Camera Exposure – A Primer in Photography’s Most Important Fundamental

Understanding exposure is the most fundamental knowledge in the world of photography. This short primer will explain all the terms and what they mean. By the end of this article, and with a little practice, you should be confident in understanding how and why to control the exposure on your camera and how it affects your image.

Understanding Exposure

In photography the word Exposure refers to how much light comes into the camera for any given image. It’s important to note, though, that it’s not measured in absolute terms. Instead, we talk about exposure in relative terms, called Stops.

Let’s say we take a picture that has this much light coming into the camera:

If we were to increase the exposure by One Stop then it doubles the amount of light:

And if we increase it by One Stop again then it doubles the light again:

We can control the exposure with three controls:

1. Shutter Speed – how long is the camera shutter open? Shutter open for more time = more light. Shutter open for less time = less light.
   
   
2. Aperture – every camera lens contains an aperture (a hole) through which the light comes. By controlling the size of the aperture, we can control how much light can get in. Wider aperture = more light. Narrower aperture = less light.
   
   
3. Sensitivity (ISO) – this means how sensitive, or reactive, the camera is to light. The more sensitive it is, the less light it needs to make an image. That means you can use a quicker shutter speed, or a narrower aperture.

Units used and what they mean:

• Shutter Speed is measured in seconds – usually as fractions of a second such as 1/100th of a second. Doubling or halving the shutter speed means changing the exposure by one stop because you have doubled or halved the amount of light getting in.
  
• Aperture is written as ‘f numbers’ e.g f1.4. The f number is calculated by dividing the focal length by the diameter of the aperture. Don’t worry about that – it’s super nerdy. What you need to know is that low numbers mean more light. The higher the number, the less light. Each incremented number is usually one stop.
  
• ISO is written in numeric values e.g ISO 100, ISO 200, ISO 400… and so on. The lower the number, the less sensitive it is and the longer shutter speed you need.

Under and Over Exposure

If the camera does not get enough light for a given shot, then the image will be too dark. This is called underexposure. Conversely, if the camera gets too much then the image will be too bright. This is called overexposure.

When a digital camera overexposes it can be a serious problem for the image. The reason is that the camera will record the brightest parts of the image as pure, even white. If this happens then there’s nothing you can do to recover detail.

Under and over exposure can also happen in parts of an image as well as a whole image. For instance, a common problem in landscape photography is that the sky is much brighter than the land. This can lead to the camera overexposing the sky, recording it as flat white rather than what’s really there.

To see if an image is overexposed we can use a histogram, which also helps us see how many stops we need to adjust it by.

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Other Effects of Shutter, Aperture and ISO

The controls that govern the exposure also have other effects on our image.

• Shutter speed can determine if a moving subject is blurry or not. For something in motion, you need a fast shutter speed to seemingly freeze it in place. For a static object, a longer shutter can be used.
  
  In landscape photography we might deliberately choose a longer shutter speed to make things look blurry. For example, moving water and clouds can look amazing when allowed to blur.
  
  The other thing to consider is how steady our hands are. With a longer shutter (above 1/30th of a second as a ballpark) the image itself might get blurry if we can’t hold the camera totally steady. This is why many landscape photographers use tripods.
  
• Aperture has several optical effects on the image.
  
  • Depth of Field: The higher the f-number, the greater the depth of field. This means the range of distance which will all be in focus. For example, if you focus on something close to you with a low f-number, the background will be blurry. If you then switch to a high f-number the background will now be in focus, or at least less blurry. Higher f-number means greater depth of field.
    
    Landscape photographers often want everything to be in focus from near to far, so often use higher f-numbers. That does mean though, that a longer shutter is often needed, hence a tripod.
    
    Remember though, you can use depth of field however you like. Sometimes a blurry background looks great. This is called ‘Bokeh.’
    
  • Sharpness: This is how clearly the camera lens can resolve details and edges. Most lenses have an optimal f-number where it will look sharpest. The higher the f-number the more sharpness tends to reduce, because the diameter of the hole is smaller. This can cause diffraction in the lens which causes the image to look a little blurry aka ‘soft’.
    
• ISO can affect not only how sensitive the camera is to light, but the overall quality of the final image. The higher the ISO number gets, the more digital noise creeps in. In technical terms, the signal-to-noise ratio gets worse. Digital noise looks like small speckles all over the image, and if there gets to be too much then an image might be unusable. For this reason, landscape photographers use the lowest ISO possible for most occasions. This does mean you need a longer shutter speed or a wider aperture.

A Balancing Act

From what we’ve learned above, we can see that Shutter, Aperture and ISO can all increase or decrease the exposure. That means that they have a reciprocal relationship. If you alter one you need to alter at least one more to keep the same overall exposure. You can get the same overall exposure with a fast shutter and narrow aperture, as you can with a slow shutter and wide aperture. That is assuming the ISO remains constant.

It also means that for any given shot, there is no single exact way to get the exposure right. It probably will depend on what other goals you have in mind. For instance:

The Good News

The good news is that we don’t have to manually adjust the exposure for every shot, but nor do we need to let the camera take a fully automatic guess every time. We can set the camera up to prioritise certain settings (i.e keep them fixed) and adjust others. More about this in my post about how to set up your camera for landscape photography.

Hopefully this helps you understand the essential points of Exposure in photography. If you have any questions then leave a comment and I’ll get back to you.

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