Shooting in Contrasty Lighting

One of the key features of digital cameras – especially compact, fixed-lens models – is their limited dynamic range and tendency to record images with blocked-up highlights when shots are taken in contrasty lighting. Such conditions are common in Australia, where clear skies and brilliant sunlight can be found throughout the year. Over-expose in these conditions and the highlight areas will contain empty white pixels with no data whatsoever, and any detail in these areas will be irretrievably lost.

Most compact digicams have a dynamic range of 5-7 f-stops and, although DSLRs usually have a wider dynamic range, they can also fail to capture highlight detail unless shots are exposed carefully. This means understanding how your camera’s exposure system behaves and how to overcome inherent biases.

Two examples of subject types with brightness ranges that can exceed the dynamic range of a digital camera. Beach scenes, even in winter, can include bright skies, clouds and surf and require careful metering (in this case -.03EV exposure compensation). Backlit scenes tend to produce silhouettes when their brightness range exceeds the dynamic range of the camera’s sensor.

Shooting Strategies
A number of strategies can be used to overcome inherent exposure biases and provide a more usable tonal range for photographers to work with when editing high-contrast shots. Note: some editing will usually be required to overcome problems created by the camera’s inherent limited dynamic range.

1. Find out how your camera’s AE system is biased and counteract unhelpful biases by using the camera’s exposure compensation settings. If in doubt, set the exposure compensation to -0.3 EV and check the results, or use the -0.3 EV point as the central point for a series of bracketed exposures.

2. Use spot metering to establish an exposure that is based on the lightest 30 to 50% of tones in the image. Select an area that is slightly darker than the brightest point – avoiding specular highlights – and take your exposure measurement from that area. Then re-compose and take the shot without lifting your finger from the shutter button.

The above shots – taken only minutes apart – show how a compact digicam whose metering system is biased towards shadow detail will react in bright, contrasty situations. When presented with a large bright area of sky, the exposure is biased to reveal shadow detail and details in the clouds are lost. Zoom in and reduce the amount of sky in the picture and you can see the cloud structure without compromising shadow detail. (Setting the camera’s exposure compensation to -0.7 EV would have revealed more cloud detail in the first shot but the second shot would be under-exposed with this adjustment.)

3. Use the camera’s histogram to establish where to position the exposure. In most cases, the best results will be obtained if the histogram peaks slightly to the left of centre in the graph. The shot will be slightly underexposed but highlight detail will be preserved. If the histogram has a highlight alert, it will flash when highlight detail is not recorded. Back off the exposure to the point where the flashing just stops.

Best exposures in contrasty lighting are when the histogram is weighted slightly towards the left of centre.

4. If in-camera contrast adjustment is available, knock the contrast back by a notch and see if that improves the situation. Success with this strategy depends on the subtlety of the in-camera adjustment – which can vary from one camera to another.

5. Shoot in Raw format and make the necessary adjustments when converting the Raw file. If you’ve used either the first or second strategies, it should be a simple matter to adjust the exposure, brightness and contrast sliders in the Raw converter to spread the data out to cover the available range and make the image look ‘right’. The advantage of this strategy is that it will allow you to minimise the noise and posterisation that can occur in shadowed areas of the image when it is post-processed in editing software. Best results are obtained when you output your images as 16-bit RGB TIFF files.

6. Use a split neutral density filter to reduce the density of the brightest part of the image. This strategy is most successful for landscape (or, at a pinch, architectural and some sports) photography, where it can be used to reduce excessive brightness in the sky and bring the overall dynamic range in the subject back to a more manageable level.

A graduated neutral density filter can allow you to balance the exposure to ensure a more even distribution of tones in both the bright and dark areas.

Post-Capture Processing
Converting Raw files to standard formats on a computer gives you tremendous control over the results. You can correct colour, adjust contrast and apply tonal response curves. Converting images to 16-bit per channel TIFF files allows you to use Levels and Curves adjustments to their fullest extent and perform wide-ranging manipulation (masking, dodging, burning, etc) without degrading the image or causing ‘banding’.

The latest versions of Adobe’s Photoshop and Photoshop Elements editors include a nifty Shadows/Highlights command that allows you to lighten shadows, darken highlights and adjust mid-tone contrast by simply dragging sliders. Live previews show you the effects of all adjustments. These adjustments should be done with a light touch as over-correction can cause the image to look over-processed. Posterisation will occur as a worst-case scenario.

Auto contrast controls, which work by mapping the lightest pixels in the image to white and the darkest to black and then adjusting all intermediate tones to fit between them, may also be used to recover detail in images that have been shot in JPEG format. However, unless the information was captured at the point of exposure, this control cannot recover it. Nor can any other post-capture adjustment.

If you use the metered exposure for some contrasty scenes, highlight details are often not recorded. Attempting to correct the tonal balance with the Shadows/Highlights tool can make some improvement but usually results in posterisation around the highlight areas. There is no way to insert detail that was not recorded into the highlight areas.

Underexposing a contrasty subject by between 0.3 and 1 EV allows more of the highlight details to be captured. Consequently, when you use the Shadows/Highlights tool, much of the tonal range can be restored without producing posterisation.

How Auto Exposure Systems Work
Regardless of whether you use a film or digital camera, its built-in light metering system is programmed to integrate all the hues and tones in a subject to mid-grey and base the exposure on those values. With a digital camera, the resulting data is processed to provide aperture and shutter speed settings for the camera, based on calculations determined by the camera manufacturer.

The results of these calculations can differ widely, as they can be influenced by the metering mode the photographer selects and each manufacturer’s bias. Since most cameras are designed in Japan, Europe and North America, most biases will reflect the common types of lighting found in those areas. These tend to be significantly less contrasty than typical Australian conditions, where beach and snow scenes (in particular) can test even high-end DSLR cameras.

AE systems in digital cameras are commonly set to record the mid-tones in a subject midway between the lightest and darkest tones they can capture. Some may be slightly biased to capture shadow detail, as this strategy can deliver optimum results in low-contrast situations and will prevent shadows from becoming too dark and noisy. This bias involves slight over-exposure, which can cause highlight detail to be lost in bright, contrasty lighting.

To overcome these biases, the camera should be set to favour under-exposure, rather than over-exposure. Many cameras – especially compact digicams – produce a higher proportion of usable shots when their exposures are set to -0.3 EV, regardless of the subject type. You should rarely need to shift the exposure by more than -0.7 EV.