From Photo Review Issue 45:
How do you set up your camera when lights are low? Switch on the flash? Adjust the ISO sensitivity upwards? Reach for a tripod? These are all valid reactions and all have their ‘plusses' and ‘minuses'.

Low light levels can occur at any time of day; not just at night. And they can be found indoors or out (eg, on overcast days and during storms). Unless your subject is near a window or under bright artificial lighting, almost any indoor situation will require longer exposures when you want wide depth of field. You may also need to deal with mixed light sources and balance daylight against incandescent or fluorescent lighting.

Shooting in low light may also mean shooting at slower shutter speeds, so you need to adopt strategies to prevent camera-shake-induced image blurring. The following options are available:

1. Supporting the Camera
A tripod is usually considered the best camera support because it provides the greatest stability. Sports and wildlife photographers often prefer a monopod, which is less bulky and easier to carry. However, monopods only stabilise the camera in one direction so other strategies are often required as well.

If you don't have a tripod or monopod at hand, anything that helps steady the camera, such as a wall, table, rock or tree branch, will give you sharper photos of stationary subjects. Many photographers carry a beanbag to put their camera on in such situations as it protects the camera body while preventing it from slipping.

A tripod is vital for really long exposures after dark. This 35-minute exposure was taken with a 10mm lens on a Canon EOS 40D body. (f/7.1 at ISO 200.)

2. Stabilising the Shot
Image stabilisation has done a lot to help photographers take sharp pictures in dim lighting. It doesn't really matter whether the stabilisation is built into the camera body (sensor-shift) or into the lens (lens-shift or optical), although lens-based stabilisation is slightly more effective. Neither type will affect the quality of the image. Depending on their focal length and/or zoom range, most stabilised lenses will provide between two and four f-stops of shutter speed advantage over an unstabilised system.

The rule-of-thumb for hand-held shooting with unstabilised lenses is to use a shutter speed at least equal to the reciprocal of the focal length of the lens you're using. For example, with a 400mm lens, you need a shutter speed of at least 1/400 to get sharp hand-held shots. With a stabilised 400mm lens, you can take sharp pictures hand-holding it at 1/100 or even 1/50 second.

This photograph by Neil Medland shows the benefits of in-camera stabilisation. It was taken with a Pentax K200D using the 18-55mm kit lens set at 18mm. The smallest lens aperture (f/22) and ISO 100 sensitivity were used to provide a slow enough shutter speed (1/2 second) to blur the moving water. Neil says: ‘I actually forgot my tripod on this day so I sat the camera on a towel on top of a rock and set the self timer to two seconds.' The added stabilisation was enough to create this successful shot.

3. Increasing the ISO Setting
Virtually all digital cameras provide a range of ISO settings that usually start at ISO 100 and range up to ISO 800 or ISO 1600. Large-sensor interchangeable-lens compact cameras may go as high as ISO 6400 and professional DSLRs can reach ISO 102,400! However, the ISO limit should be seen as the point by which image noise is quite obvious and, therefore, such high settings are best reserved for situations where the high ISO setting is the only way to get the shot.

If you're shooting with a small-sensor compact digicam, it's best to avoid using settings above ISO 400 because the resulting pictures are almost certain to look grainy because of image noise. Some shots may also become slightly blurred as a result of noise-reduction processing.

With DSLR cameras the larger the sensor, the higher the ISO number you can use before noise becomes obvious. Cameras with ‘Four Thirds System' sensors (18 x 13.5mm) can usually deliver good results up to ISO 1600 but most produce noise-affected images at ISO 3200. Cameras with ‘APS-C' sized sensors can sometimes reach ISO 6400 before noise becomes visible. But you need a ‘full-frame' (36 x 24mm) sensor to shoot with ISO settings above this point - and noise can usually be seen when shots taken at ISO 6400 and above are enlarged to A3 size.

This photograph by Tim Swavley required high sensitivity settings. It was taken with a Sony Cyber-shot DSC-HX5V camera about 70 metres underground. There is no natural light in this environment so the light source came from a portable battery powered flood lighting rig. (4.25mm focal length, 1/4 second at f/3.5, ISO 3200.)

4. Bracketing Exposures
Dim lighting and contrasty lighting can both fool cameras' exposure meters, producing shots that are under- or over-exposed. Bracketing shots involves taking one shot at the exposure you think is correct, then shooting additional frames with more and less exposure. It's a bit like hedging your bets.

Many cameras have built-in auto exposure bracketing functions, which makes this very easy to do. Most cameras cover a range from +2EV to -2EV (equivalent to a spread of five f-stops) and bracketing is usually available in half- or one-third-stop increments. Normally three shots are taken in the sequence. Higher featured cameras may cover a range from +5 to -5 EV and offer sequences of three, five or seven shots.

Having some idea about what the correct exposure should be for a specific situation makes it easier to set the starting exposure point for a bracketing sequence. The table below provides some typical exposure times for a variety of common subjects, based on setting the camera's ISO to 100 and using an aperture of f/4. (Note: These settings are not necessarily the optimal settings for all situations so, if either the ISO or lens aperture setting is altered, these times must be adjusted to compensate.)

Finding the right exposure for sunsets can be tricky because the camera's metering system tends to over-expose shots, removing much of the intensity of the colours. Bracketing shots can help you to obtain worthwhile pictures. This shot is the darkest of a set of three taken at 1EV intervals with exposure compensation set to -0.3EV. (Canon EOS 5D Mark II with EF 24-105mm f/4L IS USM lens at 85mm, 1/50 second at f/9.5, ISO 200.)

5. Using Flash
Entire books have been written about flash photography; so we will only cover use of on-camera flash briefly in this feature. The main problem with on-camera flash is the position of the flash, which lights the subject from the viewer's direction. Such flat lighting can destroy many subtle tonal modulations in the subject - and produce red-eyes in portrait shots.

There are a couple of ways to deal with these problems. The simplest is to reduce the intensity of the flash so it becomes an adjunct to the available lighting instead of the main light source.

You can also ‘bounce' the light from the flash off light-coloured walls, ceilings or other kinds of reflector.

One advantage of flash it that it enables you to hand-hold the camera in dimly-lit situations. Another is your ability to control where the light is distributed and its intensity. Many DSLR cameras can support multiple-flash set-ups in which several accessory flash units are triggered by the on-camera flash. Studio subjects like portraits and product shots are ideal for such arrangements.

Photographers need to be aware of how far the light from the flash will travel. Few on-camera flashes can provide full intensity to subjects more than five metres from the camera. Light falls off with the square of the distance to the subject so by the time the subject is 10 metres away only half the light will reach them. At 20 metres they receive only one quarter of the light intensity - which isn't enough to expose the subject correctly.

Long Exposures
Many low-light shots are captured with exposures of more than one second, which requires the camera to be tripod-mounted (or at least effectively prevented from moving). Since most cameras come with tripod sockets in their base panels, even the most basic of point-and-shoot models can be used on a tripod.

However, many cameras don't support long exposures - or require use of a special shooting mode (Long Shutter, Night Landscape, Fireworks or ‘Starry Sky') to reach shutter speeds of one second or longer. In these modes - and for all exposures longer than a second - it's wise to trigger the shutter with the self-timer to reduce the risk of camera shake blurring the shot.

More sophisticated cameras with P, A, S and M shooting modes usually support exposures of up to 30 seconds in the shutter-priority AE mode (and with manual exposure). For longer exposures you must use the B (Bulb) or T (Time) exposure settings. (Very few modern cameras include a T setting because the B setting is adequate when used with a wired remote trigger.)

The difference between B and T modes is straightforward. In the B mode, the shutter remains open as long as the release trigger is held down, while in T mode, the shutter opens with the first press of the release trigger and closes with a second press. Both allow you to achieve exposures that are as long as you wish - provided the camera's battery has enough power to support them.

Noise is an ever-present potential problem with long exposures - and the longer the exposure, the more likely it is to be noise-affected. Many cameras apply automatic noise-reduction processing for all exposures of one second or more. In most cases, dark frame subtraction is the preferred method. This system takes two pictures, one with the shutter open and the next with the shutter closed. The second shot records any image noise and ‘stuck' pixels. It is mathematically subtracted from the first picture to remove the noise pattern.

This photograph of the ‘Stairway to the Moon' phenomenon at Broome in Western Australia was taken with the camera on a tripod. Standard level high ISO noise reduction was applied to minimise noise at ISO 1600 sensitivity. (Canon EOS 5D Mark II with EF 70-300mm f/4.5-5.6 DO IS USM lens at 300mm, 1/8 second at f/5.6.)
This is an article from Photo Review Magazine Sep-Nov 2010 Issue 45.
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