Robert Boughen | BACK TO BASICS: Understanding Exposure

BACK TO BASICS: Understanding Exposure

September 27, 2016  •  Leave a Comment

In this newly launched segment, Back To Basics, I am going to be taking you through some of the simple things you can learn about and do that will enable you to take the pictures you want. If you own a DSLR, a Compact camera, or even an iPhone, you've bought an expensive piece of equipment that can provide you with high quality images.

Cameras do have a range of AUTO features that will allow you to "point-and-shoot" but, as expensive and clever as they may be, cameras are not always brilliant at judging your intentions. If you want to get the shot you want, we'll need to start to take control of the camera and tell it what we want. 

First of all, lets consider that, while a photograph might be considered artwork, the essence and science of a photograph is the recording of light onto a sensitive surface over a period of time. That may sound strange at first. After all, photographs don't move, so how can they be recorded over time?

To explain this, we need to understand what exposure is, aside from a criminal offence. Exposure, in photographic terms, is the way in which light is measured. A unit of exposure is called a stop of light. The exposure of a photograph will be determined by the amount of light that is allowed to fall onto our sensitive surface, which is the camera's sensor. The best way to show how this is calculated is as a triangle.



As you can see, there are three elements that control the exposure of a photo, and each of these elements changes other aspects of the photograph being taken. I will give a quick run down here, but we'll cover all of these elements in more detail in later posts.

First, there is Shutter Speed. Shutter speed is the amount of time the camera's sensor is being exposed to light. It is measured from very fast (e.g. 1000/1, or one one thousanth of a second) to very slow (e.g. 5", five seconds). The longer we leave the shutter open, the more light is available to the sensor. Shutter speed also affects the amount of motion blur the photograph will have. A very fast shutter speed will give us a much sharper image, but a slower shutter speed will likely give us blur. It is unusual to use a slower shutter speed without having a tripod.

Second, there is Aperture. Aperture (or Iris on cinecameras) controls the size of the opening through which light is being exposed to the camera's sensor. It is measured in what are called F-Stops from very wide (e.g. f1.4) to very narrow (e.g. f22). The wider the aperture, the more light is exposed to the sensor. Aperture also affects the depth of field of a photograph. A wider aperture will give a shallower depth of field (i.e. more blurred beyond the focus point), while a narrow aperture will give a deeper depth of field (i.e. more detail beyond the focus point).

Third, we have sensitivity, or ISO for short. ISO controls the sensitivity of the sensor. It is measured in units starting from 100, and the number doubles every time we increase the sensitivity (i.e. 200, 400, 800, 1600). The more sensitive we make the sensor, the more light it records. Every time we double the sensitivity, we gain a stop of light. However, the more we turn up the ISO, the more we get what is called digital noise, or grain, on our images. This is an unattractive spotty grain all over our finished image, and serves no useful purpose. You essentially want to keep your ISO as low as possible depending on how many stops of light you need to gain.

The best way to think of these three elements is as an equation. The first two, shutter speed and aperture, are creative tools that will affect the image we take. ISO is a way of balancing the equation to gain enough light to correctly expose our images. For example, say we wanted to take a wide landscape shot we would likely want to use a narrow aperture, since that gives us plenty of detail in our image. However, since that will reduce the amount of light the sensor is being exposed to, we will compensate for the loss of light by using a slower shutter speed. Lets say there is also a bus moving within your landscape, we will then have to use a faster shutter speed otherwise we will get motion blur from the bus. We will then have to up the ISO to compensate for the greater loss of light.




The exact level of exposure you want will depend on your creative idea for the image you are taking, though generally speaking most images will speak for themselves on this matter. As you can see in the image above, the exposure in the centre is clearly the most pleasant and closest to reality. That to the right is too light, so it is over-exposed, and the left is too dark, so it is underexposed.

Most cameras have an exposure meter built in, that looks similar those that you see at the top of each of the above images. Use this as a tool to help you judge where you need to make adjustments. We will cover in more detail next week, how and why to use shutter speed and aperture as creative effects.


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