Photography A - Z the Easy Way: T - Time ExposureWriten by Eric Hartwell
Most times, when you take a photo, the camera will make calculations to determine the correct shutter speed settings. These speeds will typically be in the range 1/15 to 1/000 of a second. At other times, especially when the light is low, the shutter speed will be longer than this. Shutter speeds greater than a second would typically be termed timed exposures.
In some cases, exposure times can reach several seconds or minutes and at these low speeds mounting the camera on a tripod is essential to prevent camera shake and blur. The lower the light, the lower the ISO and the smaller the aperture then the longer the shutter speed will need to be.
Modern digital cameras are capable of calculating the exposure time in low light and many have a built-in speed setting of up to 30 seconds. In other cases you should use your B or bulb setting to achieve the desired timed shutter firing.
In very low light, such as at nighttime, you should calculate your expose then readjust according to the results. By taking several pictures with a long exposure you can determine which is the best exposure setting bearing in mind that the camera meter (and even a hand held on) can be fooled by the low light and often high contrast scenes.
Eric Hartwell runs the photography resource site http://www.theshutter.co.uk and the associated discussion forums as well as the regular weblog at http://thephotographysite.blogspot.com.