How to test my light meter?
I have an old Gossen Luna-Pro F meter and I am trying to decide if I need to replace it.
I have tried metering a scene with different cameras and the Luna-Pro and comparing the readings but I can't seem to draw any conclusions.
I need a meter when I shot my RB67 and I am under the impression that I will get more accurate readings with a hand held meter than a TTL camera meter????
Is there a method to test the Luna-Pro that is not so complicated that I need a degree in physics to use?
I have been looking at the Gossen Digi-Pro F or the Sekonic L-308S but why spend the money if my old Luna-Pro is just as accurate? Maybe I just do not know how to use it correctly???
One testway is to shoot a roll or two in a variety of situations using the meter "normally" and see if the results match your expectations.
If the shadows and neutrals fall "in the right spot" it's probably working fine.
This assumes a "standard" is used when you develope and print too.
Mark Barendt, Ignacio, CO
"We do not see things the way they are. We see things the way we are." Anaïs Nin
A very crude check of reflected light meters is to hold the cell against a flourescent light. The reading should approximate the "sunny sixteen" exposure. A better check is to compare the meter in question against meters known to indicate the correct exposure. TTL meters can certainly be as accurate as most hand-held meters under average conditions. My first camera with TTL metering, a new Nikon F Photomic T, exposed Kodachrome a little more accurately than the Weston Master III I'd relied on for some time.
I definitely don't have a physics degree. You could try reading off a matte black card, 18% gray card and a bright white card and compare the readings. Use the 18% gray card reading and set your camera to that and photograph each of the cards. Pick a scene or subject that has a decent range of tones and take an incident reading and take shadow, mid-tone and highlight readings adjusting for the zone each falls in. Develop the film and contact print the negatives -- see what you get. You could also test by photographing the gray card having over and under exposing by half stops up to say 2 1/2 stops each way as well as the gray card reading. Cover the lens and click off a couple of frames which will give clear negatives plus the film fog. Make a test (step) print from the clear neg. see where it turns black and use that for your printing time. Cut pieces of print paper and print each negative at the same enlarger height, time and lens opening (having labeled which stop of the film it was). Then see which one of the prints is the very closest or hopefully right on matches the gray card. That will be the ISO for that meter, film, camera and chemistry.
I'm sure there are more scientific ways to do this but this is simple and doesn't take long to do. It could be repeated for other films. All of course B&W. It should give you an idea of the equipment's accuracy because it takes in to consideration the meter and camera. Which will carry over to color (I think). For color you could take transparencies once you have the accuracy of the equipment established and if they look right no worry.
All you need is a grey card and a cell phone. Hold the grey card up in front of a subject and shoot it in black and white. Find a comparable tone with the grey card and meter both that part of the subject and the grey card and see what you get. You can then meter up and down zones and see how they fall by comparing to some sort of zone strip. You could make one of those by step printing in the darkroom or just buy one on the cheap.
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Take it outside and set it for ISO 100 and check the reading. It should be 1/100 or 1/125 @f16 in bright sunlight.
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I tested my Luna-pro F against my F100, FM3A and F65 using a grey card, making sure the cameras and the light meter were the same distance from the card, from the same spot, and that the light isn't changing between readings. Make sure the lenses on the cameras match the angle on the light meter (I think the angle on the Luna-pro matched an 80mm lens). Make sure all the adjustments on the meter are set to zero. After a lot of fooling around I was satisfied that the meter was very close, but there was even a small difference in the readings between the cameras so no readings matched up exactly.
It wouldn't have to be a grey card, a towel would do just as well.
If I had been present at the creation, I would have given some useful hints for the better arrangement of the Universe.
Alfonso the Wise, 1221-1284
When it comes to using a light meter to create an accurate exposure regime, out and out accuracy is not as important as consistency.
Originally Posted by stradibarrius
A light meter may be "inaccurate" by one or more stops and yet be perfectly good enough to create an operable exposure regime for a specific camera. It would only start to go wrong if you attempted to apply the same exposure regime to the camera using a different (maybe accurate) light meter.
Creating an accurate exposure regime for a specific camera/light meter combination is, in effect, setting up a closed loop ; change one element and the loop breaks...
I have used a Luna Pro-F and have used it for years. The meter has always performed flawlessly. I have tried to check it against other meter with limited results. You have to use a uniformly lit surface with a uniform color. The Luna Pro-F seem to have a pretty wide angle of view and that makes it hard to compare the reading with other meters. I got a Vari-Angle attachment which turns the meter into a semi-spot meter with a 7 & 15 degree angle of view. Since getting the Vari-Angle it has made comparing much easier, you know exactly what the meter is seeing. I have compared the meter with my Sekonic L-558, F100, N80, D300, D50 & 645ProTL and they are all within 1/3 stop of each other. (all those cameras have spot meters)
One thing you need to do is zero in correctly, if you don't it will be off. Download the manual from the link below it tells you how to zero it.
Pardon my ignorance, my sensation is that the Gossen Luna-pro being a lightmeter with silicon blue sensor and integrated circuits, if it works, it should work just as factory new. Electronics don't "decay" with years. Selenium meters might lose sensitivity, I don't think SBC metres will show any kind of decay. So basically you are just checking for damage (needle out of calibration, "lens" displaced, who knows) that would put reading sensibly off.
Anyway, for testing any lightmeter, I would just do this:
- Find a large surface of uniform neutral colour: a white or a grey wall, that is, in shade (you must be in the shade as well). ("neutral" colour because CdS metres respond differently to different colours. SBC light meters should not have this problem, or have it very much reduced. Neutral is neutral anyway).
- Measure reflected light from a distance of at least a metre, paying attention to the angle between lightmeter and wall. Keep it orthogonal to the wall.
- Repeat measure with other cameras, and lightmeters, you have, from the same distance, paying attention to the angle.
You need a uniform wall because this way, the larger or the smaller the angle of reading, you can compare readings;
You need to be a bit distant, let's say at least 1 metre away, so that small differences in the angle of incidence does not play a role, and so that small lack in uniformity in the wall do not cause two different readings with two different lightmetre having a different angle of reading.
You need a neutral wall so that you can use CdS metres for comparison.
You need a wall in the shade so that it is more uniform, and there is less parasite light entering camera viewfinders (obviously you must be in the shade yourself).
All that said, you should have the same reading from all lightmetres, and maybe from all cameras. You might see small differences between lightmetres, but no more than 1/3 EV. I would be more careful in comparing readings from cameras, as those can be influenced by lens vignetting or lens flare (you can have slightly different readings when you change your lens) and by parasite light entering the viewfinder if you have the sun at your back.
I would not use grey cards, as a reading from a grey card is influenced by the angle formed by sun - card - camera. Grey cards are tricky, basically not very reliable, objects, unless you mount them on a tripod, stick them to a wall etc. so that you make the readings with exactly the same angle of reading and the same angle of incidence of the light. Also be careful not to project a shadow on the grey card if you use this method. With grey cards, a small movements in the card, changing the angle of reflection of the light, changes the light that is measured and this is possibly why comparing lightmetres is unreliable with grey cards, you can easily get small random differences.
A wall in the shade is more reliable because it is in a fixed position, so you only have to check that the angle of the lightmeters-cameras that you compare is always the same.
That's what I would do.