Exposure Meter testing…any affordable devices available,or any shops still doing this

John Koehrer said:

I haven't noticed anyone mention the ANSI tolerances which are on the order of +/- 25%.
When I was doing repairs we used a standard light source built into a Kyoritsu tester.
It used transmitted light not reflected and in addition to the brightness being variable there was a "K" factor that would also change the value perhaps by slightly changing the spectrum?
Anyway if you have two meters differing by 1/2 stop, each of them could be within tolerance but at different ends of the scale. If you want them to agree you would need to send them in at the same time & have them calibrated to each other.

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K is just a light loss constant that factors in such variables as the light loss as it passes through the lens and the accuracy of the exposure meter. There's a fairly large equation in the standard to calculate K, and in the 1971 standard's appendix, it recommends removing the most changeable variables from the equation to create a stable value of K, they label as Ko. This saves you the trouble of working with too many numbers. This way you are only working with the two that change most often, and they are the actual transmittance of a lens and, most importantly, the spectral response of the photo cell in the meter.

The idea of K is that when the meter is aimed at the calibrating light source and it doesn't read the light source correctly, you add a value that makes up the difference which is called K.

According to the most recent standard, there are two ranges of K. Each has a range of +- 1/6 stop.

Mike, a T.V. remote should do the trick. I just checked mine and found a 1/3 to 2/3 stop change in the meter's response.

I've checked mine also Stephen,and did get the same response around 1/2 stop. In the real world is that going to make a big or small difference. Considered all the other variables we have to deal with,in accurate shutter speeds would seem to be more of a bug-ah-boo than worry about small amounts of infrared bouncing off green leaves in the shadows.I know Pickering used that as one of the corrections made to his zone vi modified Pentax meters, but if you are a where of the the meters fault it is easy to correct it by opening up 1/2 to 1 stop.

mike c.

Please, Mike, don't use "pickering" (Fred Picker?) as a source for much of anything aside from how to sell stuff. Fred was a good salesman, he put together Zone VI - some of their stuff was quite good other stuff not so. But he was not much of a photographer and was full of a lot of hot air.

As far as light meters go, by far the best information I have found is in the out of print book by Dr. Richard Henry. If you can somehow get hold of a copy I would strongly urge you to do so. I can't always understand what he says (my head hurts with too much math) but he had a lot of correspondence with the Japanese designers and builders of light meters. I'm not really clear on the infamous (my term) "K" beyond the fact it is a constant which seems to have varied from 1.00 up to as much as 1.5, depending on the manufacturer at the time. I think the variability is no longer, but I really have no idea.

As for getting a standardised light source, again Dr. Henry had an exceedingly clever and inexpensive idea using a GE household bulb - which has been changed. While one of the posters on this thread poo-poo's Dr. Henry's method, it is described so well in his book that I was able to replicate his results exactly. He obtained a lot of very expensive equipment (micro-densitometers and the like) and, having spent much of his life being a research chemist he was very well versed in scientific methodology. Fortunately for me his book in the 2nd edition came out when I was more able to spend time and money in the dark, so I purchased two of the lamps he specified which I have carefully kept safe, and periodically I check my Pentax meter using them.

One of the interesting things Dr. Henry found was that many repair shops had very old equipment for checking meters, and that much of it hadn't been recalibrated for a considerable period. He also compared the Zone VI modification of the Pentax with unmodified Pentax and, if I read correctly, found the unmodified meter superior (slightly). So I wasted my money.

Looking at his book again I see that he says he visited and/or talked with 12 or so repair shops and found one which said they didn't know how to calibrate meters so sent them to the factory. About 3 were 'vague' about how they did it and the rest had only some of the necessary equipment and really weren't sure of what they were doing. I paraphrase all this, Apparently one needs to send a light meter to the maker for calibration. At least that is the impression I get. Unless you are fortunate, as I was, to get the information at a time when the requisite lamps from GE were on the market

Ok RJS,I will try to find DR. Henry's book if it is still available,it sounds interesting. Pickering was a salesman but he did have a few innovations to his credit that were beneficial to the amateur photo guy. thankyou for your patience am off to the dark room.

mike c.

Lucky you. Just a quick search on the net didn.t turn up a copy. Maybe you're better at searching than I.

Anscojohn said:

*****
Don't forget the Leudi!

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Here is a Leudi (end view),

Some good discussion here on light meters, but in terms of the OP, I was hoping, after 6 pages, to read some stuff like real experience with the intensity of contemporary wax candles.

There is a post in this thread about testing with candles: http://photo.net/leica-rangefinders-forum/008b1u

mike c said:

I've checked mine also Stephen,and did get the same response around 1/2 stop. In the real world is that going to make a big or small difference. Considered all the other variables we have to deal with,in accurate shutter speeds would seem to be more of a bug-ah-boo than worry about small amounts of infrared bouncing off green leaves in the shadows.I know Pickering used that as one of the corrections made to his zone vi modified Pentax meters, but if you are a where of the the meters fault it is easy to correct it by opening up 1/2 to 1 stop.

mike c.

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I was using the infrared example as an easy test to show the spectral sensitivity of the photo cell and am not implying that it is a major factor.

Most of the errors attributed to any one factor are generally small. The concern comes when there is an accumulation of such factors. Some of the time, they can cancel each other out. It's when they tend to all fall in one direction that real problems could happen. Under normal conditions, this isn't usually even much of a problem because of the slop involved with exposure. The failure most often happens when the errors combine in extreme shooting conditions where there is little room for error.

I can see your point and agree Stephen,I'm pretty lacks when it comes to equipment testing. It can get pretty expensive testing every thing so I'll sit back and follow this string and hopefully learn ways to save some money, that was the main idea of the post.

mike c.

RJS said:

As for getting a standardised light source, again Dr. Henry had an exceedingly clever and inexpensive idea using a GE household bulb - which has been changed. While one of the posters on this thread poo-poo's Dr. Henry's method, ...

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One of the reasons I hesitate to write here is the tendency of some to mis-quote and distort my intentions/ structure of my OPINIONS to agree with their preconditioning. I am not fond of "doing battle" with anyone, nor have I "poo-pooed" Dr. Henry's METHODS. I have not read his book, so I have no idea of what he has done.

Calibration of the run-of-the mill "Exposure Meter" is NOT as demanding as that of a truly accurate Cascade Photomultiplier. I've worked with attempts to use said exposure meters in place of the equipment necessary for what we were doing... each of those attempts failed miserably, but that is another story. In the evaluation process, we considered a number of factors... the most important being accuracy of measurement. We had a terrible problem in trying to extract information from the manufacturers. We did manage to wrest the claimed accuracy of Honeywell's original 1/ 21 Spot Meter - "1/2 Stop".
Currently, Gossen states that their "Ultra-Pro" is accurate (can be relied upon) within "1/3 of a stop".

One of the first requirements in ANY efficient calibration is to select a standard, and it is common to consider accuracy ratios ... the standard should be ten times more accurate than the measuring device - or at the limits of technology, four times is sometimes all that can be achieved.

Would a run-of-the mill household lamp, powered by a non-descript supply, monitored by what has to be considered - I'm trying to think of a word - I LIKE Radio Shack gear, and within its intended use, it is probably very good - but here ...) inadequate Volt meter produce energy within "1/30 stop" (ten times) or even 1/12th (four times) of a stop"? Possible - certainly POSSIBLE - but RELIABLY? I'd have to see objective results of that before I'd believe it.

I HAVE investigated the amount of light seen in "Sunny 16" with variations as high as -1 Stop to +1/2 stop.

All this is why I cringe when I hear "Uncle Fred" from the local Camera Club stating adamantly, with "anyone who disagrees with me will be skinned alive" conviction, that "my exposures are always `dead on'. I never allow even 1/10 stop either way". In truth he would not know a 1/2 stop error if it came up and bit him on his ...

Now ... a simile: "Measuring for exposing film is like using a rubber ruler to measure a bowl of jello.

Now ... wherever you go, there you are. This is where I am.
i

mike c said:

... Considered all the other variables we have to deal with,in accurate shutter speeds would seem to be more of a bug-ah-boo than worry about small amounts of infrared bouncing off green leaves in the shadows.

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-And inaccuracy of "f/stops" ...

For some reason I've always assumed the aperture was more reliable than the shutter,although most of my exposers on MF are prereleased to add a little bit of safety to combat Murphy's law ,although Ive never ask the tech guy about the aperture when taking in the camera for a CLA,(which is not very often I admit ). Ed please don't misunderstand me ,you do sound like you are very knowledgeable on this subject and I have respect and grateful for you taking the time to respond to the comments of me and others on this subject. Photography is only my hobby now,and I enjoy it alot.

mike c.

I hope I have not misquoted or distorted anyone's opinion, and that is certainly not my intent; particularly not anyone of such comparative youth! I think it terribly unfortunate that Dr. Henry's book seems to have gotten such poor circulation. He and Ctein are about the only authors in the photographic field who offer much more than opinion and, I think, rather truncated 'research'.

I still have no idea what this "cascade" thing is, although the name is quite impressive. By way of clarification, I am not anyones "Uncle Fred" nor do I threaten people who disagree with me. Sometimes they even cause me to change my mind. However in this matter of 'calibrating' exposure meters I have seen nothing to refute Dr. Henry's information. I'm sure I have mis-paraphrased him, and short of somehow faxing pages of his book I know of no other way of communicating what he wrote. I have enjoyed exchanging opinions with people posting here and have often learned. Unfortunately I have also learned there is a considerable amount of misinformation circulating which, I suppose, is harmless.

By the way; I don't think anyone I am talking about expected reliability or accuracy within 1/30 stop. I'm sure I can't see it in a print and none of the instruments referred to would show such miniscule variation. That would be a whole different level of investigation, certainly not any kind of practical photography of which I am aware. Maybe EK when they were really doing stuff had use for such, I have no idea. I was only a social scientist and these things are way beyond me.

RJS, your opinion is most appreciated, its pretty much up to the reader to decide for himself what is important. You have sparked some interest in me about Dr Henrys book,I'm a little like you about using the internet but that is one of the benefits of having younger daughters and sons around.


mike c.

RJS said:

I still have no idea what this "cascade" thing is, although the name is quite impressive.

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"Cascade" refers to a system where individual ranges can be determined, and the instrument set to read far more accurately through that particular range.
As an example, one might take a preliminary measurement and determine that the value measured, coarsely, appears to be 21 watt-seconds (I am simplifying here!), but the resolution of the scale in use is one watt-second, so it is not known whether the correct value is 20 or 22 w/s. The meter is then switched to a finer scale, with a total range of one w/s, and a resolution of .1 w/s. From that point, still finer measurements can be made by switching range. An example might be a clock with a second hand (the hour hand would by "primary, the second hand secondary), another hand to show fraction of a second, etc.
Sophisticated stuff.

BTW, the primary voltage I was talking about: 1800V. Definitely a "wake-up" call.

I am not anyones "Uncle Fred" nor do I threaten people who disagree with me.

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I did NOT intend that to be directed at you - It was directed towards ME... an attempt to support my reaction to some situations I have encountered. Whether or not there is some amount of "Uncle Fred-ness" in Dr. Henry's work, I do NOT KNOW. I can only offer my opinion of the adequacy of calibrating an exposure meter from the information given here.

By the way; I don't think anyone I am talking about expected reliability or accuracy within 1/30 stop. I'm sure I can't see it in a print and none of the instruments referred to would show such miniscule variation.

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Neither am I. "1/30 stop" measurements would be gross overkill and totally unnecessary in ordinary, or even the very finest picture taking work in which the great majority of those here might be involved. For CALIBRATION of a measuring instrument, however -- It may still be unreasonably "fine", but it should be a consideration in -- the name of the game, "Error Management".

Mike,

Ed's phrase "error management" and his advice is dead on. Best way to save yourself money is not to worry too much about the accuracy of your equipment. Unless you are using something made before the 70s, the equipment should be fine.

How accurate does the equipment have to be, or for that matter, can be? One thing many people tend to miss is that the meters we use are called "exposure" meters. "Light" meters are very different. I once looked into obtaining a light meter to calibrate my sensitometer. It would have cost over $4000.

My advice is to understand what factors are in play and what affect they will have so you can better "error manage". Understand there is no such thing as perfect exposure, and three of the biggest variables have nothing to do with your equipment. They are the values associated with the actual scene which include illuminance, luminance range, distribution of the luminance range, and flare (very overlooked), your own personal methodology or how you tend to meter, and personal taste. Believe me, your personal approach has a greater influence on exposure than anything else.

Do some research into exposure theory. The knowledge will help to elevate any anxiety you may have about the concept of accurate exposure. Photographic Materials and Processes is a good book to start. At one time, it was my bible. I used it so much I wore off its binding. Dunn's Exposure Manual goes into great detail and has excellent insight. The Manual of Photography is full of detailed technical information (although it tends to use photographic nomenclature used in England which can be confusing).

RJS,

It's been some time since I've read the Henry book. Like you, I appreciate his scientific approach. I hate the formatting of the book though. It made it harder to read. Still, the book remains one of the better popular books on photography. As you seem to be interested in some of the more technical aspects of photography, here are a few papers about exposure meters that you might be interested in. I don't know about you, but I personally like having the source documents. I've listed them before, so forgive me for the repetition.

Connelly, D, Calibration Levels of Films and Exposure Devices, Journal of Photographic Science, Vol 16, 1968.

Nelson, C.N., Safety Factors in Camera Exposures, Photographic Science and Engineering, vol. 4, no. 1, Jan-Feb 1960.

Stimson, Allen, An Interpretation of Current Exposure Meter Technology, Photographic Science and Engineering, Vol. 6, No. 1, Jan-Feb 1962.

Of the three, I like the Connelly paper best. Although Stimson chaired the ANSI committee for the 1971 Exposure meter standard and much of the appendix in the standard comes word for word from his paper so it's rather good too.

Ed Sukach said:

I have not read his book, so I have no idea of what he has done.

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Find one, it's quite an interesting book.

Thanks Stephen for all that reference material,I don't have a lot of technical back ground, and I will look into the reading material you have posted. Thanks for taking the time to replay.

mike c.

My thanks also, Stephen. Unfortunately it's been a long time since I have done any research in a library, and I have no idea now how to find journals since the arrival of computers on the scene. I spent many hours in the stacks at USC when I was in graduate school, but I'm sure it has all changed. If there is a way you could point me using my computer I would be most grateful. I do enjoy reading the source material even though it is often (if in the physical sciences) over my head.

I had forgotten Neblette's book; mine too is well worn, but I thank you for reminding me. I'm going to have a look, and also at some other books I had forgotten I had; Todd and Zakia, Lobel & M.Dubois and a James Higgins book. Perhaps you know them. It's been years since I looked at them but I'm sure they have a lot of information that I have at least some of hidden in the recesses of my aged mind.

'Cascade' sounds a little bit like the idea behind the vernier caliper. While in graduate school I worked as a machinist for several aerospace companies (I was a fraud - a rotten excuse for a real machinist) but it helped pay the expensive tuition at USC. Anyway, I think I can see how that process could work to refine measurements.

Which, I think, leaves us no closer to finding a 'standard' light source to check our light meters. Apparently these days the meters built into the better digital cameras are quite good and can serve as a standard. I don.t have a digital camera but that's not the point. I would guess that you, Mike, do have one and can work back from there. Otherwise, this thread, while interesting, entertaining and informative, has not answered what I think was the original question. Or has it?

Keep tuned in!

[
Which, I think, leaves us no closer to finding a 'standard' light source to check our light meters.

*****
True enough; unless someone can provide sourcing for sperm oil calibrating candles; or the GE bulbs no longer made but mentioned above.

That get's me back to Sol Invictus. I HAVE to believe that the sun, about high noon on a clear day, when used as a "single point" light source has GOT to be close enough to a standard that it can be used.

NB: I am not referring to "Sunny Sixteen." Lumidisk, or Lumidyne, can't remember which, gave a method for calibrating your camera with their unit in place by pointing your camera, with their disk screwed in to your lens, directly at a high sun.

RJS,

Wow, I know what you mean, I've totally forgot about the Neblette book which also happens to have a similar title as the Strobel, Compton, Zakia, and Current book. Just found the James, Higgins book too while double checking the spelling of "Strobel." Ah, the memories. While we're at it let's not forget The Theory of the Photographic Process, especially the first and third edition. I got lucky some years ago when I found a number of titles I had been endlessly searching for show up in a local used book store. They told me a JPL imaging scientist had recently died and they had gotten his library of photography books. Probably not so lucky for the JPL guy though.

I'm also a USC alum and I've never liked their library system. This maybe heresy, but I found most of what I wanted over at UCLA. Their science and engineering stacks at Boelter Hall has most of it, but they were starting to move the ancient photography journals over to the Southern Regional Library Facility on the far side of campus which is where books go to die. Actually, they're very nice there, but they have extremely limited hours and a crappy copy machine. UCLA has an excellent search engine on line if you want to track down those papers. You'll still need some shoe leather and a pocket full of dimes in order to get your hands on those papers. If you're interested, I could send you a few more titles, you know, as long as you're going to the trouble to be there anyway.

A few years ago, I found a web site that sold camera calibration equipment. They had a nice table top exposure meter calibrator. I've been looking for the site again over the last few days, but it's nowhere to be found. The researcher in me would loved to own such a toy. It was pricey, and truthfully, I really don't need it. My exposures are good enough most of the time. I figure if I had an old meter or a questionable meter, the best thing would be to take it to a professional, like Quality Light and Metric, for a quick and accurate check. For someone like Mike, I recommend him spending his valuable testing time on defining his film processing before delving into calibration minutia. Although I would still like to have a nice microdensitometer.

Unfortunately, Dr. Henry doesn't give a part number for that GE light bulb. He says GE "White 100 W light bulb producing 1585 lumens."

Walgreens lists a soft white bulb with the same lumens:
http://www.walgreens.com/store/product.jsp?CATID=303408&id=prod1159704#

And GE lists a soft white with nearly the same - 1600 lumens, the 97761 – 100W
GE Soft White with A19 base.

One thing I notice reading his setup is that there is no provision for blocking IR from the meter being tested. For linearity testing, it really doesn't matter, but if one is trying to calibrate to a particular light value, then it seems like it should be filtered out.

Stephen,advise acknowledged.

mike c.

Found something interesting. It's by Kyoritsu:
EF Series Collimators
Kyoritsu and Gokosha make the finest lens collimators
available. They are used in the repair, design and
manufacturing of film, digital, and video cameras and lenses.
The EF series camera testers are made for repair, research,
development and quality control. They include exposure
measuring testers, multiplex exposure and analysis system

The catalog can be found Dead Link Removed.
Dead Link Removed

or at Dead Link Removed
Dead Link Removed

Here is some info on sun variability. The first graph is all the radiant energy over the course of years. The second shows just the UV (to which light meters will respond) over the course of a day:




I still think a candle is going to have less variability than the sun. And I think different candles will very less than different light bulbs.

Perhaps I'll check all the candles in my home and see how much variability I get.