Learning to use your light meter

You are smarter than your light meter!

Just about every modern camera has a light meter built in. Plus digital cameras have a screen on the back where you can check your exposures and/or histogram. Do you really need an external meter, too?

The answer is a definite “Maybe.”

But before we try to answer that, let’s go through the basics of how a light meter works, the options available, and how to use the meter. We will look at both the meter in your camera and at external (usually hand-held) meters. And now there is a new meter that wirelessly connects to your mobile phone.

Light meters are color blind and unable to think. They make bit assumptions. They don’t always get things right. You have to do a lot of the work for them. Their readings are open to interpretation. You area much smarter than your meter, especially the one in your camera. Most cameras offer a few different metering modes. For most people the “evauative” or “matrix” mode can handle most situations. In this mode the camera meter reads different segments of the scene and quickly compares them to scenes programmed into the camera to try to make the best guess as to what you might be pointing your camera at. For general every day photography you should probably be using this mode. They usually also offer a center-weighted average and a partial metering pattern that only reads the center of the frame. Higher end cameras may also add a “spot” metering mode that reads an even tighter area. How big a spot varies by camera. For example, the Canon 5D Mkiii has a 1.5% area coverage, the Canon 6D Mkii uses a larger 3.2% area for its spot meter. It is the spot meter that I want to concentrate on here.

The meter in your camera basically assumes that everything you point it at is a medium tone of gray. And it wants to suggest an exposure setting that renders your subject a medium tone of gray. Luckily, many scenes that we photograph have a wide range of tones from darks to highlights that when averaged together are a mid-tone of gray. Whether that gray is 18% or 13% or 12.75% is an argument for another day. For now let’s just accept that average scenes average out to a gray tone that our meters are looking for.This averaging out is what makes the meter work. But it is also what makes us have to think about it and sometimes override what the meter tells us to do.

Let’s take some extreme examples—the proverbial polar bear in a snow storm and the black cat in a coal mine. If we relied on our in-camera meters both of these photos would be a mid-tone gray with a wildly overexposed gray cat and an underexposed gray polar bear. I am writing this at 7am in my PJs, so instead of going out to find the cat or the polar bear (or the coal mine or the snow storm) I am going to simulate the situations using a target with wide black, gray, and white stripes. Using the spot metering mode in my camera I metered each stripe separately (shown by the red targets) and made three photographs with the meter readings. I also made one more image using a hand-held incident meter, which I will get to later.

spot-meter-test.jpg

In the upper left I placed took the spot meter reading off of the gray stripe and all three stripes fall into line of black, gray, and white. In the upper right I made a spot meter reading of the black stripe (our black cat in the coal mine) and you can see that the meter made its expected assumption that the scene would average out to gray and by following along with the camera settings, that is just what we got. The black stripe turned gray and the white stripe overexposed. In the lower left I took the meter reading off of the white stripe (our polar bear) and the meter again assumed it was gray and turned the white patch to a muddy gray. The lower right shows a hand-held incident meter which reads the light falling onto the subject instead of the light reflected from the subject. More about that in the section below on flash meters.

This is where things can go wildly wrong if you don’t know what you are doing with the spot meter. Most of my photos are of people, so let’s look at what might happen in these situations. Caucasian skin tones are usually about twice as bright as the average gray (maybe 36% reflective as opposed to 18%). Darker skin tones might range from 18% reflectance down to 6% or so. If you point your spot meter at the different skin tones you will get very different readings that will lead to incorrect exposures with the caucasian skin being underexposed and darker skins being correctly exposed or overexposed. If you use the spot meter in your camera or if you use a hand-held spot meter you need to think things through more than when you use a matrix or center weighted mode and be ready to use your exposure compensation dial on your camera or work in manual exposure mode to get things just where you want them. Let’s look at some examples of where a spot meter can lead us if we blindly follow it.

spot-meter-scene.jpg

Here we have a still life set with objects of different brightness. From left to right we have a Color Checker Passport, a white index card, a Kodak gray card, a black card, a Gnomicello*, and a set of gray patches. In the upper left I made made a spot meter reading off of the gray card, giving a good overall exposure. Upper right has the spot reading made off of the black card, leading to an extreme over exposure of the scene. Lower left has the spot reading on the white index card, and the expected under exposure of the scene. Lower right is the exposure suggested by the evaluative meter mode in the camera. I actually think that the evaluative meter did the best in this series. The spot meter is not always our friend.

Extending this outside the studio, where should you point your spot meter? Point it at the white clouds and you get an underexposure. Point it at deep green foliage, get overexposure. You need to figure out something in the scene that you want to have render at a medium gray and point the spot meter only at that element, then lock in the exposure and recompose to make the photo. Bottom line, I think that spot meters have their place for photographers working with film and following the Zone System to control the tonal range of individual images. For the rest of us spot meters are useful if you want to figure out the contrast range of a scene, but you still have to work out the proper exposure overall.

For kicks, let’s compare the evaluative meter reading with the center weighted mode.

scene-meter.jpg

I think you can make a case for either exposure here. This all, of course, assuming that the monitor you are viewing this on is calibrated and set to a decently low brightness setting of around 100 cd/m2. Most monitors are set much too bright from the factory (for gaming or for watching movies), so make sure your calibrator has an option to set brightness levels.

Flash Exposures

hand-held-meter.jpg

This is all well and good for photography lit by continuous light sources like the sun, incandescent lights, LED lights, etc. What happens when you need to figure out the exposure for studio flash units that are not TTL** compatible? The meter in your camera has no concept of flash, so you need to rely on an external meter. Typically these have been hand-held devices such as the one shown here, my trusty old Sekonic L-358 (that is no longer in production). There is also a new meter, the Illuminati Meter (which I have not yet had a chance to try), that can be placed in the scene and connects with your mobile phone so you don’t have to hold it in your hand.

These meters can either be connected to your flash via a sync cable, a wireless transmitter, or they can be set to wait for a flash of light to trigger the reading. As noted, these meters are placed in the scene to read the light that is reaching them, the “incident” light. This provides a more accurate reading because it only reads the light falling on the subject and is not swayed by the brightness or reflectance of the subject or anything else in the scene. You set the ISO and shutter speed on the meter to match that on your camera, fire off the flash, and the meter tells you the aperture (F/stop) to set the lens to. For the most accurate readings the dome of the meter should be pointed at the main light illuminating the scene.

Some hand-held meters can also be set to read reflective light and/or be used as a spot meter, but then you run into the same issues that you have with your in-camera meters. And you don’t have the computer “smarts” of Evaluative or Matrix metering in the hand-held meter.

Color meters

The Illuminati Meter mentioned above and another relatively new device, the Lumu meter (iPhone only at this time and also which I have not had the opportunity to work with yet) extend the function of the meter by adding in a color meter mode. Normally color meters cost around $1,000 and only do their one thing. Having this available in a light meter costing around $300 - $500 is a big deal for those who need critical control over color.

Back to the beginning

So, back to the question of “do I need an external, hand-held light meter?” The answer remains a definite “maybe.” It depends on how you work, what type of lighting you use, how comfortable and sure you are about the display on the back of your camera, how much you know about reading the histogram on the camera, and on and on. I have friends like Tony Corbell and Matthew Jordan Smith or take multiple meter readings of every scene and friends like Joel Grimes who never touches a meter. The photographer makes the image, not the tools.

Wishing you good exposures!
John Cornicello

* Gnomicello is a birthday gift I received from artist Mike Oncley a few years ago.

** TTL means Through The Lens. Some camera and dedicated flash combinations offer a metering mode where the flash fires twice. Once at low power just before the shutter opens letting the camera meter read the flash and then again with the shutter open to make the exposure.