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dmr
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 03, 2006 - 07:22 pm: | 
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I'll ask this here since I seem to get a resounding "I dunno" wherever I've asked it other places. 
Is there a way, without resorting to super expensive laboratory grade standard light sources, to do a reasonably-accurate calibration or calibration check on a light meter, specifically those in manual cameras? In general, I've been given three answers:
1. Buy a (handheld) light meter and calibrate/check from that. Unless I buy a new or known calibrated one, same story, I don't know how accurate it is.
2. Check it under sunny-16 conditions. I'm hoping that I can get something closer to a known standard than this. I keep hearing that noonish sunlight on a clear day gives you consistently 80,000 - 100,000 lux, which is about EV 15 at ISO 100, etc.
3. Compare it with a camera that gives good exposures. This is what I have been doing, but it seems so un-scientific. The camera I used for a reference is a 20+ year old Pentax K1000 which does give consistently good exposures. I really have no clue how accurate this really is when compared to a standard.
Any suggestions?
Thanks in advance, gang. |
rick
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 03, 2006 - 08:40 pm: |
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I made a "standard" light source out of a LUXO lamp and a piece of opal glass (plus miscellaneous stuff to put it together). Then I made 2 dimming masks out of half-tone screen, with a different pitch so that I can stack them to get an additive effect. This gives me 3 light levels: full, one-screen, and two-screens down. The 3 values I happen to have wound up with are (ISO 100) f/16-250, f/5.6-60 and f/2.0-15... a good general purpose range that goes beyond full sun at the high end.
I don't know how easy it is these days to still find halftone screen; something like fine wire mesh would work well too.
I established the "correct" light values by checking it with several, relatively new and very good cameras and meters and, if there was any disagreement (there wasn't much), making a value judgment as to which was the better number.
This still leaves me with the question of whether the brightness of the bulb stays constant over time. It isn't on much, so there shouldn't be too much drift, but it doesn't hurt to recheck it periodically to make sure that you still agree with what you're getting. |
dmr
Rating: N/A
Votes: 0 (Vote!) | | Posted on Monday, July 03, 2006 - 09:36 pm: |
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Thanks, Rick. Wow, fast reply.
Stupid question here -- what exactly do you mean by a Luxo Lamp? I googled it and got Luxo Corporation, and they make all kinds of lights and such.
Yes, I do know what a halftone screen is. When I was in high school, I worked in a print shop. I have no clue where to get one now.
Your idea seems a lot more accurate than what I'm using -- the blank wall of the bathroom with the overhead and vanity lights on dimmers. I finally figured out how to physically rotate the meter on a QL17 GIII (thanks to searching here) to set the low end of the scale, and I set it to a perfect ISO 400 1/30 f2.8 on the Pentax and then my roommate went in and shut it off!  She thinks I've totally lost it tonight, going back and forth in there with one camera and then another.
Anyway, thanks, Rick, that gives me some ideas.
Oh, and may the 4th. be with you! |
Glenn Middleton
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, July 04, 2006 - 06:28 am: |
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For a very constant light source, use a LED torch to illuminate the opal screen. I got this idea when I picked up a 6 LED torch that was part of the manufacturer's display. It was dirt cheap because the display torches did not have the battery carrier in them. Removed the head complete with electronics and run the LEDs off a small plug in transformer/charger. The dimming masks are wire mesh screens. These were removed from the carbon arc illumination lamp on a Vickers metallurgical projection microscope of 1930s vintage- a true piece of furniture, non of your modern bench stuff!
Have since found that any 3/4 LED torch and a stack of ND filters will work. The size of the opal screen is the factor that decides the number of LEDs - you obviously want an even illumination.
I hope my US contacts are having a very fair day. |
rick
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, July 04, 2006 - 09:56 am: |
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great idea, glenn!
: ) = |
rick again
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, July 04, 2006 - 10:02 am: |
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the LUXO lamp that i used was just what i happened to have: an elbow-arm desk lamp with a cone-shaped reflector about 6 inches in diameter. i made a block to hold a 4x5" opal glass on it, and threw away the elbow-arm part.
(Actually, i had made all this to create a lamp housing to put on my 4x5 Speed Graphic to enlarge 4x5 negatives with, but i never used it for that... so when i needed a light source for meter testing i already had it lying around. i just cut the dimming screens to fit the negative carrier and mounted the whole thing on a table top tripod to hold it at a convenient angle. With my enlarging bulb in it, though, it gets AWFULLY hot, and once when i failed to turn it off promptly i cracked the opal glass. this makes it useless for enlarging but it's still okay for meter testing. i do like the LED idea though, constant output and no heat.....) |
M Currie
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, July 04, 2006 - 10:52 am: |
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The best Luxo lamps are the ones with both a fluorescent and an incandescent bulb. Artists like them because they have good color balance.
I usually just aim at my dining room wall, which is featureless and off-white. I compare with a couple of known accurate cameras - my wife's F100 and my wonderfully accurate little Minolta X370. These two usually agree, and if I can get any meter to agree with them, then it's close enough.
Most of the cameras I'm working with have rrelatively coarse adjustments anyway, and averaging or center weighted meters. In real use, it's rare to be able to come closer than a half stop anyway. |
Arnold Harris
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, July 04, 2006 - 01:40 pm: |
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What's wrong with testing a light meter with a second light meter, calibrated by inserting a fresh battery?
This is a question, not a statement. I know little about light meters, and perhaps my question reflects naivete. The ones I have used over 35 years are exposure meters built into the Voigtlander Vitessa-T 35mm rangefinders that constitute nearly all the cameras I have owned and operated. These are powered by selenium cells.
All but a couple of my 42 Vitessa-Ts have exposure meters that seem to function well. As I CLA and, as necessary, rebuild all these, I expect to test the built-in selenium cell units and their galvanometers against a newer and mountable Voigtlander battery-powered light meter.
Unfortunately, the digital scale on the latter device is in f-stops rather than exposure values (EVs) as I am used to. However, the Compur lenses have f-stops imprinted on the bottom and exposure values on the top of the aperature setting rings. So I expect to know with sufficient accuracy whether any of my old selenium cell exposure meters need adjustment or selenium cell replacement.
Frankly, I have been surprised to learn that these old selenium cell exposure meters still work at all after emerging from their German factories 50 years ago. But some knowledgeable people I have read think the selenium wafers are affected more by moisture and corrosion than by light. I also understand the selenium cells and their potted connections can be cleaned off with acetone. Has anyone on this forum tried that?
In any case, new selenium cell wafers can be ordered in exact sizes and shapes from Magnetron in the UK, and Quality Light Metrics in Los Angeles CA can adjust the unit (the galvanometer, presumably) for old camera owners who can remove their light meters from the camera and ship them to Los Angeles.
Arnold Harris
Mount Horeb WI |
David Ritchie
Rating: N/A
Votes: 0 (Vote!) | | Posted on Tuesday, July 04, 2006 - 03:02 pm: |
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Do not get too hung up on the calibration of light meters. A while back I bought several light meters of various makes and models, and quickly learned that no two gave exactly the same readings. These meters were high end made by Minolta down to the less expensive ones. Most of the better meters gave readings within a third of a stop of each other, but I do not recall any that were identical. I figure that this deviation is acceptable to the meter manufacturer, and for print film is ok. You would need to do a few tests to determine your own settings for slide film. Several years ago I recall returning a new meter to the manufacturer 3 times till they got it right. I was using a lot of slide film at the time. Do not forget that a meter that might be right on at a particular light level may be off at a different light level. I guess the message here is to calibrate the meter as close as you can and then test and adjust your camera settings to get the results you want. |
paul ron
Rating: N/A
Votes: 0 (Vote!) | | Posted on Wednesday, July 05, 2006 - 03:59 am: |
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I use my ceiling 8' florecent lights at a constant distance to calibrate my meters for the past 30 years. Just be aware that color temps do make a difference, I use daylight lamps but still be aware there are variables so don't use it as a definative since age of the lamps makes a difference too. Use it as a check but compare as well. |
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