Guide to aviation photography > Exposure basics
Exposure is all about the amount of light that reaches the sensor in your camera. It is important to get the right exposure – if not then your images will be too bright (overexposed) or too dark (underexposed).
Exposure is controlled by three variables: shutter speed, aperture, and ISO. The first two both control how much light reaches the camera’s sensor. Shutter speed is the length of time that the shutter is open, exposing the sensor to light. Shutter speed is measured in seconds, for example 1/100 of a second or 1/1000 of a second. The aperture is the diameter of the opening in the lens which lets light through to the sensor. Aperture is measured in f/stops. The third variable which controls exposure (ISO) does not change how much light reaches the sensor, but describes the sensitivity of sensor itself. An ISO of 200 means that the sensor is twice as sensitive as ISO 100.
So how do f/stops work?
An f/stop is a dimensionless number (it has no unit), which relates the physical size (in millimetres) of a lens' aperture to its focal length.
For example, a lens with a focal length of 100mm, and an aperture of diameter 25mm produces an f number of 4, which we write as f/4. The same 100mm lens with an aperture of diameter 50mm gives an f-number of f/2.
As this shows, f/2 is a "wider" or "faster" aperture than f/4 - it lets more light reach the sensor. With some more mathematics we can calculate that f/2 actually lets four times more light reach the sensor than an aperture of f/4.
Each aperture in the series f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 lets in half the amount of light as the one before.
The effect of shutter speed / aperture / ISO on your images
A correct exposure can be achieved by many different combinations of shutter speed, aperture, and ISO.
However, shutter speed, aperture and ISO do not just affect the brightness of your pictures. They all have additional effects on your photographs. Shutter speed effects how motion is portrayed. Slow shutter speeds will increase the effect of motion, while fast shutter speeds will freeze motion. Aperture also controls depth of field. A large aperture (for example f/2.8) will result in a narrow area on either side of the focal plane being in focus, whereas a small aperture (f/8) will result in greater depth of field (more of the image will be in focus). Increasing ISO will allow you to take the photographs with less light but will reduce the quality of your images by introducing grain or “noise”.
Exposure modes on your camera
Setting your camera to auto mode (and giving it control of shutter speed, aperture and ISO) will rarely result in good pictures. Not only are you trusting the camera's calcuations of how bright the picture needs to be (which for aviation photography are usually wrong), but the camera will decide what combination of settings to use. If you are photographing a helicopter the camera might decide to use a fast shutter speed and freeze the helicopter's rotor blades. Even if the exposure is "correct" in terms of brightness, the aircraft will look like it is about to fall out of the sky.
Of course, keeping track of all three variables (full manual mode) can require a lot of attention. Therefore there are other exposure modes that can help you. The most useful for aviation photography is shutter-priority auto exposure. In this mode (frequently designated Tv or S on the camera's mode dial), you choose the shutter speed and ISO, and the camera will select the aperture to produce a correct exposure. Shutter prioirity is my preferred shooting mode for aviation photography. As shutter speed has such an important effect on your aviation pictures, it is important to control it. Aperture is less important, so you can allow the camera to change it.
Aperture-priority auto exposure (Av) does the opposite of shutter-priority mode. You control the aperture and ISO, and the camera will vary the shutter speed in order to get a balanced exposure.
In Program (P) mode, you set the ISO and the camera will suggest a combination of shutter speed and aperture to give what it thinks is a correct exposure. However, you can then "shift" the program, by turning a dial on the camera, which selects a different combination of values that give the same exposure. So, you can either increase the shutter speed (and use a wider aperture), or decrease the shutter speed (and use a narrower aperture).