You’re ready to start taking pictures, but before you start clicking the shutter, you need to work out the correct exposure for your photos. The question is, when your camera has a built-in meter for this purpose, why do so many photographers and professionals buy a dedicated handheld meter?
The answer is simple: the best light meters(opens in a new tab) give an empirical reading of the actual light in your frame, while a camera meter gives a misleading reading of reflected light.
A handheld meter (also called an incident meter) takes an incident reading, which means it measures the light falling directly on the subject. Your camera takes a reflective reading, measuring the light that bounces off your subject. It’s a bit like measuring the amount of water in a bucket by pouring it into a measuring container, instead of throwing the water at a wall and measuring the amount that splatters.
On top of that, your camera is programmed to expose your images according to ‘medium gray’. In other words, you want the reflected light in your image to be 18% gray (see ‘Kodak Moment’ below for an explanation of why), and this causes all sorts of exposure problems.
For example, when the frame is filled with a bride in a white wedding dress, a lot of light is reflected into the camera (much more than 18% gray). So a camera’s reflective meter reading thinks your image is too bright and tells you to massively underexpose the photograph. And when her frame fills with a groom in a black suit, it tells her to massively dodge for the exact opposite reason, even though the light hasn’t actually changed. And if the light hasn’t changed, why do you need to change your exposure?
The answer is no. An incident indicator doesn’t measure reflected light, and it doesn’t care what percentage of gray is in the frame; it only measures the light that actually falls on the subject. And just like that, it always gives an accurate reading!
How to use a light meter to get perfectly exposed images
1). First, make sure the ISO on your meter matches the ISO you set on your camera. Also check that the meter is set for daylight or flash, depending on the light source you want to measure.
The Sekonic L-308 is an excellent and affordable light meter
2). Hold your meter in front of your subject, pointing it toward the light shining on it (not the camera). Now simply press the measurement button to read the light measurement. With multiple light sources, you can measure them individually by pointing the meter at each one. (And, obviously, if you’re measuring the flash, you’ll have to trigger the light to take a reading!)
3). Now just set the measured reading to your camera’s exposure settings (you’ll need to have your camera in manual mode). If you want to take a meter reading for a specific aperture, to allow you to control the depth of field, simply enter the desired aperture into the meter and it will calculate the set shutter speed for you.