Exposure is the term used in photography to measure the amount of light that passes though lens and hits the photosensitive material inside your camera. This could either be a film or in the case of a digital camera, a digital sensor. Now, the amount of light needs to be controlled. Otherwise, you’ll get a photo that is either too bright (over-exposed) or too dark (under-exposed) photo. Aperture controls the flow of light that goes into the camera. Shutter speed controls the amount of time that the light is allowed to stay on the photosensitive material. The ability to absorb light by the sensor is controlled by ISO.
Modern digital cameras are smart. They have a built in light mater and metering system. The Light meter can measure the amount of reflected light that is coming from the subject and metering system determine the correct exposure based on that measurement. Depending upon the amount of light, it can also display the exposure on camera’s exposure meter. Exposure meter is a scale with “zero” at the middle and usually located on viewfinder, top or rear LCD panel. “Zero” means correct exposure. Below “zero” or “minus” means underexposure and above “zero” or “plus” means overexposure.
Let’s understand in deeper detail on Aperture, ISO and Shutter speed, what they are and how they affect your photographs. A major advantage of understanding the key aspect of exposure can help you to get out of the ‘Auto’ mode and have more controls over your compositions.Now your camera metering system has determined the right exposure for you. In “Auto” mode the camera tries to keep the the indicator at “zero” level by adjusting the Aperture, Shutter speed and ISO. Unfortunately, with an automatic mode, you won’t be able to control the necessary settings like aperture, shutter speed and ISO. To be able to control these you will need to have a camera, either digital or film, that has manual settings. Fortunately most modern digital cameras have that options. If your switch to “Manual” mode, based on your current Aperture, Shutter & ISO settings, your camera exposure meter’s indicator will indicate your current exposure. It could be under (on the left), over (on the right) or correct (at the middle). In order to get the correct exposure, you will need to carefully balance aperture, shutter speed and ISO and bring the indicator to “Zero”. Unless you deliberately want under or over exposed picture.
It controls the light by adjusting the diameter of lens opening through which the light passes and reaches the sensor. It also controls the Depth Of Field (DOF). Aperture is denoted by f stops. F divided by a number. The larger the number the smaller is the aperture and vice versa. Yes, this is a bit confusing, but hopefully you will get the hang of it.
Smaller the opening the more focused the light is and as such more of the picture is in sharp focus or more DOF. Conversely the larger the size of the opening, the more diffused the light is and lesser is the area of the frame that is in focus or less DOF. Both have its advantages. Smaller aperture is required when you are shooting a group photo or a landscape when you need the entire frame to be in focus or more depth of filed is required. Larger apertures are required for shooting close-ups and especially when you need to blur out the background (bokeh), such as when shooting a flower or say a bird perched on a tree. Read more on aperture here
It controls how long the light is allowed to pass though the aperture. Longer the time more light can pass & vice-versa. This is critically important and often used in direct inverse relation with that of aperture. Larger the aperture the faster is the required shutter speed and vice versa.
Faster shutter speeds are required when you want to freeze the action, such as shooting a surfer riding the swells or may be when you want to capture the moment when your 9 year old son hits a home run. Slower shutter speeds are actually used for more dramatic results such as when you need to create a silky smooth effect of the water when shooting a waterfall. Read more on shutter speed here
In the days of film cameras people would purchase film of varying speeds. The faster the film is, the more light sensitive it is and vice versa. It is necessary to know the difference as otherwise the shots would get over or underexposed. Today’s digital cameras use sensors in place of films but they function more or less the same way.
So when shooting in low light conditions, you need a higher ISO and vice versa. Thankfully you can change that from within the camera according the conditions. There is however one important thing to note about ISO. The higher the ISO setting the more is the noise in the images. Sow when shooting in indoor or dark conditions try selecting an optimum ISO and complement it with flash to fill the dark areas. Read more on ISO here