I was asked recently via Twitter if I had any tips on taking photographs in low light situations. Now, I'm no photography expert but here's a few pointers that I've picked up since owning a DSLR.

There are three main elements which control how much light is needed to take a photograph: sensor sensitivity (ISO rating), aperture and exposure time. Each of these settings will have an impact on your picture in ways other than increasing the brightness.

Deciding which settings to use is entirely subjective. It depends on the subject of the photograph, the style of the photographer and the camera/lens combination. It's also a balancing act. If you change, say, the aperture to create a narrow depth of field, you may need to change either sensitivity or exposure to maintain the chosen light level. You may set an exposure time suitable for hand-holding your camera, but this may not allow enough light, so the sensitivity of aperture is adjusted accordingly.

Sensitivity (ISO):

The ISO rating is often given as a number such as 100, 200, 400 etc. Doubling the number increases the amount of light on your picture, but the side effect is more noise. In days of film, this would have been referred to as grain but nowadays digital noise is less pleasant and too much can ruin a picture.

To make things more difficult, the amount of visible noise depends on the camera. Expensive professional-grade cameras tend to have much better performance at higher ISO levels, whereas a cheaper camera may ruin pictures at ISO800 and cause distracting noise at ISO400.

If you have a DSLR, don't assume that it will cope well at the higher ends. I wouldn't take my Nikon D80 above ISO800 - and even that I'd use as a last resort.

Aperture (f-number/f-stop)

The aperture is the size of the opening at the front of the lens. Working in a similar way to the eye, a larger opening means more light will get through to the sensor compared to a smaller opening.

Again though, there is a consequence of opening the aperture. The wider it is, the narrower the depth-of-field. This means that the distance which is in focus is less. It can be used to great effect - you could use a wide aperture to take a portrait photograph with a nicely blurred background for example - but this would not be ideal for landscape photography where it would be more appropriate to have everything in focus.

The aperture is given as an f-number such as f/1.4, f/2, f/2.8, f/4, f/5.6, f/8. These numbers are 'full stops' - that is each one provides half the light of the previous. DSLR cameras will also allow the photographer to set half-stops or even third-stops.

The size of the aperture is limited by the lens in use. As a rule, more expensive lenses provide larger apertures than their cheaper counterparts. Zoom lenses will generally not provide the same maximum aperture throughout their range.

Exposure

In my mind, this is the easiest element to understand in terms of it's effect and consequences. Exposure simply means the amount of time the shutter is open and the sensor is exposed to light. It is measured in seconds or fractions of a second depending on the length of time.

Perhaps obviously; the longer the exposure time, the more light hits the sensor and the brighter the resulting image. The main consequence of a longer exposure time is camera shake or object movement.

Camera shake occurs when the length of exposure is simply to long for a person to keep the camera still. This effect is magnified for longer lenses and can be resolved by the use of a tripod. The recommendation is to look at the length of your lens and to use this figure as the minimum shutter speed. For example; if I am using a 200mm lens and am hand-holding the camera, I will not set my exposure any slower than 1/200 seconds. For a 50mm lens, the exposure will be 1/50 seconds.

Object movement occurs when something in your picture moves whilst the shutter is open. A little movement looks like an out-of-focus shot. Lots of movement can make objects appear ghostly. This can be used to great effect - think of a picture of a busy train station perhaps, where the building itself and perhaps trains are sharp and in focus, but all the people are blurry images.

Another consequence of very long exposures can be noise which can be introduced when the camera sensor gets hot due to prolonged use. This, however, is usually only seen on very long exposures of minutes rather than seconds.

Artificial Lighting

One other way of dealing with low light may be artificial lighting. The camera flash may be frequently used to make subjects brighter but it's important to consider how effective it will be. For example, when watching concerts you will often see the flashes of cameras going off around the auditorium. Often these are compact cameras or even mobile phones. All the flash will do in this case is illuminate the head of the person in front of you. DSLRs may have larger flashes but they too are limited to a few meters.

External flash guns can be purchased which have more power again, but you're not going to illuminate a landscape! The advantage here is that the flash can be mounted off-camera to create or eliminate shadows around a subject. These units also reduce the risk of red-eye because the angle of light going into the eye is higher. Light reflected from the back of the eye is not sent straight into the camera lens.

Further Reading

The internet is full of useful tips and guides for photography. Wikipedia may be an obvious starting point and I have attached some links to this post which may be helpful. Specialist forums are a good place to ask questions too. I'd recommend Talk Photography - again the link is posted in the 'Related Links' section of this post (see below)

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