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 leaves on trees can become blurred. As a general rule I like to think that anything slower than 1/100th of a second moves you out of the possibility of hand holding the camera and a tripod becomes a necessity.
As you can see from the two images, the first has a shallow DOF (f2.2 @1/400th sec) where only the foreground object is in focus. This is because a larger aperture was used allowing more unconsolidated light to hit the sensor. The depth of field is so shallow at this aperture size that even objects only inches behind the main focus are blurred. The second image has a deep DOF. The aperture size has been decreased which means the shutter has to stay open longer to gather the same amount of light (f20 @ 1/50th sec) and everything in the shot has come into focus.
A word of caution. When you are using AV or TV modes on a DSLR the camera can be left to determine a different ISO setting than the ISO you require. These modes are great for some scenarios and allow you to get used to how the ISO, Aperture and Shutter speed relate to one another to provide the correct exposure but do try to get used to using the camera in full manual mode so that you can keep a check on the ISO more effectively and gain greater control over the whole shot.
So, how does the camera 'decide' whether a scene is correctly exposed?
Either side of this central point and the photograph (from the camera's point of view at least) will be under or over-exposed. The difficulty is that if you meter the light (measure its intensity) from a particularly bright or dark part of the thing you are photographing, then this can cause the camera some issues. Snow is a particularly good example because the camera is trying to average out light to meet its preferred 18% medium grey reflectivity when all it is confronted with is almost pure white.
In the camera's semi-manual modes metering is usually done automatically, compensating with the ISO and F-Stop in Shutter priority mode and similarly adjusting ISO and Shutter speed in Aperture priority mode.
However, it is down to the photographer to choose which part of the scene the reading is taken from. Choosing which part of the scene or landscape we wish the camera to take the average from is done either via a light meter or a DSLR's internal metering
Generally there are four metering modes on a DSLR that can be chosen.
The first is: Evaluative metering. This is the mode most commonly used and is the way in which most compact cameras measure the average exposure. The camera takes an average reading the amount of light across the entire scene:
Partial metering is useful if you intend having subjects in the foreground that are brightly backlit by the sun or another strong light source. Take the exposure value from the most correctly exposed part of the subject. This method omits a good proportion of over or under-exposed areas covering the major area around the frame.
Spot metering is useful if you want the exposure reading from a small, correctly lit part of the image some distance away or from the point of focus on a landscape. Spot metering is one of the most commonly used methods of metering for landscapes because it only takes an exposure reading from one small area of the scene; no more than 5-7%. Usually several readings are taken to give an average. Because most landscapes are very high contrast you can spot meter small areas where light is reflected to the correct degree.
Center weighted average metering. This is where the meter concentrates up to 80% of its sensitivity reading towards the centre of the frame. If, for example, you have very dark or brightly lit objects towards the edges of the frame then this can be a useful method of metering the scene.
It'd worth noting that if you meter the available light in one of the camera's semi-automatic modes then the exposure will be measured from the centre of the frame. To take an exposure reading from a different area you need to use the AE-lock function to maintain that exposure reading before moving the camera or recomposing the shot. Be aware that some cameras will release the AE lock after a number of seconds, therefore working in full manual mode is best of you do not want to be rushing the shot after the metering is complete.
Nature has quite extreme variation in light readings. There can be the brightness of the sky right through to the deep shade of foliage. It is always worth taking a few exposure readings using spot metering to give yourself an an idea of the range of levels that exist. Try to meter from the Goldilocks zone: not too bright, not too dark, not too garish, not too muted but just right. In other words try to meter from parts of the scene containing correctly exposed mid-tones.
When you buy a DSLR the factory setting will set the metering to evaluative. This is the usual default for most cameras. Metering is one of the most critical parts of attaining a correctly exposed photograph but it's surprising just how many people never change or even look at this setting.
Thinking about how your camera will meter a particular scene is equally important as the other exposure settings. Of course, nobody wants to have to go into the settings menu and decide on a metering system every time they take a shot and for general use just revert to using Evaluative metering. But if you are going to the trouble of getting all of your photo gear to a particular spot and you have a clear idea of how you would like the shot to look then metering the scene can really make the difference between an ordinary and extraordinary finished result.
METERING FOR ND GRADS
You can also use metering to find the correct ND Grad filter to use. First take a reading from the brightest part of the composition, then the mid-tones, carefully making a note of these readings. The ND Grad you should be using will be the number of stops between the brightest part and the mid-tone.
If the difference is 3 stops then useND8, for 2 stopsND4 and for 1 stop use ND2. It is also generally a good idea to bracket the shot 1 stop either side of the correct exposure. Not only will this give you the opportunity of seeing the exposure results across 3 shots but also gives you the further option of using exposure fusion in post-processing or to bring out darker and lighter tones at the extreme ends of the scale using HDR software.
It's good to remember that metering is not only the best way to ensure the correct exposure but can also be used to render the photograph with more artistic quality. Although, for example, you may hear it suggested to always meter from the mid-tones, it may not be the best way to get the effect you want. You may have foreground objects that you want to keep in deep silhouette as framing for a composition.
Some foreground elements are as easily deciphered by their shape and you may wish to use shape rather than texture or color and place the majority of the composition in stark contrast. In this case you will want to meter from a brighter part of the image to get the the darker areas really dark.
METERING FOR GND FILTERS
When it comes to landscape photography, you'll be using GND Filters (grad filters). Typically, there's about a 2 stop lighting difference between the sky and foreground. Grad filters come rated by stops (the rating system depends on the brand of GND Filters--HiTech, Cokin, Lee, or Singh-Ray).
You will typically meter your exposure on the foreground (with the grad over the lens). Since the GND Filters will have a darker section over the sky, by metering your exposure for the foreground, the foreground is balanced while they sky's exposure is held back by whatever the stop rating your grad filter is.
For sunsets, you'll typically want to use higher than 2 stop as there may be between 2-5 stops difference between the foreground and the brightest part of the sky.
My recommendation for grad filters is to have
One 2 stop Soft Grad
One 2 stop Hard Grad
One 4 stop Soft Grad
The above setup will cover most contrasting light conditions.
A WORD ABOUT WHITE BALANCE
White balance is an area of the camera that many people choose never to touch, preferring instead to allow the camera's auto white balance to do all of the work but it can be critical to have some working knowledge of this to attain correct exposure and color balance.
It is vital that the camera has some information on what type of light source is being used to illuminate a particular scene so for many situations using just the pre-sets will be enough to prevent odd color casts or over-exposure of high key areas. Really though, white balance is best adjusted manually and tailored to any given situation.
White balance is pretty a pretty simple concept but there has been so much written about it that it can be easy to get bogged down in the technical detail of incident light and relative intensity.
In its most basic form white balance is simply giving the camera a better idea of how we see color by saying to the camera “This is White.” The camera can adjust the color temperature cast by a particular light source by using this information as the foundation of how it sees color Grey card with an 18% reflectance of the spectrum is usually the best way of creating a custom white balance because (if you buy the correct card) it will reflect the entire light spectrum equally giving the camera perfect information for correct white balance.
Camera manuals will often tell you to point the camera at something white or a flat grey surface but most white objects favor the reflection of a particular color temperature. What may appear to the eye as a grey, stone wall can, in fact, contain silicates that are reflecting light with an imperceptible severity.
Grey cards are commercially available and even come as key-rings so no matter where you are you are assured of setting the white balance correctly. These are well worth the investment and this simple tool may make the difference in making sure you images are exposed and colored correctly.