Light Metering

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Lexicon, Knowledge Base

Light measurement is one of the most elementary requirements when capturing successful images and, without this or an external means of calculation, it fundamentally becomes a guessing game. It is how a camera determines the correct shutter speed, ISO and lens aperture to be used.

The way cameras measure light has evolved considerably over the years; from models that had no on-board method (requiring an external light meter, or application of the ‘sunny 16’ rule) to todays multi program marvels that can make full assessment of the environment and expose an image perfectly with little or no input from the photographer.

As all objects reflect varying amounts of light, cameras are calibrated with a standardised amount of light in mind – 18% grey (also known as mid grey) being the norm. With this in mind it is easy to see that if a camera is pointed at objects reflecting light that is brighter or darker than this, under or over exposure will occur. In other words, it is easy for the meter to be ‘fooled’ into giving the wrong exposure value.

A good example of this is when photographing a snow scene. The light reflected from snow is so intense, the camera interprets it as 18% grey which severely under exposes the scene, leaving you with disappointingly murky grey results. Similarly, a scene with mysterious large deep black areas will be overexposed as the camera attempts to render the scene mid grey, causing the richness of the black tone to be lost in a sea of dark grey.

Glancing at today’s crop of cameras, it is not difficult to notice that nearly all boast more than one method of metering. So, if program metering is so accurate, why are alternative, more archaic methods still available? To a large degree, the answer lies in a photographer’s personal preferences. We each have our own ideas as to how a final image should look and, composition aside, exposure is of huge significance; slight under exposure can give an image a more saturated look – but that look is not to everyone’s tastes (myself included). Conversely, over exposing a scene a little can lighten the colour (but run the risk of blown out highlights). Sometimes our intention is not for a technically perfect exposure as this can detract from the atmosphere or mood we try so hard to create.

Taking this into consideration, it is easy to see that there are still valid reasons for a cameras specification to include differing metering options, and the purpose of this article looks at a few of the most enduring and useful methods. It is by no means exhaustive and serves as a basic guide for the novice to gain an understanding as to how their use affects results.