How to check camera's meter?

[Ed Sukach]

Maybe you can answer this, and this has been bugging me lately.

Is there anything readily available for a reasonable cost that will produce a light of a known intensity, kind of a "standard candle" sort of thing.

I mean, for other values we have known physical standards. For length we have rulers, for volume we have measuring cups or graduates, for temperature we have thermometers or lacking that, the freezing point of H2O, for time we have quartz watches or lacking that the high point of the sun.

For luminance or illuminance, it appears we have nothing more precise than "Sunny 16" or "check it with a known good meter".

I consider the following to be minimal:

1. A Standard Lamp. Ours were in a constant cycle with the National Bureau of Standards - to try to insure constant accuracy. Not inexpensive in themselves and cost of calibration - monthly - was HIGH.

2. An AMPERAGE - Controlled Power Supply. Most PSs are voltage controlled -- and the output of the Standard Lamp is affected more by variatons in current than voltage (by 3X). Also NOT inexpensive

3. An optical Bench - or some other apparatus allowing fairly accurate distance relationships.

4. An atmosphere controlled room.

5. Sundry other "stuff" - a PC to crunch all the data would be nice.

That is a "nutshell" description of what was in use in the Metrology Lab where I worked. There may well be some sort of Commercial Device available now for Exposure Meter (a.k.a. "Photometer") calibration.
Considering the demand there must be for those out there, I would be surprised out of my socks if their price was anywhere near what us mortals could afford.

Uh... "Rulers"?; "Freezing point of water? (actually it is the "Triple Point" of water, defined as +.01 Celsius) ... "Sunny 16"?
I don't know ... IMHO, they are all pretty "Rough" to be considered as "Standards"

 

[Chan Tran]

Ed do you know who makes these instruments? I know that us mortal can't afford them new but what if they show up on ebay and nobody bid on them?

 

[dmr]

Ok, let me bounce this off the gang here. I posted this on another board and nobody really supported it.

A co-worker pointed out to me that they produce bright white LEDs rated for so many millicandelas, like 1000 and 2600.

Now I know these are not accurate absolute numbers, and probably minimum numbers, but we think that if you took one of these and ran it with a constant voltage source and constant series resistance, the light output would be very consistent, although not calibrated or traceable to any standard.

Then make kind of a "shadow box" of a known length with the LED at one end and an opening at the other.

Then measure the light output at the end of the box with a known calibrated meter. This should then be more than close enough to check the accuracy of in-camera meters, at least at one point on the scale.

I think this is something that mortals can afford.

Any comments on this approach?

 

[dmr]

Uh... "Rulers"?;

I daresay that the common ruler or meter stick is accurate to well within 1%, correct.

"Freezing point of water? (actually it is the "Triple Point" of water, defined as +.01 Celsius) ...

Hmmmm ... As I think back, I do recall that term from physics years ago. I don't really know how accurate the common thermometer is, but my guess is that they are usually well within a few percent of being correct.

"Sunny 16"?

Now this is the one I question. From another thread on another system about the Johnson Exposure Calculator, it's obvious that there are several variables involved that "sunny 16" does not take into account.

I'm just looking for something that is reasonably close, kind of like the ruler, measuring cup, or thermometer. I guess that is a recently-calibrated light meter, huh?

 

[Ed Sukach]

Who made these things ... ???

We had so much equipment from so many suppliers ... I remember General Radio for their Sound and Vibration Meters, Federal for their Gage Block Comparators, International Light for their Photometers, Ealing for Optical Test equipmment ... I think many power supplies were made by Hewlett-Packard. Whooosh!! Memories - and how much I have forgotten...

Now ... as far as Exposure Meter Calibration ... I can't recall EVER seeing anything like a Calibration Certificate specifying anything else other than "Complying with Manufacturer's Tolerances" or "Meeting ISO Standards".
Before I'd even attempt to determine the accuracy of another meter, I would need *some* information about deviation from a known standard at various points of the scale - i.e. "At a Reference Point of f/8; 1/125th second; ISO 100, this meter reads f/8.3". That should be done at at eight or ten ...(? maybe more..?) points through the measuring range.
Even at that .. there would be a HOST of other factors - spectral sensitivity for one - that could have a MAJOR effect on the results.

Now ... eBay ... I don't know - somehow I think eBay is too much of a gamble as far as calibration equipment goes. Even power supply calibration is important - if not critical.

 

[Roger Hicks]

I've been thinking quite a lot more about this, hence the long post.

A: Accuracy

We expect and get surprisingly high accuracy we expect from a foot-ruler (maybe +/-1% at 1 inch or 25mm, as little as +/-0.15 per cent at 12 inches or 30cm for a finely graduated engineer's steel rule) and from a quartz clock: a mimute a week, entirely commonplace, is 1 part in about 10,000. These are WAY greater than the accuracy we expect or get from an exposure meter.

B: Standard source

The original standard candle was of spermaceti wax, weighed 6 to the pound, and burned at 6 grains per hour. By modern scientific standards it was hopelessly inconsistent but as far as I am aware a modern paraffin wax household candle of similar weight, with the wick trimmed for maximum brightness without smoke, is very close to a standard candle, and a standard candle exhibits remarkably little variation, probably less than the +/- 1/3 stop of an 'in-spec' light meter: the Ainger Hall spot meter used a candle flame for regular calibration. With a standard candle and a darkened room, it should be feasible to meter the flame directly; or to construct a transmission target without further assumptions (measuring the opacity of the diffuser directly); or to make a reflection target with the assumption that bright white paper reflects 90 per cent of the light falling on it.

C: The usefulness of such calibration

First, it is at a single (low) colour temperature. From memory the standard white light for photometry is filtered to 4850K, and variations in colour temperature could make for significant variations in meter reading, depending on the cell's sensitivity.

Second, it is a single fixed point. The intelligent way to use it would be for shadow speed point calibration for negative film and highlight speed point calibration for colour slide, but many would want to use it to calibrate to an 'average' reflectivity of 14 per cent and a good few would use 18 per cent because they know no better. The big problem is that the meter might have very different sensitivity at different illumination levels, and there is no calibration for linearity.

Third, the exposure meter equations make a LOT of assumptions. The first, concerning image brightness at the film plane, includes object-to-lens distance, lens transmittance, camera flare, lens barrel vignetting, and the average angle of off-axis rays. The film speed equation also incorporates a constant K and a safety factor. K in turn was based on psychophysical testing, i.e. popular acclaim.

Better still, many of these assumptions have changed over the years with new generations of standards: for example, as well as K (now represented by K0) we have K1 (sorry, can't do subscripts) and the 1971 ANSI standard makes specific reference to different values of K for projected film.

All assumed values are based on 'typical' values and may depart quite widely from the actual values encountered by a specific photographer using specific equipment: this is why manufacturers clearly state that ISO (and earlier ASA or DIN) values are a starting point only. Anyone who wants to see the equations in all their glory, with an explanation of the associated assumptions, can find them (among other places) on page 79 et seq. of Perfect Exposure, Hicks & Schultz, David & Charles/Amphoto 1999.

All this explains why such things as the ZS and BTZS exit, and why personalized EIs are a good idea. It also explains why I suggested early in this thread that the best idea was to go out and take pictures using the meter. I should have added, 'after the most casual of tests to reassure yourself that it is working at all, whether you use Sunny 16 or compare it with another meter.'

Cheers,

R.