Correct exposures make good photographs so it’s important to understand the controls that determine image brightness.
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Correct exposures make good photographs so it’s important to understand the controls that determine image brightness. Exposure consists of two components: lens aperture (the hole through which light enters the camera) and shutter speed (how long the light is allowed into the camera). A correct balance between them creates pictures in which all tones in the subject are recorded correctly. All digital cameras have exposure metering systems for measuring the intensity of light reflected from the subject. Most provide at least two metering patterns that take measurements over different parts of the camera’s field of view. Understanding how metering patterns work will help you to decide which one to use in different situations. Multi-pattern evaluative (or matrix) metering is found in all cameras and is the normal default setting for cameras with more than one metering pattern. This system divides the field of view into five or more segments and individually evaluates the light levels within each segment.  Multi-pattern metering is ideal for landscape shots ans subjects with varying brightness levels because exposure calculations take account of bright skies and dark foregrounds. A microprocessor in the camera takes the readings from each segment and biases them according to the difference in overall brightness (and often contrast) within each segment and between adjacent segments. It then calculates which aperture and shutter speed settings will deliver an optimum exposure. Some cameras also include distance information from the autofocus system and/or colour data. Multi-pattern systems are good all-rounders that cover most types of scenes (including backlit subjects). However, because they deliver an ‘averaged’ exposure setting, they may not provide the best exposure for subjects that are backlit, dark subjects against bright backgrounds or bright subjects against dark backgrounds. Spot metering takes a single reading from a small section of the field of view. To use a spot meter, simply center the spot on the area you want to measure and press the AE lock button and/or press the shutter release half way down. This locks the exposure (and focus), allowing you to re-compose and take the shot by pressing the shutter all the way down.  Spot metering is ideal for backlit subjects and other tricky lighting situations. Centre-weighted average metering is offered mainly in DSLR cameras and a few Advanced digicams. It works by integrating readings from all over the field of view, placing more emphasis on the centre of the field. It’s best used for subjects with an average brightness range where the main area of interest is central. This pattern is unsuitable for beach and snow scenes and wherever there are large differences in brightness between the top and bottom of the picture.  Centre-weighted average metering works best with subjects where brightness is evenly distributed across the field of view.
Dynamic Range  The illustration above is an example of a subject that exceeds the camera’s dynamic range. No detail is recorded in the brightest highlights on the right side of the picture or in the deepest shadows in the centre, even though the picture is sharp and correctly exposed for the average brightness level. (Photographed with a 10-megapixel, smallsensor digicam, ISO 200, 1/60 second at f/5.6.) If a photosite doesn’t capture enough photons (light ‘particles’) to produce any signal – or a very low signal level, the result will be a totally black pixel. The camera’s image processor may attempt to amplify low signals in order to produce pixels that are not quite as black. However, at the same time electronic noise will also be amplified and this can produce coloured spots in shadowed areas. An example of shadow clipping is shown below.  The red circle shows areas of this image where no shadow detail has been recorded, despite a correct exposure overall. (Photographed with a 12-megapixel, small-sensor digicam, ISO 400, 1/800 second at f/4. The histogram display can provide a useful guide for setting exposures because its shape reflects the tonal distribution in your subject. When the graph encompasses the entire scale without showing high peaks at either end, the photograph has recorded the full subject tonal range and highlight and shadow areas should contain detail. To avoid blown-out highlights, make sure the graph does not build up at the right hand end of the scale. To avoid blocked-up shadows, try to keep the graph low at the left hand end of the scale.  The histogram for this image, which has a limited brightness range, shows subject tones are evenly distributed around the middle of the brightness scale. All cameras have problems recording subjects with an extremely wide brightness range, as shown in the two illustrations below, both taken with spot metering. 
If you expose to capture detail in the dark areas of the subject (shown in the picture on the left), the bright sky will be over-exposed and no details will be recorded. However, metering exposure on the sky causes the entire foreground to be recorded as black.  An example of an image captured with incameradynamic range adjustment. Although the foreground in the right side of the shot still looks a little dark, the overall balance between bright sky and dark foreground is much more natural than shots exposed for either highlights or shadows.
DSLR and Advanced digicam users can also minimise shadow clipping by shooting raw files. They can be adjusted when you convert them into editable formats (JPEG or TIFF) to restore shadow detail that may have been marginal at point of capture. Unfortunately, you can never recover detail that was lost through overexposure – or shadow detail lost through gross under-exposure.
Sensitivity Settings Modern image processors include noisereduction processing so it usually takes a fair amount of amplification before noise becomes visible in digital photos. However, with cheaper, less sophisticated digicams, noise can often be seen in shots taken at ISO 200. Cameras with more powerful image processors and larger sensors may produce noise-free images at ISO 400 but, for most digicams, noise will be noticeable at ISO settings of 800 and higher. Photographs from DSLR cameras usually remain noise-free up to ISO 800, with a gradual increase in noise thereafter. However, some models can produce almost noise-free images at ISO 3200 and higher. Currently the highest ISO setting available on a DSLR camera is ISO 25,600, which is available in two professional DSLRs. Shots taken with this setting aren’t totally noise-free but the noise looks more like film grain than electronic noise and images can be printed with little visible deterioration.  The characteristic pattern of coloured dots that identifies image noise from high ISO settings. Note the stuck pixel, circled in red, where neither image data nor noise was recorded, due to the long exposure (30 seconds at ISO 6400). Superficially, most image noise looks a little like the graininess produced by high-sensitivity films, although white and/or coloured speckles may be visible, either all over the image or in shadowed areas. Some noise-affected images have a pattern of bright and coloured dots that is replicated in all shots taken under the same conditions.
Speckling is usually associated with high ISO settings, while the dot pattern is more common with long exposure times. Both can be exacerbated by high ambient temperatures. As with most camera settings, photographers should balance shooting convenience against potential for noise when adjusting ISO settings. Choose a low ISO for the majority of shots – including long exposures. But be prepared to increase the camera’s sensitivity when conditions demand it, taking into account the potential for noise in the resulting shot.
High and Low Key Photography High key images are obtained by overexposing – or exposing to position the shadowed areas in the subject close to the mid-point. Simply set the camera to spot metering and position the spot on a shadow.  A typical high key image with exposure metered on the shadows. The subject’s eyes were darkened in post-capture editing to give the picture more impact. (Exposure details: ISO 125, 1/250 second at f/5.6.) The reverse applies to capturing low key shots, where metering should be on the brightest parts of the subject. Concentrate on areas that contain some visible detail – not on the brightest areas of white.  A typical low key image with exposure metered on the red and light green tops of the people near the centre of the picture – avoiding the bright white top. (Exposure details: ISO 100, 1/50 second at f/16.) Some recently-released DSLR cameras have pre-set modes for shooting high- and low-key pictures. However, most cameras will require deliberate under- or over-exposure to achieve these effects. Be wary when shooting high and low key photographs, particularly when using a small sensor compact digicam and capturing JPEG images. Both under- and over-exposure can result in lost highlight and shadow details, particularly in subjects with a wide brightness range. Shooting raw files will provide a little more scope for exposure adjustments but, in general, subjects with a wide brightness range can be more effectively converted into high or low key images with image editing software.
Silhouettes and Backlighting Flare occurs when light entering the camera’s lens is reflected backwards and forwards by the optical elements in the lens. This reduces image sharpness and contrast and creates a veiling effect over the scene. Modern lenses are coated to reduce the effects of these reflections and flare is relatively rare. However, when shooting with the light behind any subject, try to position the camera so the subject actually obscures the light source. To produce a silhouette, simply expose for the background, using the spot metering pattern (if your camera permits). Your aim is to record the outline of the subject so make sure the metering takes no account of the foreground. Using a small lens aperture (f/11 or smaller) will help to minimise the chances of flare.  Centre-weighted average metering was used for this shot. The exposure was based on the average brightness level of the sky, leaving no detail recorded in the foreground or the trees. (Exposure details: ISO 250, 1/60 second at f/13.) Backlit subjects differ from silhouettes in having detail recorded in the subject itself. The challenge is to prevent the camera’s exposure meter from being overly influenced by the bright background so spot metering is normally required to ensure subject details are captured. Where there is a large difference in brightness between the subject and the background, fill-in flash is often used to brighten the subject and provide a better balance between the subject and the background. Whichever technique you use, balancing the exposures is the key to success. With strong backlighting, flash fill is usually the best solution. Best results are often achieved by reducing the intensity of the flash by 0.7 or 1.0 EV. Watch out for flare if bright light spills around the edges of the subject and enters the camera’s lens. 
A typical backlit shot, taken with spot metering. (Exposure details: ISO 200, 1/250 second at f/11.)  Visit epson.com.au for the latest Epson printers.
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