I use the hair for metering shadow detail. Mid tones and shadows on one portable device.
Portable, but a wasting asset in my case and with many other middle-aged/old men. No longer as dark as it was, either.
Originally Posted by jmal
Maybe you can answer this, and this has been bugging me lately.
Originally Posted by Ed Sukach
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 entirely agree with you, Roger, but let's recall that the OP's question was how he could judge whether his camera's built-in meter is working correctly within its design limits. The question was NOT whether the meter is delivering perfect exposures for both slide and negative film in all cases without operator intervention of any kind, which as we all know is impossible. The reading from any meter will need creative interpretation to give an optimum result.
Originally Posted by Roger Hicks
Well, I was making the not unreasonable assumption that the reason he cared about his meter's accuracy was that he wanted to take pictures with the camera.
Checking it against another meter using a sheet of white paper or (as another reader suggested) an eggshell white wall or even a grey card will not tell him that his meter is within specification: it will merely tell him how well it agrees with another meter. Unless the other meter is known to be accurate AND reads the same way, this does not tell him a great deal. And even if both meters are within specification, as Ed (I think it was) pointed out, they might disagree by 2/3 stop.
Hence my advice to take pictures, and hence my observation (which many people do not realize) that a meter which gives perfect exposures for trannies (chosen for their lack of latitude) will not be giving the optimum exposure for negative at the same reading.
Yes, I'd do a quick-and-dirty comparison test with a large sheet of white paper, but I wouldn't rely on that to do much more than tell me that the meter was working reasonably well, not that it was accurate.
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I consider the following to be minimal:
Originally Posted by dmr
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"
Ed Sukach, FFP.
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?
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?
I daresay that the common ruler or meter stick is accurate to well within 1%, correct.
Originally Posted by Ed Sukach
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.
"Freezing point of water? (actually it is the "Triple Point" of water, defined as +.01 Celsius) ...
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?
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.
Ed Sukach, FFP.