Exposure Triangle? - The Exposure Pyramid of Photography
The Exposure Pyramid
There are three basic variables that help to determine how bright or dim a photo will be; shutterspeed, aperture, and ISO. The shutterspeed, the speed which the sensor or film is exposed to light, is the one variable that is thought of most often. The aperture is the diameter of the hole where light passes through. The ISO refers to the sensor or films sensitivity to light. Each of these three variables effects the way that a photograph is exposed. When used correctly, this knowledge can help to get the proper exposure to your photograph.
ISO - Sensitivity to Light
ISO was traditionally used to measure the sensitivity of film. The higher the speed of the film, the less light was needed to expose the film and capture an image. Low speed fill, with a speed of ISO 100, is used to take pictures primarily in sunny outdoor settings, or at other bright events.
ISO could not be changed easily with film cameras, it the photographer had to change the film itself. When digital SLR cameras came into existence, it become possible to change ISO on the fly, making it a viable option for photographers to change on any settings. This has been especially useful to wedding photographers, who have, on a vast majority of occasions before digital, had to waste numerous frames on a roll of film in order to get the right speed into their camera.
ISO settings can improve the ability of a photographer to take low-light photographs. Many digital cameras have settings that go to ISO 3200, or even ISO 6400. Increasing these settings will allow you to take a photograph with a smaller aperture or higher shutter speed, but it also introduces noise. When deciding whether or not to increase ISO to get a proper exposure, the photographer needs to determine whether or not noise is an acceptable element to have in the photograph.
Shutter Speed - The Time of Exposure
Shutter speed is the element of photography exposure with which most people are experienced. The shutter speed refers to the amount of time that the sensor or film is exposed to light. Whether a film camera or a digital camera is used, the premise is the same. You set the exposure to determine the amount of time light will let onto the capture mechanism.
The longer the shutter speed, the longer the light hits the sensor or film. It is measured in seconds, and is done in fractions. The photographer can take a photograph where the shutter speed is 1/8000 of a second, or where the the photographer has to choose when the shutter is closed (known as bulb mode). I have taken photographs where the shutter was open for 30 minutes.
The trade off for shutter speed is motion blur. The average shutter speed for any sort of portrait should be 1/250 of a second. This is to help prevent blur of a subject moving. There is also a general rule that says the minimum shooting speed, especially for a new photographer, should be 1/focal length. For example, if the photographer is shooting with an 800 mm lens, the shutter speed shouldn't go more then 1/800. This is to prevent the natural shake of a hand when taking a photograph.
Aperture - The Size of the Hole
Aperture is the size of a hole in relation to focal length of the lens that permits light to be inside the camera body. The size of the hole is determined by the number. The smaller the number, the larger the hole. A lens with the aperture set to 1.8 has a larger opening in the lens than a camera with an aperture set to 11.
The trade off of opening the aperture is the loss of depth of field. With a larger aperture, much less of the photograph will be in focus. This has it's merit, and is often referred to as bokeh. Sometimes, though, it is not seen as beneficial. Portraits are often taken with an aperture setting of 11. This allows the eyes to be in focus, while still keeping acceptable focus on the face. If the aperture setting is too low, then aspects of the face would be out of focus.
Using the Settings
The right exposure can be achieved with the correct settings. Changing ISO is the first step to any situation. Making sure you have the right ISO setting for the environment is helpful. If the photograph is taken in a darker environment, a higher speed is the place to start. From there, setting the aperture and shutter speed will depend on the amount of the photograph that needs to be in focus, as well as the amount of motion blur that is acceptable. Using a light meter, whether in camera or handheld, will also give you an idea of the settings needed in order to obtain the proper exposure.