The exposure in the photographic sense means exposure of the film or the image sensor to the light. Exposure is a critical and single most important fundamental of photography. Exposure rests on the three pillars of the aperture, shutter speed – the time the aperture remains open during the shot and the ISO which is the sensitivity of the film or image sensor. The correct grasp of these concepts enables you to get the right exposure in challenging circumstances. Let’s get to it.
The camera lens has a diaphragm which has an opening through which the light comes into the camera. This opening in the called the aperture or the lens aperture. Again, Lens > diaphragm > opening = aperture. In most mid to high-end cameras this aperture can be adjusted to vary to control the amount of light that comes in through it. The aperture of the camera is measured in f-stops or f/number. A higher f-stop means a smaller opening and thus less light entering the camera. You can do a lot by varying aperture alone. See my previous post on An introduction to Aperture Magic.
The shutter speed
The shutter speed determines the length of time the aperture remains open to allow light into the camera. This adds another dimension to the process, the aperture opens to a certain f-stop and the shutter speed determines how long it remains open before closing the aperture. Thus a higher(faster) shutter speed means less light entering the camera.
The ISO traditionally is the sensitivity of the film being used in the camera. When the cameras went digital, there was no ISO. To fill the gap they implemented ISO as a level of amplification of light received on the image sensor. An image shot at ISO-200 will be twice as bright at the one shot at ISO-100 given the rest of the factors remain constant.
The concept of exposure rests on these three pillars. However there’s a fourth to be considered. This is the lens itself. The lens has certain properties you cannot change and some are better suited for one application while the others for their own. So photographers tend to have multiple lenses in their kit and use them as the situation demands. Different lens have varying apertures amongst other specs. A brighter lens will have a larger aperture (smaller f-number) than that of a less brighter lens. A brighter lens thus is a popular choice for new photographers. It allows more light and you can leverage this in by being able to shoot at higher shutter speeds. Also there are other things like the quality of material, the diffraction etc. are a big consideration before buying a lens.
Let’s review and also learn some of the effects of varying the aperture, shutter speed and ISO on the shot. A higher ISO means more sensitivity. You will want to use a higher ISO when shooting in dim environments. A slower shutter speed means allowing the light to enter for a longer time. What if the subject moves while the shutter is open? You will see that the image comes out to be blurred as the camera tried to capture the image throughout the action. However lower shutter speeds are ideal when you are shooting a still subject like composing a portrait etc. A higher shutter is warranted when you want to shoot a moving subject. A good example of this is a sport or action photography where you want to freeze the moment into a perfect picture.
While the aperture is also another element to control the amount of light, its effect on the image are complex. A detailed article covering the aperture is An introduction to Aperture Magic.
Photo by brentbat