How to Check Your Meter for Accuracy
by James Ollinger
A lot of people email me and wonder if their meters are accurate. Here's a relatively simple way to find out:
Test it against another meter that you trust.
It's easy to forget, but if you have modern camera, you likely have a meter you can use for this test. Almost any SLR made since the early 60s will have a built-in meter. Many recent digital cameras can tell you the exposure info even if you're shooting in automatic. All you need to do is read it.
Here's an example using an old-ish camera. I'll pick my Canon AE-1, which is an SLR made in the late-1970s. It runs in a semi-automatic mode called "shutter priority," which means that if I choose a shutter speed, it'll pick an aperture to go with it.
So what I'm going to do is this. I go to an outside wall of my house which is lit by the sun. It's painted light gray and it's blank. I take the Canon and I put a 50mm lens on it, which is "normal," meaning that it covers the same angle of view as my eyes. By design, most hand-held light meters do the same (though many are actually a little narrower).
I stand about 3 feet from the wall, and position myself so that I don't throw a shadow on it. I focus the lens to infinity so that all I can see through the viewfinder is this gray blur. But this gray blur is the same gray blur everywhere in the viewfinder. This will remove any bias the camera's "center-weighted averaging" might have on the exposure, since there's nothing to give extra weight to.
I set the Canon's ASA speed dial to 100. I could pick anything I wanted, but 100 is as good as any. And I'm going to pick a shutter speed. Just for the hell of it, I'll pick 1/125th. Now I aim at the wall, make sure everything is the way I want it, and I take a reading by pushing the shutter button half-way down. Inside the viewfinder, it reads about ƒ/11.
Now I put the camera down and I pick up the hand-held meter I want to test. Let's say it's a Weston Master IV. I set the calculator dial to ASA 100. Standing in the same place, I aim the meter at the same part of the wall that I used when I metered with the camera and I slip the needle lock off and I take a reading. I says about 200 footcandles. I dial that into the calculator and I get 1/125th at ƒ/11. It matches the AE-1, so I'm good to go. At least at 200 footcandles.
If you really want to work it out, you can try the same thing on an object that's less well lit and one that's more brightly lit and see how far off you are.
You're not limited to reflected light meters here. You could also try out an incident meter, like a Norwood. If you're sure that the subject is flat and evenly illuminated even in a small area, you could check spot meters too.
The important thing about using a wall, or some other featureless object that's evenly illuminated across the area you're metering (a piece of blank posterboard would work), is that most camera meters aren't just "averaging" meters the way most hand-helds are. My Canon AE-1 is center-weighted averaging, so it takes whatever's in the center of the image and gives it extra bias. Recent cameras usually use matrix-metering, where they break the area up into small pieces and the camera tries to figure out what the subject is based on the meter readings of each of those little blocks, and it puts together a reading based on that. The result may be something that will be significantly different than what a plain, averaging hand-held meter would come up with, even though they're both properly calibrated and working the way they were intended to when they left the factory.
What If My Meter Hand-Held Meter is Off?
It depends on how far off it is. If it's grossly off, then chances are your meter's no good. Look at what your camera meter's telling you—does it seem reasonable? If it is, then assume the camera's good and the hand-held's bad. If the camera meter's telling you something wacky, then maybe you need another camera to use as your standard. Or something else is wrong (you have it in the wrong mode, you're shooting through 8x of neutral density filters, etc.).
What if the meter is just a bit off? Like 1 stop? Try checking it across different light levels. If the meter is consistently 1 stop off, then you can make adjustments. Let's say the hand-held consistently reads 1 stop slower than the camera across the board. Okay—when you use the hand-held, you mentally add 1 stop to the reading. Or you know that you need to set the calculator dial to twice the ASA to get an equivalent reading.
The Bracketing Method
Here's another way to check/set your meter. This requires burning a roll of film if you're using a film camera, but you can do it with digital too.
Set your camera up on full manual mode and compose your test image. Pick a subject that you normally shoot. Pick something that you would normally shoot using your hand-held meter. In other words, if you're going to take your Kodak Stereo and shoot landscapes and you want to use a hand-held meter, put a roll of film in the Kodak and set up for landscapes. That way you're getting real-world data and you can compare apples to apples.
Set your meter up. Pick a film speed as your center; let's say we're using Ektachrome 100, so our center speed is 100.
I set the calculator dial on the meter at ASA 50 and take a reading. I set the camera for these readings and shoot an image. Then I change the calculator dial to ASA 64. Transfer those settings to the camera and shoot. Reset for ASA 80, read and shoot. Reset for ASA 100, reading and shoot. Same for ASA 125 and ASA 160 and ASA 200.
Another subject. Compose. Meter on ASA 50. Read, set camera, shoot. Meter on 64, read set and shoot. Repeat through ASA 200.
Continue until film runs out.
Now I have the film processed and I look at the results. I pick the best shot of each group and I should notice that there's a consistent pattern of which ASA gave me the best results. For our trouble, let's say it's a toss-up between ASA 125 and 160, with 125 being a little more common. Now I know: when I'm using this meter and this camera, I want to set the meter at ASA 125 even though I'm shooting ASA 100 film.
That's called determining an Exposure Index. The difference between an E.I. and ASA is that the E.I. is personal to you. It's what works best for you and your equipment. If you read much photographic literature on exposure, you'll come across this concept often.
The thing about ASA (and ISO) is that it's a "standard" that is really meant for manufacturers to build to, but it doesn't work perfectly in the real world. There are too many variables. Camera shutters are often slower or faster than their ratings. Meters drift or are mislead. Film speeds change with age and the chemical process used to develop film. Determing your E.I. factors all of that into the equation. It works with the equipment you've got, even if it's not dead-on-spec.
One thing: if you use the bracketing method, you have to be careful what you use to evaluate the results. Slides are great because they're direct-from-film. Prints, however, can be "corrected" when they're being made. Let's say you're doing your own printing: if you've got a negative that's 1/3rd stop underexposed, you can correct for it and get a good print; likewise you could louse up a "correctly" exposed negative. Worse: if you've given your film to a printer, you have no idea what you're going to get back. Most prints are made automatically by machine and they're all tweaked to give an "ideal" print. If you want to use the bracketing method with film, you'll need to evalulate the negatives, not the prints.
What About My Meter Which Doesn't Have ASA/ISO Numbers?
Doesn't matter, you can do the same thing. You have a Weston Master II which uses Weston numbers? Got a General Electric DW-48 with GE numbers? Got a Russian Leningrad that's calibrated in GOST? Hell—maybe you have an old Bertram set up in Scheiner. Compare it to a meter you know is good set on an ASA that you know and see what gives you the equivalent reading. Your camera says 1/125th at ƒ/11 at ASA 100. Your original Weston Master reads 1/125th at ƒ/11 at Weston 64. Okay—you now know that your Weston Master's 64 is equivalent to ASA 100. Your DREM is reading the same at Scheiner 14? Okay, Scheiner 14 is equivalent to ASA 100 on that meter.
Same with GE, same with GOST, same with whatever system you care to name.