The idea of giving a "correct" exposure to a photograph means letting the image formed by the camera lens act sufficiently on your digital sensor to give a good quality picture. The fact is there is no magical correct setting to find the balance of neither too much nor too little light falling onto the sensor.
When a photograph is correctly exposed, it has plenty of detail recorded in both the darkest shadows and the brightest parts of the picture and therefore produces a good print.
An image that is overexposed has received too much light in the lightest parts of the image also called highlights and they appear white and lost on print resulting in poorer image quality.
In contrast, an image is underexposed when it has received too little light. The dark parts of the image are thus converted to pure black, losing detail in the shadow areas. This leads the image to appear heavy and featureless when printed.
The real world generally represents a wider array of tones that can be represented on film or digital sensor and therefore you have to make an artistic decision as to where you place those tones. In this process inevitably some of the details of the image will be lost but read on to find out how to control and predict which details are lost.
The graph below shows how the aperture (intensity of light) and shutter speed (time) interrelate and at what settings they give the same overall exposure level to the image. The relationship between aperture and shutter speed is know as reciprocity.
The actual choice will depend on which part of the image you want in focus (the depth of field you want) and whether you want to record any movement as frozen or blurred.
As an example, both mages below are correctly exposed but at the same time differ greatly. The image on the left has very little depth of field but frozen hand movement achieved with shutter speed set at 1/30 second and aperture setting at f2.8 . The image on the right had 1 second at f16 and shows nearly all keys in focus but the moving hands appear blurred due to the slower exposure.
On many of the digital cameras, there are four basic exposure shooting modes that provide different approaches for setting the camera's aperture and shutter speed to suit the light in the scene.
The Manual Mode is the most basic and flexible option. It allows the photographer to set both the aperture and shutter speed settings. Because of this it takes longer to set than any other mode.
In the Aperture Priority (Av) mode the photorapher sets the aperture and the camera adjusts the shutter speed automatically to suit the light in the scene. This option can be quite helpful for close-ups, where the depth of field is critical, as well as night photography. The mode will handle a very wide range of lighting conditions, since cameras offer far more potential shutter settings than f-stops. You just have to pay attention if the camera is picking a slow shutter speed thus resulting in blur.
Shutter Priority (Tv) mode works the other way. The photographer sets the shutter speed and the camera selects the aperture automatically to suit the light in the scene. The mode is useful in sports and any action photography where you must maintain control over the appearance of movement and image blur.
Auto or Program mode allows the camera to determine the shutter and aperture settings automatically based on the light in the scene. The fully auto program is handy for rush situations.
Learning to recognise situations where special care over the way light is read will greatly improve a shot and here are seven steps to follow in order to get a better image exposure .