The traditional triad of ISO, aperture and shutter speed is the determinant of exposure. Appropriate selection of these three parameters will ensure the optimum exposure to record a scene as desired. Selecting exposure settings that allow less light to be recorded by the sensor creates darker, or underexposed images and selecting settings that allow more light to be recorded creates lighter, or overexposed images. The exposure controls are all conveniently related. The traditional values denoting step-changes in ISO, aperture, and shutter speed can all be considered using the concept of stops; f -stops when describing aperture, or just stops when describing changes in the other two variables or combinations of the three. Changing the aperture by one f-stop in either direction or doubling or halving the shutter speed or ISO all result in changing the exposure settings by one stop, doubling or halving the amount of light to be recorded.
Perfect exposure for any given scene can be achieved by using many different combinations of this triad that balance with each other to ensure that a given amount of light (governed by the size of the aperture within the lens), is allowed to fall on the sensor for a specified amount of time (governed by the shutter speed), creating a pre-determined electronic response (governed by the ISO sensitivity setting). For the digital SLR user in the quest for the perfectly rendered landscape image, the ideal values for two of the three variables are actually quite limited: increasing the ISO comes at the cost of increasing noise; using too wide an aperture, like f/2.8 results in less depth of field, and using too narrow an aperture, like f/22, causes a loss in quality due to diffraction. The ideal ISO for overall quality is the native ISO of the camera, usually 100 or 200 depending on the manufacturer, and the ideal aperture is usually somewhere between f/8 and f/16. My favourite technique is to use aperture priority mode and Matrix metering, so that the camera makes a reasonably intelligent guess in selecting the ideal shutter speed to create a ‘perfect’ exposure when I use my favored default values of ISO 100 and an aperture of f/16. Most images can be made using such settings provided we use a tripod to eliminate camera shake.