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Understanding and Using a Photography Light Meter
Find the perfect exposure with your camera light meter
A photography light meter is the clever little device that calculates exposure. How does your camera know what exposure value to suggest when you point it towards a subject? There's got to be some kind of measurement going on there.
Well, that's where the camera light meter comes in. It works out how much light there is and lets you know the correct exposure value - the combined effect of aperture and shutter speed - that's required for a good picture.
So photographic light meters do a really important job! When the aperture and shutter speed are set to amounts that will produce a dark, under exposed image, digital light meters display a negative reading. Meanwhile, a positive reading will be shown when too much light is being let in, and a neutral display of zero is shown when the exposure is set just right.
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Quick background facts
In the days of film, many keen photographers owned independent, handheld photography light meters. Why, didn't they have enough kit/hassle already? Well, basically it was more important to be accurate in exposing every picture.
If they messed up, that was a bit of film - and money - down the drain! So painstaking effort was well justified. An independent photography light meter can offer a greater degree of accuracy than built in ones (more on that in a mo).
In fact, some digital photographers still make use of independent light meters. Each to his own, but I reckon this is a tad loopy! Digital photography is great because everything has become so streamlined.
An independent light meter is surely unnecessary in circumstances where an instant preview of the photo can be seen on the back of the camera. Looks a little dark? Delete the pic and open up the aperture a stop!
How does a photography light meter actually work? The built in camera light meter on your digital SLR camera measures the reflected light in a scene. Reflected light differs from what's called incident light. Incident light is the actual light falling on a subject from a given light source. Reflected light is the light that's reflected back off that subject.
There's a problem here! Different things have different reflective qualities: lighter objects reflect more light than darker ones. So with built in light meters there can sometimes be a discrepancy between the measured reflected light and the actual overall brightness of a scene.
But a bit of experience, and good use of the metering options in cameras, makes this an easy problem to solve. (Independent light meters are capable of measuring incident light).
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Using different photography light meter modes
This is often the default setting for digital light meters since it is quick and convenient, producing broadly accurate results. Basically, the photo light meter takes a reading from all areas of a scene, averaging out the amount of light and suggesting an exposure value based on this. It actually involves quite a complicated system, since lots of bits of the scene contribute to the overall reading, but it is the simplest metering mode to use.
When to use it:
Matrix metering is useful for shoots where it is important to be quick and opportunistic. If your subject is constantly changing or moving around, you may not have time to calculate the exact exposure with a more precise metering option. Matrix is a photography light meter setting can usually be relied on for decent results. Combining it with 'Auto' exposure mode really does leave you free to focus on the subject.
Matrix metering is safest when there is a fairly even level of light, without strong highlights or shadows.
Shooting in a Raw file format is more forgiving on 'in-camera' exposure errors. So the dangers of an incorrect matrix meter reading are lessened with this system.
When not to use it:
In shots where it is important to find a balance between really dark and really light areas, setting a photography light meter to matrix mode is not a good idea. For example, sunset shots can often turn out too dark, with only the sun correctly exposed.
Equally, where there is strong backlighting and the main subject is in the foreground, matrix metering will lead to a silhouette effect, rather than ensuring the subject is well exposed.
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On this setting digital light meters draw their reading from the central part of the frame. An area in the centre of the viewfinder, which feathers out towards the edge, is selected and used to measure light and inform the exposure.
So it's more accurate than matrix metering. The central area that you focus on will, by itself, have more clear cut lighting than the entire frame, so results will be reliable. It's possible, of course, to take a reading of your subject - positioned centrally - then set the correct exposure before recomposing the scene. (Tips on photography composition)
When to use it:
Centre-weighted metering is great when there is a strong contrast between the foreground and the background light. This is often the case with portrait photos. Where someone is standing close to the camera with a strong light source behind them, matrix metering would suggest an exposure far too negative to capture the subject clearly - leaving only a silhouted outline.
But setting your camera light meter to centre weighted in this situation allows you to ignore the background light and just deal with the important part.
When not to use it:
Whilst this metering mode is accurate, it requires slightly longer to set up the shot than when matrix metering is used. Usually that's no big deal, but if your subject is moving, as in wildlife or sports photography, problems can appear (tutorial on understanding shutter speed).
It's not possible to keep up with a fast moving subject and take a precise light reading (in any case as it moves the required exposure will likely change), so matrix metering is much the best option here.
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Spot metering is the most accurate setting available on a photo light meter. A tiny area in the very centre of the frame is measured and taken as the guide for exposure. This little area will usually differ in brightness from any number of other parts of the scene.
You can be sure of precise results when using this setting, since the reading cannot possibly be affected by mixed light sources. Just like with centre-weighted, it's easy to recompose your shot having found the right exposure for the relevant area.
When to use it:
Spot metering is extremely useful when you are able to identify a small area that needs to be prioritized for accurate exposure. It allows you to isolate a subject from the surrounding scene and set the correct exposure for it.
This is great in shots where there is lots of contrast in the light. Perhaps you're trying to capture a flower that is catching the sun's light whilst those around it remain in shade; or shoot the light pouring in through a church window; or take a close up shot of an interesting texture with light falling on it in various ways. Spot metering is the solution and is hugely valuable for many 'creative' shots.
When not to use it:
Spot metering does not tend to be used as frequently as the first two photography light meter options, just because it is more specific. As with centre-weighted, it's a bad option when shooting moving subjects or when you only have a split second to seize an opportunity.
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