Commercial Photography Technique – Light Metering
If you want great exposures in awkward lighting conditions you need to learn how light meters work and how to compensate for their failings.
Your camera’s light meter is the heart of your photographic system, helping you to determine the amount of light needed to correctly expose every shot you take. Because certain scenes are much brighter than others, the amount of light reaching your sensor must be carefully regulated. Too much light and the image will appear washed out or ‘overexposed’; too little light and the image will appear too dark or ‘underexposed’.
Although the light meters built into cameras are highly sophisticated, they’re by no means a failsafe way of determining exposure. In fact, they get fooled by a wide variety of common lighting situations. Ironically, it’s these situations that often yield the most exciting shots. To avoid ending up with a portfolio of poorly exposed shots you need to know when your meter is being fooled and how best to intervene, often quickly when shooting commercial photography. That means learning not only how light meters work, but also the pros and cons of the metering modes available. This knowledge will also empower you to take creative control over your exposures and interpret scenes in your own way for your commercial photography.
Light meters are made up of light-sensitive silicon cells that measure the brightness of your subject in accordance with the selected metering mode. Half-depressing the shutter button activates the meter and the light reading is then fed to the camera’s central processing unit. Before calculating the optimum exposure, this microchip also gathers information about the ISO setting, exposure compensation and flash settings; as well as the aperture and/or shutter speed chosen.
Compact camera meters are frequently built into a window alongside the lens or viewfinder. SLR meters, on the other hand, are situated inside the camera body and measure the light from behind the lens. This ‘through-the-lens’ or TTL metering is far more accurate because it takes proper account of the diffusion, diffraction and dimming of light caused by lenses, filters and close-up work. The proportion of the scene it reads is also adjusted according to changes in focal length. Many TTL meters also control the output of dedicated flash units, particularly those used for commercial photography.
“Your meter only measures the light reflected back from a scene not the light falling on it”
Remember it’s reflective
In terms of how your camera’s light meter works, there are two crucial facts to memorise. The first is that your meter only measures the light reflected back from a scene, not the light falling on it. This means that the reflectivity of your subject strongly influences the accuracy of the final reading. In the same lighting conditions, for instance, the amount of light reflected from dark or matt black surfaces will always be less than the amount reflected from light and/or shiny surfaces.
The second key fact is that light meters assume that all scenes have a pretty even distribution of highlights, midtones and shadows. When these tones are blended together, the overall value reflects the same amount of light as a spectrally neutral mid-tone grey with 18 per cent reflectance.
Due to this standard calibration, your meter is at risk of under or overexposing any scene featuring a predominance of very light or very dark toned subjects, as it mistakenly tries to pull them back to a uniform grey. Intelligent metering is critical for commercial photography because detail lost through overexposure is lost forever, and although some underexposed shadow detail can be salvaged, doing so reduces image quality - an unacceptable situation when shooting commercial photography.
Broadly speaking there are four types of metering systems. Each mode measures different areas of a scene in different ways:
This is the simplest system of all, and the default setting on most cameras. It averages the exposure for the whole scene assigning a slightly greater weighting to the central portion of the frame (typically 40-75 per cent of the frame). The exact layoutof the sensitivity map varies from camera to camera and some allocate slightly more weighting to the bottom half of the frame to reduce the influence of skies. This setting is of particular use in commercial photography. This mode can be very effective with mid toned or low-contrast subjects, but it’s easily confused by small subjects and patches of light and dark. User intervention is often necessary.
Partial / Selective-area
Partial restricts metering to a small, central portion of the frame (typically between 6 and 15 per cent), making it ideal for backlit or high contrast subjects. This setting is of particular use in commercial photography. If your key subject isn’t in the centre, place it temporarily during the metering process, fix the exposure using the camera’s auto exposure lock (AEL) facility, then recompose the shot with your subject in the correct position.
Used correctly, a spot meter offers the most accurate way of calculating exposure. It enables you to meter the light falling exclusively within a tiny circle in the centre of the frame, (typically between 1 and 3.5 per cent). Spot meter from a small area of the main subject with a mid-tone grey. This setting is of particular use in commercial photography. Remember to choose your metering point with great care, as minor errors can result in wildly inaccurate readings. Spot meters come into their own when faced with extremely high contrast scenes (eg. Spotlit or backlit) and are invaluable for determining the exposure within very specific subject areas. They are also great for on-location work because you can take selective readings from inapproachable subjects as if you were close by.
Here the frame is divided into a honeycomb pattern of between 5 and 270 segments. Individual light readings are taken from each segment and the exposure calculated using complex algorithms. The sophistication of this mode increases dramatically as cameras become more advanced. This setting is of particular use in commercial photography. SLR matrix metering is remarkably accurate in almost all scenarios, even backlit (although strongly backlit or spotlit subjects are best dealt with using spot or partial metering). This is because exposure is calculated using a vast array of data including the focusing point; the size, brightness and distance of the main subject; back lighting and colour. Advanced SLRs often boast image banks containing thousands of subject field trials from commercial photography. These are compared with the results from each segment for greater precision.
Whichever metering mode you employ you’ll be faced with problematic conditions. When this happens, try these remedies:
1 Decide what your key subject is and move in to fill the frame with as much of it as possible, so that
overly light or dark areas are excluded from the metering measurement. Then use AE lock to fix the exposure
2 Another good tactic, for your commercial photography, is to take a reflective reading from a spectrally neutral
grey card placed in front of and receiving exactly the same light as your subject. Although not always practical
on location for commercial photography, grey cards should provide better results than random grey point
selection, and aren’t influenced by the subject’s tonal distribution or reflectivity. Fill as much of the frame as
possible with the grey card and angle it carefully to avoid picking up reflections or shadows that could influence
3 Take an incident reading using a handheld meter. Here a light-sensitive cell inside a white dome is held in the
same lighting as the subject and pointed towards the camera. Because the reflectivity or tonal distribution of a
subject has no bearing on the reading, incident metering offers simplicity and accuracy (provided you can move
back and forth from your subject). It’s best suited to sidelit or frontlit rather than backlit subjects, and it’s often
used for commercial photography studio work when there’s time to compose carefully. The image-dimming
effects caused by filters, close-up conditions or lens aberrations must be considered separately.
4 To preserve both highlight and shadow details in subjects featuring large areas of light and dark tones, spot
meter off both and average the two readings. If the shadows and highlights are over five stops apart, you
may be unable to record the entire range. In this case you must decide which details to sacrifice in your
commercial photography. If, say, the highlight details are more important than the shadow details, spot meter
off the highlights. This will give lots of highlight detail, saturated colours and deep, detailess shadows.
Alternatively bracket the image with a variety of exposures (suggest at least 5) including images metered for
highlights and lowlights and mid range and then combine them during post-shoot production using the HDR
function within Photoshop. This 'ideal world' approach is becoming popular in commercial photography.
This will produce an image where all areas of the subject appear correctly exposed and with real depth.
5 An alternative option is to override the metered exposure by dialling in a different EV value (usually in the range
of -5 to +5 EV in 0.3 EV or 0.5 EV steps). In the official EV scale, an increase of one cuts the amount of light
reaching the sensor in half, but on your camera, dialling in a positive EV value increases the exposure. There’s
an element of trial and error in finding the exact amount of compensation required, but in-camera histograms
enable you to monitor changes in brightness distribution easily as part of your commercial photography. Finally
for that extra bit of security it’s always wise to kick your camera’s auto-exposure bracketing facility into gear as
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Copyright - Adam Coupe Commercial Photography 2008