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Exposure: Measuring the Light


This article covered the various way photographers can measure the amount of light available to take a photograph. Light is measured in Lux (metric) or foot candles (imperial units). Once you know this, look at Getting the Light Right to see the various camera settings which will give the correct exposure, and how they affect the finished picture.

The exposure meter inside your camera

The exposure meter inside your camera is an amazing bit of technology, but it doesn't know what you're looking at. It assumes your subject is in the centre. You'll get a more interesting composition if it's not (more on this in a future article). But if you're main subject is the archway of leaves round the edge, the camera's light meter will still expose for the sunlit sea in the middle. Granted, that doesn't happen very often. You can solve this by walking right up to your subject until it fills the frame, then setting the exposure. Be careful your own shadow doesn't fall on the subject, though.

More seriously, the built-in light meter assumes your subject is a mid-grey tone, and that you want it to come out as a mid grey. So if you're shooting a white cat in the snow, or a black cat on a black rug, the camera's exposure meter will do it's best to produce a picture of a grey cat. You can compensate approximately for this by exposing a stop or two more for light subjects, and less for dark ones. Yes, really that way round.

There's a better solution. A black object reflects only about 2% of the light falling on it, whereas a white one reflects about 95%. Rather surprisingly, the shade of grey agreed to look exactly halfway between the two reflects 18%. Kodak sell something called an "18% gray card" (American spelling of gray) which is exactly what it sounds like - a card which is exactly mid-grey. For a front-lit scene, you put it facing halfway between the sun and the camera, and meter off it, and add half a stop. Remember to remove the card before you shoot! They cost around $17.95 for a pack of two, and they're usually much more accurate than guesswork.

They're still not perfect, though. Some include sparkly bits which cause underexposure. And that's where hand-held meters come in. Most of them can perform several of the following functions

Spot meters

These meter off a tiny portion of the scene, typically just one degree, so that you get the same result as walking up to the subject, and you're unlikely to have problems with your own shadow. Some will open up a little more. This allows you to measure the exposure for the cat's eyes, say, or to measure the highlights and shadows separately and use Ansel Adams' zone system. If you know exactly what you're doing, this guarantees perfect exposure of the whole scene, every time, but it isn't simple.

Incident light meters

If your subject isn't contrasty, but isn't mid-grey either, you'll get the most accurate result by measuring the light falling on the subject, rather than the light reflected off it. You do that with an incident light meter. There's an added advantage, too. There's always more incident light than reflected light, so an incident-light meter is more sensitive.

Flash meters

These are special meters which will meter flash. Duh!

Colour meters

These measure the colour temperature of the light. Put simply, the hotter the colour temperature the bluer it is, and the colder the temperature, the redder. If you have a digital camera, you can probably set the white balance to match the colour temperature, or adjest it afterwards with a computer programme. If you're using film, you can adjust the colour temperature with filters, or change the light source.

If Your Light Meter's Broken

If your light meter's broken, or you don't have one (unusual these days, but not impossible) you can still get correct exposure.

  1. Set the shutter speed as close as possible to the ASA. For example, if you have 100 ASA film, set 1/125th of a second.. If you have 400 ASA film, set 1/500th (cameras don't have a 1/100 or 1/400th setting)
  2. Set the aperture as follows:
    • If the weather is sunny with clear shadows, set f16.
    • If the weather is hazy, with soft shadows, set f11.
    • If there is light cloud, with shadows just visible, set f8.
    • If there are heavy clouds, with no shadows at all, set f5.6
    • At sunset or in open shade, set f4
    • If it's raining, go home before your camera gets wet.
Since you're judging the amount of light by eye, you might not be exactly right, but you'll be close.

To summarise: a separrate light meter will help you expose correctly, even when your in-camera meter can't be trusted.

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