An image that appears as too bright is classed as overexposed and an image that appears too dark is classed as underexposed. In digital photography, 3 factors control how bright or how dark an image is – Shutter speed, aperture and ISO
On your digital camera, the light (or image) is captured on a digital sensor. The sensor as a film replacement. Covering the sensor is something called a curtain which opens and closes when you press the shutter on your camera. Below you can find your digital sensor controls the amount of light that hits it.
•Shutter speed refers to the amount of time the image sensor is left open for. The longer it is left open the brighter the image will be, and vice versa.
•If you wish to capture droplets of water or use your camera for sports the fast shutter speeds (or short exposures) will allow you to do this Use slow shutter speeds to create light streaks (see image above), or to make water appear like steam.
•General DSLR cameras will have shutter speeds from 1/4000 of a second to around 30 seconds.
•Bulb function on a camera allows you to keep the shutter speed open for as long as the shutter button is held down for. You can purchase shutter release cords for most DSLRs which will allow you to lock the shutter open until you unlock it, which will save you having to keep your finger on the shutter button.
•Think of aperture like a window. if a room contains a large window and another room of the same size contains a smaller window, the room with the larger window will allow more light to pass through thus producing a brighter view
•So aperture is effectively the size of the hole that allows light to pass through the lens to the image sensor (or film)
•Aperture is measured in F-stops. This is where the confusions begins. F-stops are written as fractions so the smaller the F-stop the more amount of light that will be allowed to pass through
•Full F-stops largest to smallest – F1, F1.4, F2, F2.8, F4, F5.6, F8, F11, F16, F22, F32
•ISO refers to the sensitivity of the sensor. The more sensitive the sensor is set to, the brighter the image will be. Increasing ISO will amplify the light that is captured by the sensor to effectively produce a brighter image.
•Increasing the ISO, is fantastic for low light conditions but comes at a price. Because increasing the ISO amplifies the information that is captured by the sensor, the resulting image will be full of noise or speckled pixels.
•Therefore in most scenarios, you should use the lowest ISO setting possible to maximise image quality. I try not to go any higher then ISO800 (see below for full ISO numbers). But even at ISO800, for some cameras this can result in poor quality images.
•Full ISO – 100, 200, 400, 800, 1600, 3200.
Combining shutter speed, aperture and ISO
•For most exposures or shots, you should use the cameras built-in exposure meter to calculate the correct exposure. But say if the camera meters an exposure with shutter speed of 1/20 at an aperture of F11. This would result in a blurred shot unless the camera was placed on a tripod. But by increasing the aperture by 1 stop (so using a lower F stop, remember fractions), making it F8 (see full F stops above). You can then take the exposure with a shutter speed of 1/40. Double the previous time.
•So increasing the aperture by 1 F stop, will allow you to take a shot with a shutter speed twice as quick and vice-versa.
•If you are already using the largest F stop your lens can handle. But the built-in meter on the camera is showing a shutter speed of 1/20. Again this is too slow in most situations. By increasing the ISO, say from ISO200 to ISO400, you can then use a shutter speed of 1/40.