Understand: the exposure triangle
In photography, exposure is the quantity of light detected by the photosensitive element (photographic film for analog cameras, or a digital sensor for digital cameras). The higher the quantity of light detected by the photosensitive element, the higher the exposure. A photograph can be over-exposed, under-exposed, or properly exposed. Too much light detected by the photosensitive element leads to an over-exposed image (i.e. the image is too bright). On the other hand, too little light detected by the film or sensor leads to an under-exposed image (i.e. the image is too dark). Exposing properly means making sure that the photosensitive element detects the right amount of light.
The exposure triangle
The following parameters form together what we call the exposure triangle :
Aperture: the size of the opening in the lens when a picture is taken.
Exposure time: the amount of time during which the shutter remains open.
Sensitivity (ISO/ASA): the measure of the photosensitive element’s sensitivity to light. For digital photography, we talk about the digital camera sensor’s sensitivity to light which is noted ISO. For analog photography, we talk about the photosensitive film’s sensitivity to light which was noted ASA and then ISO in the later years.
Light enters your camera through the lens, through an opening called the entrance pupil. The bigger the aperture, the more light enters your camera. Behind the entrance pupil lays the shutter. It works like a little door that prevents light from entering your camera. When taking a picture, that door opens for a given amount of time. The more time the shutter remains open, the more light enters your camera. Behind the shutter, inside the camera, lays the photosensitive element that will capture light. Its sensitivity can be more or less important and the more sensitive it is, the more light gets to your image.
All things equal, if you increase aperture, more light will be detected by the sensitive element. If you increase the time during which the shutter remains open, more light will be detected by the sensitive element. If you increase the sensitivity of the sensor, more light will be detected by the sensitive element. Note that if increasing aperture or sensitivity will increase exposure, increasing shutter speed will decrease exposure. You have to decrease shutter speed (i.e. to increase exposure time) to increase exposure.
Why three and not just one?
At this point, you could ask yourself why there are three different ways to increase the quantity of light detected by the photosensitive element. Actually, depending on the parameter you choose to increase (aperture, exposure time or sensitivity), side effects will occur. Each of these parameters has its own pros and cons, and depending on the situation and what rendering you want to get, you will choose to increase one or another.
