Everytime you push the shutter button on your camera, three major decisions are made. Those three decisions are the shutter speed, film speed and aperture value. These three things are the factors that determine proper exposure of your picture. You may have plenty of light, but if you don’t have these settings right the photograph will not come out right. Even if you don’t shoot in manual or priority modes, understanding this exposure triangle will help you improve your photography. Getting the basics of this triangle down will also enable you to anticipate what your camera is going to do in different settings, allowing you to have more control over the image you capture. The goal right now is not go deeply into the three elements, but rather to give a brief overview of how they work together to create the right exposure.
The first part of this triangle is the film speed. This is indicated by an ISO number (ISO is just the abbreviation for the International Organization for Standardization). The lower the ISO number, the less sensitive your “film” is to light. Sensitivity to light is how intensely your sensor records the light hitting it. Most cameras have ISO capabilities covering the range of 100 to 1600, many point and shoots have ISO capabilities of 50 and 80. Many higher end cameras have capabilities way beyond 1600. Doubling the ISO value doubles the sensors sensivity to light. So ISO 100 is twice as sensitive as ISO 50 and half as sensitive as ISO 200. ISO 800 is four times as senstive as ISO 200. Simple math really. This means that one way to brighten an image’s exposure is by upping the ISO value. A high ISO can help make the picture brighter, but it will also increases noise in the image. This noise is the added grain and speckles in an image that take away from the quality and clarity of the picture. This means there is a trade off between the ability to properly expose the picture and the clarity of the picture taken.
The second part of the exposure triangle is the shutter speed. This is probably the easiest of the three elements to explain. Between the lens and the sensor is a gate that opens and closes to allow the light through to the sensor. The length of time that gate is open is called the shutter speed. This speed is usually measured in fractions of a second. The larger the second part of the fraction, the less time the gate is open. Some images require shutter speeds as slow as a second or longer, but many pictures are taken with a shutter speed 1/60th of a second or faster. The longer the shutter is open, the more light hits the sensor. As the shutter speed doubles, the amount of light hitting the sensor is cut in half. A 1 second shutter speed lets in half the light of a two second shutter speed. A 1/100 shutter speed lets in twice the light of a 1/200 shutter. As with the ISO, there is a caveat to consider. Shutter speed communicates motion, either through freezing the action or blurring the action. A slow shutter speed can blur the movements of your star soccer player, adding to the picture the feeling of speed and motion or just turning him into an unrecognizeable blob. A fast shutter speed can take that same image and capture it mid stride, freezing the action as it happens. A shutter speed that is too slow can also result in a blurred picture through camera shake. The accepted rule of thumb to help prevent blur from camera shake is set your shutter speed no briefer than your focal length. If you have your camera lens zoomed out to 100mm, your shutter speed should be no slower than 1/100. If you zoom out to 35mm, you can probably get away with a shutter speed under 1/50. The new VR lens have allowed photographers to get crisp shots at even slower speeds, but as a general rule, if you want to ensure a sharp photo in those instances where you have to shoot slow, use a tripod.
The last part of the exposure triangle is the aperture value. To my mind the aperture is the most confusing of the settings. Not because the concept is difficult, but because the expression of the aperture values seems backwards. The aperture is the opening within the lens that allows light through the lens. The diameter of the front of your camera lens is the not the diameter of the opening that allows light through. That opening is often quite a bit smaller. The measurement of that opening is expressed in an aperture value. Here’s where it starts to gets funny, the smaller the number the larger the hole. An aperture of f/2.8 is much larger than an aperture of f/8. These numbers also do not follow an immediately apparent pattern, so that it is not easily discernible how much difference in light passage there is between f/2.8 and f/4. The reason for this has to do with a ridiculous equation that measure the lens focal length and the actual diameter of the aperture and then expresses it in a confusing alpha-numeric fraction thingy. What is important to remember is the larger the f- number, the less light comes through to the sensor. The other thing to be aware of with aperture is that the size of your aperture affects the depth of field in the image. Depth of field is how much of your picture is in focus from front to back (depth). The larger the aperture (smaller number) the less depth of field. The smaller the aperture (larger number) the more depth of field.
These three things work together in every picture to provide the finished exposure and greatly impact the final composition. When you are shooting, think about how these three elements are working together to produce the final exposure of your shot. If your camera will allow to switch modes, switch to something that gives you control of these three features and start experimenting. You may find your camera will do stuff you never dreamed.