Exposure

Fundamentally a camera is a device designed to capture and record light. One of the first steps towards better photography is learning how to control the amount of light that the camera captures.  There are two main ways to do this – one is to capture light for a longer or shorter time, and the other is to increase or decrease the rate at which light enters the camera.

This idea of the amount of light contained in an image is called exposure. An over-exposed photograph has too much light, and an under-exposed photograph does not have enough light. As seen from the series of photographs below, correct exposure can have a big impact on how the photograph looks.  On the left we see an underexposed image – notice that it looks dark, and also a bit grainy (this is noise).  In the center we have a correctly exposed image, and finally on the right we see a heavily overexposed image – notice that it is far too bright and that it is very difficult to pick out any details.

From left to right: under exposed, correctly exposed, over exposed.

In an analog camera a photograph is created by exposing camera film to light. The film contains chemicals that react to light, and then this film is used to recreate the image the film was exposed to. The more light the film is exposed to the bigger the chemical changes will be. As a result, allowing too little light into the camera will mean only small chemical changes will take place, and the resulting image will appear dark. Allowing too much light in may saturate the chemical film, and the image will appear too bright (or even completely white) and lacking in detail.

Today with digital cameras the chemical film has been replaced with an electronic sensor.  This sensor is made up of a grid of millions of points (called pixels), each of which is sensitive to incoming light. The sensor keeps count of how much light hits each pixel, and then converts this into the image that you see on the LCD screen or on your computer. The same principle regarding exposure still applies – too much light and the image will be too bright, too little and the image will be too dark.

An important concept to understand is saturation. In digital cameras each pixel has a maximum count, beyond which any extra light coming in will be discarded. When this maximum count has been reached the pixel is said to be saturated, and will appear as pure white in the photograph. A small amount of saturation is usually acceptable, but too much will severely impact the quality of the photograph. Take a look at the image below. The sky appears to be an unpleasant white colour, due to over exposure. As the pixels representing the sky became saturated, any details in the sky have been lost and in the resulting image appears only white.

In this badly exposed photograph the sky has saturated and appears to be an almost uniform white.

The difference between the maximum and minimum possible brightnesses of a pixel is known as the dynamic range of a camera. This is an important thing to understand when taking a photo with a wide range of light conditions. You may find that no matter how you adjust the exposure there will always be parts of the image that are too bright or too dark. Looking again at the photograph above, notice that although the sky appears too bright and overexposed, the trees in the foreground appear dark and underexposed. In this case we are limited by the dynamic range of the camera – increasing exposure may make the foreground brighter but will worsen the saturation in the background. And decreasing the exposure may reveal details and colour in the sky, but will make the foreground even darker.

If you run into this problem you have a few options to address the issue. First of all, you could find and use a camera with a better dynamic range (note that dynamic range is a property of the camera’s sensor, not its lens). Alternatively, you could adjust the composition to reduce the impact, for example, in the image above you could reduce the amount of sky in the image, or perhaps find a different angle which is better lit. Finally, you can use a technique called High Dynamic Range (HDR) photography, which involves taking and combining (in post processing) two photographs with different exposures (I will not cover this concept in this course, but there is plenty of information available on the Internet).

Now that you have some understanding of what exposure is and why it’s so important, it’s time to start thinking about how to control exposure. The primary three ways are through shutter speed, aperture and ISO, and each of these concepts will be covered in depth in the next few lessons. Shutter speed provides a way to control the length of the exposure in time. Aperture gives control over the rate at which light enters the camera, and ISO determines how sensitive the camera’s sensor is to light.

It is also useful to look at some of the standard camera modes, and how each of these affect the control the photographer has over exposure. There are four standard exposure modes: Program (P), Aperture Priority (A or Av), Speed Priority (S or Tv), and Manual (M). Some cameras also have additional exposure modes designed for specific scenarios (e.g Beach, Sport, Landscape). These additional modes are sometimes useful for beginner photographers, but in general it is better to stick with the four standard modes. On a DSLR camera these modes can usually be selected from a dial on the camera body.

Program Mode. Program mode is the most automatic mode. In this mode the camera will automatically select an aperture and shutter speed that it believes correctly exposes the image. The photographer has a limited amount of control (usually by spinning a small wheel on the camera) and can adjust the shutter speed and aperture slightly. This mode is mostly useful when you need to quickly take a photograph and don’t have time to worry about settings like aperture and speed.

Aperture Priority. In aperture priority mode the photographer manually selects an aperture, and the camera will automatically try to find a shutter speed that gives optimal exposure. Aperture priority mode seems to be the most commonly used mode by photographers. The reason for this (and we will go into more detail later) is that controlling the aperture allows you to control the depth of field.

Speed Priority. Speed priority mode is the opposite to aperture priority mode. In this mode the photographer manually selects a shutter speed and the camera will automatically try to pick the best aperture. Shutter speed is mostly useful in situations with fast moving objects, for example, sports or wildlife photography.

Manual. In manual mode the photographer is responsible for setting both aperture and shutter speed. In other words, the photographer is entirely responsible for setting the correct exposure and will have no help from the camera. This is the most difficult mode to use! But it can also offer the most flexibility, and there are situations where you will get a better photograph using manual mode. I recommend beginners to try manual mode only once they are comfortable with shooting in the other modes.

Take some time to experiment with each of the four camera modes. Try changing the available settings in each mode and see how they effect the exposure of your photographs.

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