Exposure Control and Compensation
Photographers usually strive for what is known as a correct exposure when taking a shot and a 'correct' exposure is one where the subject being photographed is captured in the way the photographer intended
There are occasions when you want to override the camera's suggested exposure settings and typical examples of this include shooting objects as silhouettes against a bright sky and high key portrait effects when photographing people. For silhouettes the object being photographed is deliberately under-exposed and when creating the high key portrait effect you deliberately over expose the subject
To master exposure a photographer needs to understand how to interpret measurements suggested by the camera's light metering system and how to interpret histograms which are diagrams providing information on light levels ranging from black to white and covering all the mid tones in between. Most cameras have a histogram function for viewing the light entering the lens and also for viewing the exposure information in captured shots. Histograms are a life saver in tricky shooting conditions because they tell you whether your exposures are in the ball park or completely off the scale
Automatic Exposure
When a camera is in point and shoot mode it will take an average meter reading and attempt to make a faithful recording of the scene. This is known as Automatic Exposure and the keyword here is 'average' as most cameras do just that and do it pretty well but to master exposure and use it effectively you need to understand the three variables involved in exposure control (Shutter Speed, Lens Aperture and ISO Rating) and how they can be manipulated to your advantage
Units of Measurement
Exposure is measured in Stops or Exposure Values (EVs) and it can be tweaked in camera to expose for the part of the scene which you deem to be the most important. This is called exposure compensation and it can be adjusted in full stops or fractions of a stop such as 1/3 stop and 2/3 stop. +1 stop doubles the exposure and -1 stop halves the exposure. When you change shutter speed or aperture value (see below) you are performing exposure compensation
Exposure Variables
Exposure is controlled using three variables - Shutter Speed, Lens Aperture Value and ISO Rating.
Shutter speed is important when photographing moving subjects like water, sports and wildlife.
Aperture value is important for controlling how much of the scene is in focus (depth of field).
ISO Rating controls the sensitivity of the light sensor and acts like a light amplifier.
Shutter Speed
Shutter speeds can go all the way down to 1/4000s and less which is useful for high speed photography and at the other extreme you can lock the shutter open indefinitely for taking night time exposures of star trails.
Shutter speed is measured in seconds and fractions of a second:
... 1/120s, 1/60s, 1/30s, 1/15s, 1/8s, 1/4s, 1/2s, 1s, 2s, 4s, 8s ...
Each step In the sequence above the either doubles of halves the exposure. Moving from left to right each step doubles the exposure and when moving from right to left we are halving the exposure. This is pretty intuitive because we all understand seconds as units of measurement but with aperture values the number used are not as intuitive
Aperture Value
Aperture Value is measured in F-numbers and these numbers refer to the size of the lens diaphragm opening during an exposure:
f/22, f/16, f/11, f/8, f/5.6, f/4, f/2.8, f/2
With aperture values each step moving from left to right increases the size of the aperture opening while moving from right to left we are decreasing the size of the aperture opening
As with shutter speeds moving from left to right each step doubles the exposure and when moving from right to left we are halving the exposure. F-numbers denote the ratio of the focal length of the lens divided by the physical size of the opening of the lens diaphragm. You will agree that F-numbers are not as intuitive as shutter speeds but after a while they begin to make sense
ISO Rating
This is the third variable and it is concerned with the sensitivity of the camera's light sensor. A light sensor is an amplifier of light and you can increase or decrease the ISO rating to match the shooting conditions. In good light a low ISO rating is fine but in low light you sometimes need to crank up the ISO rating in order to use your preferred Aperture Shutter speeds settings
A higher ISO rating amplifies the light but it also amplifies the inherent noise in the sensor and this is something that most photographers find unacceptable because it degrades image quality. In Landscape Photography noise has to kept to an absolute minimum and that is why you should select the lowest ISO rating on the camera. If the lowest ISO rating means slow shutter speeds then you need to use a tripod. I never use the ISO setting on my camera because it is always on the lowest setting and I use the Shutter Speeds and Aperture Values for exposure control
Semi-Automatic Exposure Modes
Good Cameras have semi-automatic shooting modes which allow you to manually set either the shutter speed (Tv mode) or the lens aperture (Av mode). Sometimes these modes are called Shutter Priority mode and Aperture Priority mode
If you manually set the shutter speed the camera will automatically adjust the aperture value.
If you manually set the aperture value the camera will automatically adjust the shutter speed.
Shutter Priority
When photographic a waterfall you need to select a slow shutter speed if you want to blur the water and the camera will take care of choosing an aperture value which will result in a correctly exposed image for your chosen shutter speed. Likewise if you choose a fast shutter speed for photographing sports events
Aperture Priority
When photographing people or flowers you often want the background to be out of focus (shallow depth of field) and to achieve this you need to use a large aperture value to throw the background out of focus while keeping the subject in focus. You select the aperture value and the camera takes care of the shutter speed. A small aperture value such as f/16 is ideal for Landscape Photography because small aperture values ensure that all of the scene will be in focus. In Landscape Photograph it is important to have the foreground as well as the background in focus and for this reason Av mode is good for shooting landscapes
Highlight and Shadow Problems
Digital cameras can record shadows better than they record highlights and for this reason it is important to pay particular attention to the highlights when shooting. In Landscape Photography bright skies and reflections on water can often give you a headache when trying to take a shot which records a full range of tones where the sky, midtones and shadow areas are all correctly exposed
If the dynamic range of light in a scene (noon daylight for example can have up to 12 stops of light) is much greater than the camera is capable of recording (about 5 stops for a digital camera) then there are techniques for handling such situations such as using a grey graduated filter to tone down a bright sky. Advanced techniques involves exposure bracketing where you take an over-exposed shot and an under-exposed shot using a tripod and then layer blend the two images together in your imaging editor to increase the dynamic range. This is a popular imaging technique used by Landscape Photographers
Lingo - Blown Highlights and Blocked Up Shadows
If an image has no details in the highlights (over exposed) then this is known as 'blown out highlights' and when the shadows are completely black (underexposed) then the shadows are said to be 'blocked up'