Exposure
Exposure basically means how much light enters the camera and is affected by several factors:
1 The aperture setting.
2 The shutter speed (how long the sensor or film is exposed to light)
3 How bright or dark the light is.
4 The ISO (sensitivity) setting on your Camera.
Since unless you are a professional photographer with tonnes of special lighting equipment you cannot control the amount of light you have (with the exception of a flash which will only work over a short distance) exposure is going to be controlled by shutter time and aperture setting.
First let’s look at 3 examples of the same picture, one underexposed, one with approximately correct exposure and the third over exposed.
With the under exposed image you will see it is too dark, not enough light has entered the camera and it has not been able to record an accurate image of what we see, detail is lost in the shadows and dark areas.
The correctly exposed image is neither too bright or dark, all the information available has been captured, no detail is lost in shadows or highlights.
In the over exposed image no detail is lost in the dark areas, but those which approach pure white are “burnt out”, too much light has entered the camera and its sensor has become over saturated.
The major problem with over or under exposure is that even though you can correct it to some extent using software on your computer (or by over/under developing your prints in a dark room) you will never regain the detail that has been lost.
Exposure control by adjusting the aperture, you will normally have to make the aperture smaller (higher f-number) if it is a very bright day, or if you want the shutter speed to be slow enough that you can record motion blur. Closing the aperture will give a higher depth of field however and you might not always want that.
Conversely Controlling exposure through shutter speed runs into problems in low light levels when you have to expose the sensor for too long the small movements you make whilst holding the camera will show up more dramatically, and if you want to capture moving people etc. they will be blurry as the sensor picks up part of their image in several different places!
In reality unless you have an ultra steady hand or an image stabilisation system on your camera shutter speeds under 1/200 of a second will give hand shake and blur in an image.
In reality correct exposure, and achieving the feel of the image you want are a compromise between aperture and exposure time.
But just remember: if your shots are blurry looking open the aperture if you can (or over short distances use the flash), and make the shutter open for less time.
If you cannot reduce exposure time any further on a bright day make the aperture smaller.
To explain the ISO setting on a camera, just think of it as a stereo, the higher you turn up the volume the louder the music is. This means that by using a higher ISO you effectively turn up the volume, like you would if you are listening to a quiet recording. Unfortunately just like when you turn up the volume on a stereo you start to get a hiss in the background (this is called noise), the same is true of a camera sensor (it is the equivalent of grain in film which also goes up with ISO). The noise is evident as speckles of different colours all over the image, particularly in the darker areas.
This means that where possible you want to always use the lowest ISO you can to achieve the cleanest results, but if you really want to take a picture and cannot get enough light for a reasonable exposure using aperture and shutter speed it is another variable at your control.
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