light meter calibration

[C. D. Keth]

You're on a photography forum when you want a photometry forum. You're worrying way too much about it.

[BrianShaw]

Kevin. When I last worked with lab-grade photometers, calibration as you speak of it was left to specialized labs who had the equipment, know-how, and procedures that trace to NIST. Metrology is a science (or engineering disipline) all in itself. For lab/field adjustments we had Calcium Carbonate disks that were used to "calibrate" the photometers for consistent and repeatable readings. I know that you don't want to hear it, but that is the precision vs accuracy discussion someone previously hinted at. The one thing we never did was try to do highly technical processes like calibration to a standard by using "less-than" processes and techs who didn't understand the complexity of the situation... like relying on sunlight as a consistent light source. If this is your concern, I suggest you call QLM (or someone like this: http://www.prweb.com/releases/2012/1...979450.htm)and ask for a quote... and plan a budget for doing that periodically if you need to know how well your meter is retaining its calibration. They have the equipment to measure and the know how to adjust the meter circuitry if it is out of spec.

[djdister]

As someone mentioned earlier, if all you do is calibrate your meter you are not guaranteed perfect exposures. The entire imaging "system" must be calibrated or fine-tuned for maximum results and repeatability and predictability - meter, film, processing and perhaps even method of printing.

For example, if I know that setting my meter at ISO 32 for Tri-X film, and using a particular camera and developer combination works EVERY time, I don't care if it is not calibrated to some absolute constant. If it works, it works.

[Nathan Potter]

Kevin, I understand what you are getting at here. In fact I use the sun as a standard check on the variation of the meter calibration over time, not an absolute calibration. As some have pointed out absolute calibration is not necessary - it's changes in calibration that are worrisome.

I use a grey card in sunlight, at 45 degrees angle to the sun, the sun 45 degrees above the horizon on a clear non hazy day to obtain an initial EV reading. I do the same for all my light meters and cameras. Then I periodically repeat the process every several months (not as regularly as I should) and generate a trend chart to make sure each unit has not changed significantly (they don't - yet). I think this is what you are suggesting. Works for me. BTW all my meters and cameras vary by about +/- 1 EV.

Of course each meter reads slightly differently but one can take that into account by various logical means. The tracking of repeatability has nothing to do with exposure to film. Correct film exposure is determined by zoning in a variety of ways and the calibration of the film is only valid if the meter sensitivity doesn't change over time - that's the rub.

As I recall the greycard reading under the above conditions averages around EV 16 or so.

Nate Potter, Austin TX.

[Heroique]

I think there should be greater confidence in simple, periodic "meter check-ups" while in the field. If one is careful, such check-ups are really all one needs if not seeking scientific results for publication in a peer-reviewed journal, but rather working as a practical photographer w/ a basic understanding of equipment and a sensible tolerance for evaluation results.

A bit of reasoning can account for naturally occurring variables.

Dry granite is a convenient & dependable "check-up" target in my volcanic region; sandstones are also convenient, but not dependable.

[Leigh]

calibration is changing the value that the computer assigns to the electronic signal it receives.
if this is not correct, what is correct?

Very fundamental error in your assumptions:

Except for a few of the most modern meters, there is no computer.

Exposure meters use a very simple analog circuit to measure the information from the cell.
The info is converted to the display by a dedicated chip that is not a computer by any definition of that word.

-----

If you want to achieve a level of accuracy comparable to that done at the factory, you need to buy and use
the equipment used by the factory.

The sun is NOT a reliable source due to high clouds and air pollution, which can attenuate sunlight by a full stop.

- Leigh

[Bill Burk]

I have set up an 18% Gray Card on a Copy Stand; Two lights illuminate it at 45-degree angles.
If the meters have a Zero-adjust, I check it and set it if necessary.
With a spot-meter, measuring from above, I check to make sure that the illumination is even all over the card (center and all corners).

Now there is a set-up that is easy to understand. Plus you can use it for Incident and Reflected-light meters - spotmeters and regular.

[Bill Burk]

So assume there's two adjustments inside the meter, sort of like film works... one adjusts the contrast, and the other adjusts the brightness. Some meters like the Luna-Pro have a low and a high pair. The brightness adjustment is like a zero adjust and you can't mess things up too badly by adjusting it. But the contrast adjustment - there's an opportunity to really get into trouble if you don't know the procedure to adjust. Here you go back and forth checking bright and dark until the meter reads correctly over the whole range.

[Leigh]

So assume there's two adjustments inside the meter, sort of like film works... one adjusts the contrast, and the other adjusts the brightness.

All linear analog measurement circuits have two adjustments available, properly called "slope" and "intercept".

Think of a straight line drawn at an angle on a piece of graph paper.

The "slope" adjustment controls the angle of the line WRT the horizontal axis (your "contrast").

The "intercept" adjust controls the point at which the line intersects the horizontal axis (your "brightness").

Those are the only two adjustments available for that type of circuit.

- Leigh

[Bill_4606]

Kevin,
In your Original post you asked, "what else is there?" There have been many fine replies to your question in the string. I think they can be summed up in that most people develop their own standard system rather than purchase one as recommended from the manufacturer.
For example, periodically, I use a 4x5 step wedge and a gray card. I set up a light source to give me a standard meter reading off the gray card and I expose a piece of film with the step wedge sandwiched in its holder. I can then see if I have a deviation from previous measurements by as little as one third stop. For small deviations I wouldn't consider bothering to recalibrate my meter. I simply recalibrate how I interpret or use the measurement from my meter.

In another post, you compared the cost of getting a light meter recalibrated to getting your oil changed. You listed your assumptions for how A light meter is calibrated and you seem to be pretty much in the ballpark. As far as the cost goes... Different services command different prices in different markets. Sixty dollars for a light meter calibration may be the going rate for that service, while $35 is the market rate for an oil change. I'm not sure that there is a good comparison between those two.
Regards,
Bill

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.
n
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?