light meter calibration

[Kevin J. Kolosky]

Not many people own a lightsource recommended by the meter manufacturers with which to calibrate their meters.

I am just curious what other people do to keep an eye on calibration, short of sending it in very often.
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I know many will say to compare against another meter with the warning that there is the possibility that both meters might be off.

What else is there?

Isn't it true that the way in which ISO speeds are determined relates to a reflective meter providing an (zone v) exposure of f 16 at the ISO speed of the film when read off a gray card in direct sunlight?

If so, do you watch calibration of your meter by making a reading of a gray card every so often to see if this holds true? Or is this not true?

And then, if true, should not an incident meter provide the same reading in the exact same light?

[Bruce Barlow]

I have never found the rule of 16 to be accurate. My typical exposure for Tri-X is 1/60 at f16. If I used 1/400 at f16, I'd be way underexposed. So, for me, it doesn't work.

I can tell when my meter goes off because my proofs look different. I standardize my proof exposure (Fred Picker's Proper Proof), and when my usually well-exposed negatives start to look goofy, I know something's wrong somewhere, and one of them might be the meter.

I have also noticed speed variations between boxes of the same brand and type of paper, and years ago I knew they had reformulated 35mm Tri-X because all of a sudden my proofs were way too light. Retesting for film speed, the Tri-X I used to expose at ISO 200 was all of a sudden a full 400. Otherwise there had been no change to anything else I was doing. Three months later Kodak fessed up.

Proofs are a great way to figure out what's going on.

[Kevin J. Kolosky]

Hi Bruce

I certainly agree that the "f16" rule (suggestion) may not be accurate as far as exposing one's film, due to all of the other things going on. But what I was suggesting related only to the meter. So, if a person took a kodak gray card, laid it out in direct sun, placed an ISO of say 125 on the meter, and metered the card, the meter would "suggest" the exposure of f16 at 1/125th of a second.

And then, since an incident meter supposedly provides a middle reading, it would not matter whether there was a gray card or not. Just holding the meter in the exact same light should give the exact same exposure as the reflective reading?

What I am trying to get at here is that we don't have these so called "lux" lights, but we do have a pretty consistent light source in the sun to calibrate against, and if the readings are different than the f16 "rule" then perhaps the meter needs recalibrating.

[Pawlowski6132]

It's all relative. You don't need to calibrate your meter to a standard, just to your equipment and processes.

[Kevin J. Kolosky]

I didn't ask about relativity. I asked if it would be correct.

[vinny]

you can buy a second meter that you never take out and do comparisons to that. Or, mark a bright spot on a wall with tape (lets say in a closet) and meter that. Write the results on the tape. Do the same with a spot in the same area. At least then you have a reference to go off of as long as you always have the same make/model bulb in the fixture.
I got a pocket spot meter a couple years ago after a long wait. All of it's readings were about 2/3 stop off compared to every other meter I tried (including recently calibrated and brand new meters). They gave me the "it's all relative" speal and I asked how they calibrate new meters. I never got an answer and I returned the meter for them to keep. I'm sorry but 2/3 stop is way too much when it comes to my kind of work.

[BrianShaw]

I didn't ask about relativity. I asked if it would be correct.

"Correct" really needs to be defined. If correct means that two out of three meters agrees, then the test is obvious. If correct means within spec per the NIST/ANSI (or ISO) standard then one needs to test per the standard. I've heard that Quality Light Metric (and probably other repair shop) calibrate using known standards and calibrated equipment. For me correct means reasonable, and reasonable means that the negs look about what one would expect a decently exposed neg to look like. My method, of course, may leave lots open to discussion, debate, and contrary opinion about what a decently exposed neg should look like.

[Heroique]

What else is there?

I was about to share a few remarks about accuracy vs. precision.

Then I thought better of it. ;^)

Walk-in camera shops commonly offer free on-the-spot meter examinations. What’s fun is to get an opinion from two different shops. Over time, two technicians keep telling me my Pentax digital overexposes by 1/3 stop. So I believe it really does – and ignore the problem.

[John Koehrer]

ISO is determined by a manufacturer in a controlled situation, not by going out & using sunlight & gray card.

So to calibrate your meter You may try a calibrated light source at a camera repair joint but it probably won't match the factory setup.

When you adjust for your properly exposed negative you're using an exposure index. IE

[Leszek Vogt]

I believe it's called a spectrometer. It's a pretty reliable gadget and if the meter or your development deviates....it will show up. My Nikon F2A has been off about 1/2 a stop....consistently for 30yrs.

Les