Exposure, it's not exactly misunderstood, but there's more to it than meets the eye. By understanding exposure, or at least what the meter on your camera is doing, you'll get better and more consistent images.
Incident readings, reflective readings, 18% grey, centre-weighted, spot metering; what does it all mean and how does it effect your photography?
In a nutshell, your camera's metering system, or a hand-held light meter, assumes one thing: that the world is 18% grey. Middle grey, as it is also known, is halfway between black and white and has a reflectance of 18% or, put another way, reflects 18% of the light falling on it.
Whether it's a moonless midnight sky on a starless night or a snow covered ski field at midday on a sunny day, your camera's meter thinks it's looking at middle grey. What's more, it is designed to give you an exposure reading that will make that assumed middle grey appear middle grey in the photograph, whether it is or it isn't.
Why middle grey? Well, by being halfway between black and white, it's the median point and allows room for brighter and darker areas. For all intents and purposes, it works pretty well. But it's not perfect.
When metering a scene, many photographers use a grey card: a purpose-designed grey coloured card that reflects 18% of light falling on it. By placing the grey card within the scene, or close to it and under the same lighting conditions, and taking a reading from it, you get a very accurate 'average' exposure for the conditions.
Modern on-camera meters (that meter through the lens) are a type of reflective meter, ie they meter the light being reflected from the scene before it. Another type of meter is an incident meter, the hand held meters you often see studio photographers and cinematographers using.
An incident meter measures the light falling on a scene and is used by placing the meter at the subject and facing it back towards the camera. In a way, an incident meter assumes it is a grey card and tells you what your exposure needs to be in order for it to appear mid grey. Once again, an incident meter gives you an accurate 'average' reading.
Most cameras nowadays, especially SLR's, give the photographer a few metering options. These are generally spot, centre-weighted and matrix. With a very small (often a 1? field), spot meters can measure very specific parts of a scene. By measuring the darkest part and the lightest part separately, for example, the photographer can expose halfway between the two or anywhere towards either end of the scale, depending on their vision of the final print. And we just took a step towards Ansel Adams' well-known 'Zone System' that I won't go into any further here.
Centre-weighted meters were the standard for years and in many photographers experience offer the most consistency. They work by giving preference to the central part of the image area as seen through the lens.
Their consistency is due to the meter measuring enough area to give a balanced exposure without getting tricked by large areas of light or dark.
Matrix metering arrived in the late 80's and has gone from strength to strength. It works by dividing the scene into segments, measuring each and then giving what it calculates as the best exposure.
A good test to help with understanding exposure is to zoom in and out while looking at a scene and keep an eye on the exposure readout. As you zoom, the given exposure will change; as the lens widens, more variations in the light occurs, as you zoom in, less.
The light falling on any particular part of the scene isn't actually changing. The exposure readout changes as the meter averages the reflected light before it. As you zoom out, you may bring more bright sky into the image; as you zoom in, it might be towards a fairly dark part of the scene. If that's the case, two very different exposures will be given, even though the light on the scene hasn't changed.
Say, for example, that there are some trees in the middle ground that will be in the final image regardless of whether you zoomed in or out. The light falling on the trees hasn't changed at all, yet they will be rendered quite differently depending on which of the two exposures you use.
Something to think about is whether your chosen framing is predominantly dark or light; in other words is it noticeably either side of an 'average' (equal amounts of light and dark).
If the scene contains predominately bright areas (snow, sky, reflections on water etc) your meter will want to pull that brightness down to replicate middle grey. You will therefore need to compensate by giving one, two or even three stops more exposure than indicated. If the scene contains a lot of dark areas (shadows, dark colours etc) your meter will want to bring it up so you will need to give less exposure.
Naturally, the instinct might be to underexpose in bright light and overexpose when it's dark but remember that your meter is trying to find that middle (grey) ground.
Those two examples are also perfect situations where you could take a reading from a grey card or, if spot metering is an option, meter the brightest part followed by the darkest and expose halfway between (or slightly to either side).
When in doubt, you can always bracket your exposures (take additional images above and below the indicated exposure). As useful as it is - and we've all done it - bracketing is something of a cop out. Better to understand exposure and apply what you've learned by shooting a few tests. Your photography will be the better for it.
Matthew Smeal is a photographer and journalist from Sydney, Australia. To contact Matthew or to view his work, visit his website at http://www.matthewsmeal.com
