Some tips on getting to grips with the vagaries of exposure, white balance and ISO settings

Modern cameras have a range of programmed exposure modes; indeed, some have nothing else: they automatically set the shutter speed and aperture depending on the exposure reading made by the camera’s built-in light meter. There’s an automatic mode for portraits, another for landscapes, one for sport and action, another for close-ups, and several options for shooting at night, with or without flash–there seems to be a different exposure mode for every eventuality. For each mode, the chosen exposure is determined using different combinations of aperture and shutter speed.

Such point-and-shoot automation brings ease of use to the photographer, but placing this much trust in the camera to define the ‘correct’ exposure and set the right combinations of shutter speed and aperture every time is to forsake control. In many situations, it’s far better to implement some manual control over the choice of shutter speed and aperture for your desired result.

But what is meant by ‘correct’ exposure? Although the product of changing ratios between shutter speeds and apertures controlling the amount of light reaching the film plane or image sensor, a correct exposure is as much an aesthetic value as it is a mathematical one. For many photographers, it’s where the resulting photograph renders a level of detail in both the highlight and shadow areas that is acceptable to the eye of the photographer.

Many camera meters are fooled by scenes of high contrast or extreme brightness, such as high direct sunlight reflecting off sand or snow. In such scenes, a built-in meter reading, if left alone, will inevitably lead to underexposure, so the photographer needs to override the metered reading to achieve the ‘correct’ exposure. However, camera technology can help in this quest thanks to built-in spot meters that allow the photographer to make individual readings from different parts of the scene to check exposure variance within the frame, thereby helping you to decide which reading to use to best render the contrast range within the frame. Even then, an average reading of all these different levels won’t always provide the ‘correct’ reading.

Camera meters are calibrated to reproduce a scene as an 18 per cent shade of grey. As a result, an all-white scene such as a snow-covered field will be recorded grey when exposed at the camera’s metered reading. In these situations, additional exposure is needed, and this can be supplied via the camera’s exposure-compensation facility. Most cameras allow you to compensate exposure in third- or half-stop increments. How much exposure compensation you give depends on the amount of white or highlights in the frame. For example, beach sand beneath a bright sun reflects a lot of light and will fool your camera meter into underexposing, so keying in exposure compensation of +2 stops might well be necessary.

A less extreme and more common exam pie is the summer landscape with a clear blue sky occupying the top half or third of the frame. Here, the meter is likely to give a reading bias towards a perfectly exposed sky, resulting in the foreground subject matter being too dark. Exposure compensation will also be required but only around a half or two thirds of a stop. For a balanced exposure that reproduces highlights and colours accurately and reveals shadow detail, a single compensated exposure may not always give the best possible result. So, given that ‘correct’ exposure–like beauty–is in the eye of the beholder, a productive course of action is to take a bracketed sequence of images.

Bracketing is the practice of taking two or more additional shots either side of the ‘correct’ reading, at values above and below the metered exposure. Whether you bracket by a third of a stop, half stop, one stop or more around the metered light reading depends upon your personal taste and experience. Like so much of photography, it boils down to trial and error based upon myriad situations.

As well as shutter speed and aperture selection, there’s another camera setting that helps determine the exposure value of the scene you’re composing in the viewfinder. This is the ISO rating, which is a measure of the light sensitivity of the recording media used by the camera. In the case of film, the higher the ISO rating, the more sensitive it is to light. For example, an ISO 200 film is twice as sensitive as an ISO 100 film, the same is also true of ISO ratings on a digital camera. Consequently, 1/500sec at f/8 with ISO 200 will deliver the same exposure value as 1/250sec at f/8 with ISO 100 and 1/125sec at f/8 with ISO 50.

Whether using digital or film, a characteristic of higher ISO numbers is a coarser image and deterioration in colour saturation. Grain becomes more visible on the finished photograph made on film, while increased noise is the comparable trait of digital images made using higher ISO settings. The prime advantage of using ISO settings on a digital camera is that you can select a different ISO value for every photograph; with film, you need to stick to the same ISO value for the whole roll.

White balance is a feature you won’t find on film cameras, but it has an influence on the quality of light recorded with a digital camera. There are three primary colours existing in varying proportions in a light source, depending on its colour temperature. These are red, green and blue, usually abbreviated as RGB. When the colour temperature is high, there is more blue light visible; when it’s low, there is more red light. As the colour temperature increases from low to high, the colour casts rendered change in the following sequence: red, orange, yellow, white and blue/white.

While the human eye can adjust automatically to variations in light quality brought about by changing colour temperature or mixed light sources, our cameras aren’t quite so adaptable. When we all used film, the emulsion was daylight-balanced, but special film types were available for different light sources, such as infrared to record those otherwise invisible wavelengths of the spectrum, or tungsten-balanced film for shooting indoor scenes lit only by household lamps. Cameras have improved in this digital age, as they have a plethora of programmed white-balance settings to match the type of light illuminating your subject. Typical white balance settings include auto, sunlight, cloudy, fluorescent, tungsten and incandescent.

Unsurprisingly, the automatic white balance setting is the most popular among photographers, but unless there is an actual area of white, such as a cloud, in your scene for the auto setting to refer to, then your resulting image will appear flat and lacking in true colours. The reality is that most light sources aren’t 100 per cent pure white: they have a range of colour temperatures that produce different colour casts (see The colour temperature scale), but this in itself isn’t an obstacle. Instead, it reminds us that light is the source of all colour and how we depict that colour is in itself another key factor in the overall impact of photography.

Equipment selections

Accessory option: handheld light meter

Contrary to popular opinion, sophisticated in-camera light meters haven’t made the separate handheld instrument redundant. Many professionals still swear by the finer accuracy of the light readings achieved by modern meters such as the Sekonic L758D Digital Master. This meter was made purely for digital photography and can be calibrated to match the sensitivity of a digital camera’s sensor at all ISO settings.

Cleverly, it includes and exposure-latitude warning that flashes on the analogue scale if a measured highlight or shadow exceeds the dynamic range of your camera.

Compact option: Sigma DP1

The recently launched DP1 is more than just another digital compact camera: it’s the first to use an APS-C-sized image sensor, normally found on SLR cameras. More than this, the DP1 uses a 14-megapixel full-colour sensor made by Foveon, with the array of pixels arranged in three separate layers–red, green and blue–for the three primary colours found in every light source. Initial tests have produced superb image quality and it’s expected that quite a number of professionals will add this little camera to their kit.

Camera option: Nikon D300

A pro-specification SLR, the D300 is only eclipsed by the D3 in Nikon’s stable of award-winning digital cameras, but shares most of the flagship model’s features. Number one is Nikon’s revered 3D matrix metering system, using a 1,005-pixel RGB sensor with readings linked to the camera’s 51-point AF system. With a 100 per cent field of view in both the viewfinder and the LCD monitor and exposure compensation of +/- five stops in either 1/3- or 1/2-stop increments, the D300 is one of the most accurate and versatile digital SLRs on the market when it comes to focusing and metering.

DOS AND DON’TS OF EXPOSURE

DO

* Take spot readings of the brightest and darkest areas of the frame in order to measure the exposure variance (contrast) in the scene

* Bracket your exposures. Check the results on the LCD monitor and vary the amount of over- and underexposure between frames to find the result that pleases you best

* Meter off the main focal point of your composition: as the point that draws your eye into the picture, this area should be both sharply focused and correctly exposed

DON’T

* Use the automatic white balance setting if there is no area of white in the scene for your camera to refer to

* Rely on your camera’s automatic metering in scenes dominated by bright skies or reflective surfaces such as water, snow or wet sand. Instead, overexpose the meter reading by up to two stops

* Use high ISO settings in low light unless there is no alternative. Image noise and colour degradation become visible on most cameras at settings of ISO 400 or above

Recommended reading

* Understanding Exposure by Bryan Petersen, Amphoto, pb, pp160.

* Understanding Raw Photography by Andy Rouse, Photographer’s Institute Press, hb, pp144,

* Exposure and Lighting for Digital Photographers by Michael Meadhra and Charlotte K Lowrie, John Wiley & Sons, pb, pp368,

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