill: the light table is more or less in the right colour temperature range for daylight, which is why I am using it. As for testing, I am using Ralph Lambrecht's spreadsheet which does most of the work for me. I am one of those guys who has only the very weakest understanding of what you said in your second paragraph. Btw, the film I am testing is Bergger Pancro 400 which has the highest b+f I have ever seen (0.22)!

b+f 0.22 is no issue, so long as it's mostly film base. That's probably deliberate to help with anti-halation.

I guess what I meant by the 2nd paragraph is... you hardly ever know anything for sure. But there is one time when a test really reveals something. And that's when you hit the ASA parameters, then you know how much light hit the film where Ralph Lambrecht's graph hits 0.1 above b+f

Now if you take test shots with the same film and develop it at the same time. And in that test you expose exactly as the meter says for 400... read the density then you can work out backwards how far your light meter is off. This only gets you one point of reference so still might need calibration over the whole scale. And it mixes camera and lens factors etc. But it might tell you about the meter if everything else is right.

RalphLambrecht · Jun 24, 2017

Doc W said:
What about a spotmeter?

same thing;a spotmeter is a lightmeter too.

dmr · Jun 24, 2017

I've been asking this for several years now.

I find it kinda strange that we have very accurate and cheaply available tools for measuring things like length, volume, temperature, mass, but we don't have a similar tool for light*.

I've been told the following by net.experts in response to questions like this.

1. Send one meter in to a calibration lab and check everything with that.

2. If all of your cameras/meters agree within 1/2 stop or so, they are about as close as you can get. (LOL -- "If you have a watch, you know what time is is. If you have two watches, you're never quite sure.")

3. Do a sanity check under sunny-16 conditions.

* I actually regard 3 as the best, as they tell me (the ubiquitous "they") that the illuminance of mid-day sunlight in an unobstructed mid-latitude urban setting will be very consistently just over 100,000 lux, which translates to sunny 16 or slightly over EV100 of 15. If I ever question one of my cameras (or my only meter) I just do a sanity check outside in the neighborhood under sunny-16 conditions.

Chan Tran · Jun 24, 2017

dmr said:
I've been asking this for several years now.

I find it kinda strange that we have very accurate and cheaply available tools for measuring things like length, volume, temperature, mass, but we don't have a similar tool for light*.

I've been told the following by net.experts in response to questions like this.

1. Send one meter in to a calibration lab and check everything with that.

2. If all of your cameras/meters agree within 1/2 stop or so, they are about as close as you can get. (LOL -- "If you have a watch, you know what time is is. If you have two watches, you're never quite sure.")

3. Do a sanity check under sunny-16 conditions.

* I actually regard 3 as the best, as they tell me (the ubiquitous "they") that the illuminance of mid-day sunlight in an unobstructed mid-latitude urban setting will be very consistently just over 100,000 lux, which translates to sunny 16 or slightly over EV100 of 15. If I ever question one of my cameras (or my only meter) I just do a sanity check outside in the neighborhood under sunny-16 conditions.

And when meters agree within 1/2 stop or so they are within 40% of each other. Do you have another measuring tool that you could accept a 40% difference between one tool and the next?

dmr · Jun 24, 2017

Chan Tran said:
Do you have another measuring tool that you could accept a 40% difference between one tool and the next?

No! And this is why I still don't get that warm fuzzy feeling when dealing with the accuracy of light measurement. My only consolation is that I know that the film I usually shoot (almost always color negative) can usually be overexposed or underexposed a stop or so with good results. I want to be within the tolerance and latitude of slide film, however, which can't take a joke, exposure-wise. I'm also in the habit of bracketing on any semi-important shot.

ic-racer · Jun 24, 2017

One can argue that film is manufactured to exact standards, so name-brand film can be used as a standard on one's lab. Thus, one's meter can be calibrated so it gives the correct reading when exposed to the same light that caused name-brand film to yield an in-camera 0.1 log d film density when processed to the ASA conditions.

jim10219 · Jun 24, 2017

There are a ton of problems when it comes to measuring light. First off, an artificial light source will always be nonlinear across the spectrum. Even two light sources of the same color temperature manufactured from different technologies will produce different results across the spectrum. Add to that the fact that our commercial light meters don't actually measure light. They measure electricity that's influenced by light (using a photovoltaic, photoresistor, or photodiode), and there is no good way to turn pure light into pure electricity. So coming up with a circuit that can measure light at all colors and levels accurately is impossible in a hand held device with our current technology. Besides that, film itself doesn't have a linear response. And if you want to get even further into the weeds, your shutter, aperture, and film speed are almost certainly off. I've never seen a shutter, electronic, pneumatic, or mechanical, that was dead on at all speeds. And if you take the time to measure and calculate your lens's aperture settings, you'll probably notice most of them vary a bit from their marked settings.

So it's foolhardy to worry about absolute values, unless you only plan to take the same photograph under the same conditions for the rest of your life. What's important is finding a workable process that allows you to make good photos, and accept small variations as inevitable without compromising your artistic goals.

I have a bunch of light meters, and most all of them are off by a stop or more. They're still usable because I'm familiar with them and know how to use them to get a usable reading. I understand each ones strengths and limitations. As long as you can get a good photograph from one, that's all that matters.

Doc W · Jun 24, 2017

Jeff Bradford said:
... Now I have a nifty sticker that says I'm all calibrated.

You smarty-pants, you!

Doc W · Jun 24, 2017

It is a bright, clear day here today with lots of blue sky and a few fluffy clouds. My Pentax V gives me an EV of 13 1/3 - 13 2/3 when I point it north, away from the sun.

Light Meter Calibration

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