The dictionary defines exposure as “an act or instance of being uncovered or unprotected.” In photographic terms the thing that is being uncovered is the film or, with modern DSLR, the sensor. What the film or sensor is being exposed to is light.
If the sensor is exposed to too much light then the resultant image will be bleached or washed-out (over-exposed) and, conversely, if it is exposed to too little light the results will be dark or black (under-exposed). Exposure, then, is not about some fixed standard, but rather the skill of using the camera to create the exposure that you feel best suits the photograph you are taking.
Over and under-exposure can be used to your advantage if you understand them and apply them for artistic ends. Therefore, in order to attain a correctly exposed picture we need to control the amount of light being allowed in to the camera and on to the sensor.
The amount of light permitted to the sensor is controlled by three key factors:
Shutter Speed Aperture Size (f-stop) ISO
The first is shutter speed which refers to how long the shutter is left open, allowing light to pass to the sensor. The second factor is the F-Stop or aperture, which refers to the size of the hole through which light can pass. Finally there is ISO which determines the sensitivity of the sensor to light. It is by balancing these three measurements correctly that we can attain a properly exposed photograph.
Shutter speed and aperture size affect the photograph in two distinct ways. For example, if you take a photograph of a fast moving object then you will need a fast shutter speed. This means that the shutter only opens for a short length of time and you can freeze the action without blurring the image.
However, if you wanted a photograph of the very same moving object to create a blurred effect then you would need a slower shutter speed so that some of the movement is registered by the sensor during the time the shutter is open.
Shutter speeds range anything between 1/4000th of a second (for some wildlife and sports photography) right up to 30 or 40 seconds, although there are plenty of examples where shutter speeds can last minutes or even hours. For, example, the silky texture you will often see on moving water is usually captured by opening the shutter for between 3 to 10 seconds.
Aperture Size is a particularly important part of the calculation in landscape photography because the smaller the aperture, the greater the plane of focus, or Depth of Field. This is how much of the image you want to fall in focus. However, by decreasing the size of the aperture the shutter is required to stay open longer to expose the sensor to the correct amount of light.
ISO: One of the drawbacks with film cameras was that light was metered (measured) around the film ISO because each roll of film had a predetermined sensitivity to light. This film ISO would then be the measurement around which aperture and shutter speed were decided. With DSLRs the sensitivity of the sensor (ISO) can be changed from shot to shot.
However, higher ISOs come at a cost because the sensor's sensitivity to light is altered by amplifying how the sensor detects photons. Just like when you turn up an audio amplifier you become more aware of the background noise or buzz, so too with sensors because noise is created on the image causing a grainier, less well-defined shot. Therefore, an increase in ISO gives you the option of using smaller apertures and higher shutter speeds but with the disadvantage of less definition and clarity.
Correctly exposing a photograph is all about the art of balancing of these three factors.
These values are infinitely variable in how they relate to each other so it is possible to get a multitude of images using different settings, all of the same thing and all with precisely the same relative exposure values. However, there will be clear and marked differences in their depth, clarity and the overall visual effect.
USING SEMI-MANUAL MODES FOR CORRECT EXPOSURE
As well as some fully automatic shooting modes and a mode for entirely manual operation, most DSLRs offer some semi-manual operation to help with getting the right exposure for the type of shot being taken, allowing the user to concentrate on either aperture or shutter speed while the other functions are dealt with by the camera's internal computer to determine the correct exposure
Shutter Speed / Time Value
For most photographers then (and most styles or genre of photography) either shutter speed or aperture become the dominant measurement (priority) around which all other measurements are based. For example, many wildlife photographers prefer to work in Shutter Priority (TV or S) where a high shutter speed is the critical measurement because objects are often moving fast.
Even the imperceptible twitching of an animal's fur or feathers would cause blur if shot at a speed too slow. In most cases it is worth taking advantage of one of the camera's semi-manual functions where the camera will evaluate the correct exposure by setting everything except for the function that has been prioritized.
If capturing fast moving objects or if creating a time related effect is the main objective of taking the photograph then the photographer would choose Shutter Priority and allow the camera to sacrifice some of the depth of field and (sometimes) raise the ISO to achieve their goal. The camera will always look to base these measurements on achieving the right exposure. However, for landscape photography it is very often the aperture size used as the key measurement.
Aperture / F-Stop / Aperture Value
The aperture is, basically, the size of the hole through which light can travel to the sensor and is an important measurement in landscape photography because it controls Depth of Field (DOF). The F-stop numbers and aperture size are inversely proportionate so an f-stop of 1.4 would give a larger aperture, letting more light in.
An f-stop of f22, on the other hand would mean a smaller aperture, letting less light through to the sensor. Therefore working at f22 would mean leaving the shutter open for a longer duration and at f1.4 the shutter would only have to open for a short duration. You could balance the equation in favour of either shutter speed or aperture size yet the the sensor would still be gathering the same amount of light.
The reason why the aperture size is particularly important for landscape photography is because the aperture size controls the depth of field. On a DSLR, Aperture Priority (AV or A) would generally be considered a good semi-manual mode, most useful for Landscape photography.
NB: ISO (in one of the main semi-manual modes) can still be pre-determined by the user, leaving the camera to deal with the one remaining function. Alternatively, if you are prepared to sacrifice a image quality to achieve your aims then ISO can be left in Auto mode giving the camera more leeway.
LIGHT AND APERTURE
Nothing is straightforward in the quantum world and light is no exception. It doesn't travel in a straight line but rather zips about randomly. When we close down the size of the aperture (increase the F-stop value) what we are doing, in essence, is letting light through to the sensor that is travelling in a (more or less) straight path.
This has the effect of filtering out, what is referred to as, unconsolidated light. It is unconsolidated light that causes objects outside of the focal length to blur when the aperture size is increased. By using the f-stop correctly it is possible to vary how much of a photograph we want to be in focus, giving the photograph a shallow or deep depth of field.
A shallow depth of field is where objects are in focus across a shorter distance and deep depth of field is where objects are in focus across a further distance.
Of course there is always a trade-off. When you decrease the size of the aperture for greater depth of field, the shutter is required to stay open longer which means that any moving objects such as tall grass swaying or the movement of