Meter this.

We’ve all seen the portrait photographer holding a white-domed object next to the subject’s face to take a light reading. So why is that being done? Why not just let the camera do its automatic exposure thing and take the picture?



 

It’s a good question in this digital age, where we can instantly review our shots to see if we need to re-shoot with a little exposure compensation. After all, the histogram in the camera tells us just about everything we need to know about the exposure in the shot – is it under- or over-exposed and is the exposure evenly distributed over the entire tonal range?

The short answer is that the handheld meter is mostly used to measure the light falling on the subject from studio strobes or flashes mounted off-camera.  In this scenario, the camera doesn’t typically have the ability to control the flash output, as it does in ETTL mode when the flash is mounted on the camera’s hot shoe.  Hence, studio photographers usually work with the camera in manual exposure mode, where they manually set the shutter speed to a value that will synchronize with the flash and the aperture to a value that gives the correct exposure. Alternatively, they can use the  meter reading to set the brightness of each flash in a multi-light setup to give a particular effect while ensuring a proper exposure.

Here, I’ve used the words ‘light falling on the subject’, meaning incident light. Hence, the incident light handheld meter measures the ambient light and ignores how much light the subject is likely to reflect.  The metering system in our camera, on the other hand, does rely on the light reflected from the subject to determine exposure.

Which is more accurate? The incident meter is, because it is calibrated so that it will give camera settings to make an 18% grey card look like 18% grey (middle mark on the histogram) when photographed.  The reflected meter, however, relies on the subject's tone averaging out to 18% grey in order for the exposure to be accurate. If the subject is overly dark or bright (such as a snow scene), the reflected meter gets fooled because it tells the camera to expose as if the subject is mid-grey. The incident meter would read the light falling on the snow rather than the light reflected from it, thereby resulting in the snow looking like bright white snow rather than grey slush.

I started out stating that the incident meter is used in the studio to measure the light from strobes or flashes, but obviously it has a function in outdoor ambient (continuous) light photography as well. Some photographers work in manual exposure mode all the time and therefore rely on an incident meter for camera settings.

So why bother? Part of the answer may be a throwback to the film days, where you never knew until it was developed whether you got the exposure right or not. An incident meter (and a little exposure  bracketing) gave you the confidence that you got the shot. That way of working may still influence the photographer who has made the transition to digital, partly because it’s good practice.

The bottom line, though, is that anyone seriously considering turning pro using studio lighting setups should consider working with a combination flash/continuous incident meter, if for no other reason than to save time. The alternative is to shoot, tweak, shoot and then tweak some more.

Combination meters can be a bit of an investment, so they’re not for everyone.  If you’ve got the spare change, though, a meter can help you understand the exposure mystery a little better and hit the bullseye every time.

Getting Exposure Right

For those of you who attended one of my beginner digital photography courses, you may recall me harping on about the camera's metering always trying to achieve 18% grey average exposure.

So what does that mean, really? Well, if you were to examine the histogram (that funny looking graph on your camera's LCD screen) for a picture you just took, 18% grey falls smack dab in the middle. When your camera evaluates the light in the scene before you take the shot, it measures the light over several points in the scene and makes a decision about the settings required to make the average value of the exposure fall at the middle of the histogram. This is called evaluative (Canon) or matrix (Nikon) metering.

The reason for this is to try to ensure that the shadow and highlight areas of the scene remain within the left and right ends of the histogram to prevent loss of detail. Remember, the left end of the histogram represents the darkest shadow detail that the camera can capture, while the right end represents the brightest highlight it can handle. If the histogram is not centred, and the graph is rammed against one end or the other, then chances are you've lost some detail. In short, the best histogram is one whose graph trails off smoothly at both ends, barely touching the sides.

There are times when this brainwork by the camera can do a disservice, however. If you're shooting a scene that's 90% snow or white sand, for example, the camera will try to average the exposure to 18% grey. That means your snow or sand comes out a dull grey because it dominates the exposure, and the camera wants to expose it to mid-grey. Not only that, but if there is some deep shadow detail in that remaining 10% of the scene, it may be pushed against the left end of the histogram and be lost.

Knowing this, camera manufacturers provide an exposure compensation button, pictured below. Typically, you push and hold this button while you turn the command wheel and read the exposure compensation on the LCD screen. In the snow scene example, you may need to overexpose (+) by 1 to 2 stops. Now you're ready to take the shot and check the resultant histogram to see if the white end of the graph ends up close to, but not touching, the right end of the histogram. If not, adjust the exposure compensation and try again.

Remember, most cameras do not reset exposure compensation to zero after you turn off the camera. Reset it manually to ensure you don't wreck the next day's shoot.

You can also use the camera's other metering modes, such as partial or spot, to meter the light only in the very centre of the scene. A typical use for these is where you are taking a picture of someone who has a strong backlight directly behind them. Evaluative or matrix metering would read the backlight and most likely make the face very dark. You could use exposure compensation in this scenario, but partial or spot metering will do the same thing by measuring the light reflected from the face (assuming you centre it in the viewfinder) and make it mid-grey. Again, in these metering modes, the camera takes what it measures and tries to average the exposure to 18% (mid) grey.

As with exposure compensation, don't forget to reset the metering to evaluative or matrix when you're finished with partial or spot metering.