Photography is all about light. The key to mastering the art of photography, therefore, lies in mastering light and how to control it. Luckily, there are only three ways by which cameras control the intensity of light hitting the sensor or film.
The three variables referred to as the exposure triangle are; ISO (sensitivity) which works different in film cameras, aperture (amount) and shutter speed (time). If you know and master these 3 variables you will be on your way to taking pictures, that are properly exposed.
1-Film Speed/sensor sensitivity (ISO)
ISO is how sensitive your sensor or film is to light. The best way to understand ISO in this digital age is to think back to the days of film cameras. Photographers back then had to use different film depending on the light conditions they were working in. For example, if you were shooting indoors with low light you will use film that was very sensitive to light as opposed to shooting outdoor with lots of light where you would use less light sensitive film. Photographers back then had to physically change the roll of film to suit the light conditions they were working in. With digital cameras, there is no need to physically change anything all you must do is dial in how sensitive you want your camera sensor to be. As with film, if you are working in a low light situation you will have to increase your sensors’ sensitivity to light and vice versa. Note that ISO scale is progressive and proportional to light sensitivity. As you increase the ISO number the more sensitive the sensor becomes. Depending on your camera, the base ISO could be 100 (less sensitive to light) and it can go to 6400 (more sensitive to light) or higher in newer cameras.
A word of caution ISO should be treated with care. The more you increase your ISO the more you run into the problem of noise in your image. It is advisable to try, where possible, to use the lowest ISO setting of your camera for cleaner and sharper images.
The second way to control light is by controlling the time the sensor is exposed to light. When we trigger the camera to take a picture all we are doing is opening the shutter to expose the sensor or film to light. Think of the shutter as a curtain on your window. You open and close your curtains depending on whether you need light or not. The same principle applies to the shutter in your camera. When you want to take a picture, you open your shutter to expose the sensor or film to light. The duration of time it takes for the curtain to open and close is what we call shutter speed. The speed with which you open and close the shutter will determine the amount of light reaching your sensor hence your exposure. The more you leave the shutter open the more light will reach the sensor and conversely less light will reach the sensor if the shutter is closed quickly.
The unit for measuring shutter speed is time either seconds, minutes or even hours. The speed with which the shutter open and close can range from a fraction of a second to as long as you want (Bulb mode). Since shutter speed determines the length of time your sensor or film is exposed to light, it is imperative to pay very close attention to the available light when dialing in the shutter speed. Opening the shutter for a long time in bright light risk overexposing the image. On the other hand, in a low light situation short shutter speed my yield underexposed images. You, therefore, need to evaluate the light you are working with to determine how long you are going to keep your shutter open in order to get a properly exposed image. One tool that can help you to determine the amount of light and hence a proper shutter speed is a light meter.
What are the effects of shutter speed? Shutter speed controls motion in your pictures. If you use a slow shutter speed your camera will capture movement resulting in a blurred image. To void blurred images (unless it is your artistic vision), use a short shutter speed for example if you are shooting handheld you need a shutter speed of at least a 60th of a second this is a fast-enough speed to eliminate any shaking from registering in your picture.
The final away to control light is by regulating the amount of light entering the camera. The gate or door that regulates the amount of light entering the camera is called aperture. Aperture is the opening in the lens which, depending on the amount of light that you need, can be opened wide or closed-down. To make it easier a scale was developed which indicates the size of the aperture - how wide or closed the aperture is referred to as F-stop. The F-stop scale ranges from wide open F1.2 all the way to closed down F32. The scale can be confusing to beginners as the relation between the F-stop number and the size of the lens opening is inverse. As the F-stop number gets bigger the amount of light entering the lens is reduced. A trick to help you remember the relationship between the F-stop number and how open the lens is (i.e. the amount of light) is to regard the F-stop number as a fraction. F1.2 will, therefore, be 1/1.2 which is obviously bigger than F32 (1/32).
So how can you use F stop to control light? Well because F-stop is the opening in the lens it will allow different amounts of light depending on its size. When you need a lot of light for example when you are shooting indoors you will open your lens wider. On the other hand, if you are shooting in the middle of the day in bright sunlight you will obviously close-down the lens to reduce the amount of light reaching your sensor.
The effect of F-stop is nevertheless not just limited to the amount of light it also affects the area that is in focus or what photographers call depth of field (image below).
The reason for this effect is a little technical for our discussion all you need to remember is that the more you open you the lens (smaller F-number) the shallower the depth of field and vice versa. Landscape photographers, therefore, tend to us smaller F-stop numbers to have images that are sharp from front to back.
If you master these 3 variables and their relationship you will be on your way to take properly exposed images.