OK, so now we know what ISO media we’re working with, we understand shutter speeds, and we understand apertures – but where do we base our exposure? We accomplish this by taking a measurement, or “metering”, the light falling on our subject. Cameras have a built-in light meter to help with this measurement, but to get the most from it, it helps to have an understanding of how it works.
Metering – What A Meter “Sees”
Light meters only “see” one thing: light intensity. They don’t know what you’re subject is, or even what color, how bright, or how dark that subject is. So, how does a meter know if you’re shooting a light subject or a dark subject? Fact is, it doesn’t. Meters assume that everything you’re shooting is medium-toned, and they’ll try to expose your photograph that way. This works fine most of the time, if you consider the average tonality of most photographs. There are light areas, dark areas, and medium areas, which give an overall average of medium – but not all subject matter is created equally. Have you ever taken photos at the beach, or in the snow, and had your pictures come back looking really dark?. They’re dark because they’re underexposed – the camera didn’t allow enough light to strike the media – because the meter told it not to! I know what you’re thinking, “If my scene is brighter than average, then shouldn’t my pictures look too bright instead of too dark?”. The meter is calibrated for a medium toned subject, meaning it will try to make anything it meters appear medium toned in the final image. When you’re shooting bright sand with bright reflections on the water, or in stark white snow, the meter sees all that brightness and tries to make it medium – so in your final image, your snow looks like a medium gray instead of stark white – it’s too dark, or, underexposed.
So, how do you avoid this problem? You either need to meter off of something that is medium toned and shoot based on that reading, or you need to know how much to correct the meter reading to get subjects to fall into the correct tonality. There are a couple of ways to do this (we’ll dig into it deeper a bit later), use the Exposure Compensation dial or setting, which adjusts the exposure by a controlled amount that you determine – or, remember that “full manual override” we wanted? Using manual, you can set you exposure settings however you wish. Again, we’ll get way deeper into this in a bit
Metering – More Than One Kind Of Meter?
Besides the meter built into your camera, there are several other types of meters – all of which can be broken down into three categories: Reflective, Ambient, and Flash.
Reflective Meters, such as the built-in meter in your camera, meter the light reflected off the subject. They come in several flavors as well, from an averaging meter which averages the light reflected from entire scene to base its reading, to a center-weighted meter which averages the entire scene but placed more emphasis on the center of the scene, to a spot meter which takes a reading from a precise point within your scene. Many cameras allow you to change the metering pattern of the built-in meter, allowing you to use a spot meter for one scene, then a center-weighted meter for the next, etc.
Most manufacturers have developed complex “combinations” of metering pattern options – such as multi-spot readings, or an option to set how much bias a center-weighted averaging meter uses. Regardless, all reflective meters are calibrated to expose as if your subject were medium tone. The advantage of a reflective meter, particularly one built in to the camera, is that it takes into consideration any accessories that you may have on the camera while you’re metering. For example, if you have a filter of some sort on the lens, or you have a lot of extension added – even simply zooming most zoom lenses will change the amount of light passing through them – a built-in meter will take all of that into consideration.
Ambient Meters read the light falling on the subject as it passes through some form of diffuser – usually a white domed piece of plastic. The advantage of an ambient meter is that, since it’s metering the light falling on the subject instead of the light reflected from the subject, you don’t have to be concerned with the tonality of the subject – it will meter correctly regardless, assuming you’re making corrections for any added filtration or lens extension. The disadvantage of an ambient meter is that it has to be placed in the light that is falling on your subject – hard to do if your subject is on the other side of the Grand Canyon and is not in the same type of light as your camera!
Flash Meters, as you might have guessed, meter light from an electronic flash. They’re designed to meter a rapid burst of light and remember that reading so you can make the adjustments to the camera. Here again, flash meters are available as reflective or ambient, or both, although the vast majority of flash meters are of the ambient variety.