Exposure – The basics explained

Every image you take is simply a record of the light hitting your camera sensor when you pressed the shutter button. The amount of light varies with almost every single image and unless you’re in a studio, generally you have not got much control over it.

This is where the settings on your camera come into play. On “Full auto”, most cameras do a pretty good job of getting a well exposed image. If you want more control over the shot, you must understand which settings to change, and how they effect each other and the image.

Essentially, there are three settings which dictate the exposure of the final image:

  • Aperture
  • Shutter Speed
  • ISO

First I will explain how these settings affect each other, then I will explain what each has on the final image.

Let’s assume you have a bottle of wine (and why not!) which contains 1 litre of Beaujolais’ finest.

Now you have 3 friends, Usain who is an athlete, Mr. A. Hole, a politician and ISObel. They each have a glass.

Usain is obsessed with Time and going fast, so he’s the shutter speed.

Isobel is very sensitive, so she can be our ISO.

Mr A. Hole represents Aperture, no idea why.

The wine, well that’s how much light we have available.

So, in an ideal world each of our friends would have the same amount of wine, and we end up with a well exposed image and happy friends.

But what if Isobel had a bad day and you want to give her extra? There has to be a compromise, so Usain and/or Mr Hole end up with less.

The same applies if either of the others have more than their fair share.

Essentially, if any one of the settings are changed, the other two must also change by the same amount in order to still get a well exposed image. These changes are measured in “Stops”.

Still with me? Good.

So what do these settings do, and why not just leave the camera on Auto?

This is a VERY brief explanation, and there is much more to each setting which you will understand as your experience grows. For the purpose of this guide, I’ll just cover the basics.


ISO (Isobel)

This dates back to the days of film cameras (remember those??). The ISO (or ASA as it was sometimes known) represented the sensitivity of the film to light. A lower ISO number (e.g. 50 or 100) meant it was least sensitive to light, but the grain on the film was smaller giving a sharper image. Higher ISO (400 or 800) was more sensitive to light, but was slightly more “grainy”. These days, the ISO number just represents the sensitivity of the sensor in your camera. The range of sensitivity varies between camera models, but on most D-SLRs you can expect at least from 100 to 3200 for entry level camera, 100 – 25000 on mid-level and anywhere up to 204000 for professional cameras. There is one big compromise when increasing the ISO setting, and that is digital noise in the image. Generally, the more expensive the camera, the higher you can get away with. On entry-level models, I would rarely go over 800 unless it’s just snaps to stick on facebook you’re after. On a professional D-SLR, ISO 6400 can produce perfectly usable images, even for printing.


Aperture (Mr A. Hole!)

This is the size of the hole which lets light through the lens and onto the sensor. It’s fairly obvious – the bigger the hole, the more light gets through. The setting if represented by “f” numbers, and the range available is dictated by the lens. On a fixed lens, you are normally shown the maximum aperture (e.g. F/2.8) and on zoom lenses, you are shown the maximum aperture at both ends of the zoom. For example, a 15-85mm lens might be f/4 – f/5.6. One thing to remember about aperture is that the lower the number, the bigger the hole. So f/2.8 lets more light through than f/22. You may be wondering why you don’t just set aperture to the maximum possible and leave it there. There are occasions where you have too much light, on a bright sunny day for example, and need to close the aperture to get the correct exposure. More importantly is the effect the aperture has on the image. It is the main factor in how much of your image is in focus. This is known as the “Depth of Field” and deserves an entire tutorial of its own. For the purposes of this guide, I will simply give a couple of examples. If you want a nice portrait of your beautiful wife/husband and want the background to be out of focus, so as not to be distracting, then choose the maximum aperture.

If you are taking a landscape photograph of a lake with mountains in the background, you want the image sharp from the closest stone to the furthest cloud, you must choose a much higher aperture such as f/22.


Shutter Speed (Usain)

Last but by no means least, we move on to Usain the athlete, to whom time is everything. Out of the three settings discussed, this is the one that makes most sense and you can have a pretty good stab at guessing what it does just by the name. It’s not rocket science – Shutter speed is simply the length of time the shutter is open and allowing light to hit the sensor. So, if it is dark, you can leave the shutter open for longer so the sensor gets all the light it needs to record the image. This is great if your camera is on a tripod and your subject is dead. However, if you want to take a picture of Lightning McQueen whizzing past in his shiny new racing car, a long shutter speed will simply get you a blurry streak across the image. You can test it using your eyelids. Look towards something which is moving, close your eyes and then open them for 1 second. You will see the thing you are looking at move across your field of view. Now try the same thing but blink really fast. Each time you blink, the object hasn’t had time to move much and this is what your camera sensor would have captured at a faster shutter speed. Useful examples often used to demonstrate shutter speed are flowing water, or waterfalls. A fast shutter will freeze the action and show all the detail is the water droplets, whereas a slow shutter would smooth out a flowing river or make a waterfall look like cotton wool. Both are desirable, but you need to know how to control the camera in order to get the shot you want.


As a general rule, you start each shot by deciding how you want the end result to look. This helps decide which setting out of Shutter Speed or Aperture to give priority. Once that has been set, you can adjust the others as required. Normally ISO is the last to change as you want the best quality image possible.

e.g. You decide you need a shutter speed of 1/1000th second to photograph your dog catching a ball. The background is distracting, so you want to set the maximum aperture to help blur the background and really make the dog the focal point of the image. But, it’s a dull overcast day and the camera says “no” –  your exposure indicator is saying the image will be too dark. Here, you would simply increase the ISO until the exposure meter (normally a little line in the viewfinder with notches on) sits happily in the middle. If you were to decrease the shutter speed instead, the exposure would be OK, but your dog would be a blurry mess where he’s moved across the frame while the shutter was still open.

I hope this has helped understand the basics of exposure a little more and how the three settings which affect it most, also affect each other and the final image. This is by no means an exhaustive or comprehensive guide as there are many other factors which affect the image and exposure. It is simply meant to help you understand and visualise how the different aspects affect your images so you can use them to creative effect. If I have completely misinterpreted something or confused the hell out of you, please let me know.






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