The 3 Factors of Exposure – Part 3 – Aperture

<- Back to Part 2 – Shutter Speed



In part 1 we covered the first of the 3 factors of exposure; Sensitivity – which is controlled using the ISO on your camera or film. In Part 2 we covered Duration – adjusted by the Shutter Speed on your camera. Now, in part 3, we take a look at Amount. The Amount of light you use in exposure is controlled by the Aperture, and we measure this value in f-numbers.

It’s with all 3 of these factors; Sensitivity, Duration and Amount combined, that you’ll be able to fully understand and control the exposure on your camera, to get the results you’re after, every time.

Aperture control however is often the trickiest for people to understand and master. There are some complex mechanics and physics at play that overwhelm some people at the beginning. The reality is, that you don’t really need to know how it all works, you just need to know what will happen when you adjust the values.

I’m going to simplify the physics quickly for everyone to understand, and you can go off and do some further reading if you want to know more.

What is the Aperture?

The Aperture is basically a hole that lets light through from the lens to the camera, but I like to think of the Aperture as a water tap. This is the main reason I call this factor the “Amount”, because I often think of controlling how much light is getting into the camera like screwing the handle on a tap, letting more, or less water out of the tap. If you open up the Aperture really wide, the light will come flooding into the camera, and if you close the aperture down to something narrow, only the smallest trickle of light will get through.

Depth of Field

As with ISO and Shutter speeds, changing the Aperture size doesn’t just change the brightness of the image, you’ll also impact a “side-effect” called Depth of Field or DoF for short.

This is where things get a bit tricky to follow. Depth of Field is essentially how “much” of the image is in focus. A deep DoF means that lots of things, no matter how close or far away, are all in focus. A shallow DoF means that only things at exactly the right distance from the camera, will be in focus, and everything else will be out of focus.

To give you an example, if you wanted to take a photo of your garden as a whole, and have everything from the grass in front of you, to the shed at the end of the garden, all sharp and in focus, you’d want a deep DoF. If you wanted to take a photo of just 1 flower right in front of you, and have only that flower sharp and in focus, and the rest of the garden behind it out of focus, you’d want to have a shallow DoF.

So, as you open up that Aperture to let more light come flooding in, what you’re also doing is making the DoF shallower. If you reduce the size of the Aperture, letting less light in, you’re also going to get a deeper DoF.

Here’s our herb jar from the previous pages again. In this first photograph you can see that only the jar is in focus. Everything else in the background is out of focus, and even the wood of the worktop in the foreground is also out of focus:


I’ve had to take into account that the Aperture was open wide to achieve the image above. That means that lots of light was let in through the lens, and therefore I had to lower the ISO and speed up the Shutter Speed, to make sure that the image didn’t come out too bright, or “Over Exposed”.

In this next image, I’ve set the Aperture to something much more narrow, therefore lots more of the “scene” and the jar itself is now is in focus. Look at the radio and fridge-freezer behind, you can clearly see what they are now.


The problem then being, that to achieve the image above, not a lot of light was let in through the lens, so I had to slow the Shutter Speed down, and risk getting blurring if I didn’t hold the camera steady enough, and more importantly I had to increase the ISO to something very high, and accept that I would get a lot of noise.

Here’s my attempt at explaining what’s happening with a simple (ish) diagram. This is a diagram showing from above the photographer, the Depth of Field, first when it’s shallow, and second (on the right) when it’s deep.



Click on the diagram if it’s a bit too small, it should open up a bit larger then.

The blue part is the Field of View, essentially everything you can see through the viewfinder and eventually on the photograph. The orange part is how much of what can be seen in the photograph that will be in focus, i.e. the Depth of Field.

A clear example of a shallow depth of field. Only the parts of the image the same distance away as the eyes are in focus

A clear example of a shallow depth of field. Only the parts of the image the same distance away as the eyes are in focus

What F-Numbers should you use?

I’ve included some example f-numbers on the diagram. The first thing you might notice is that the wider the aperture, the lower the f-number. It seems a bit odd to some people, and it’s all to do with where the number comes from. Without getting too technical, it’s a ratio of the focal length of the lens, to the size of the aperture opening; the more you open the aperture, the closer it’s measurable size is to the lenses focal length, therefore the lower the ratio. If that doesn’t make sense, or isn’t easy to quickly remember, do it the way I do instead… I think of it as “the lower the number, the less things will be in focus. The higher the number, the more things will be in focus” – simple.

It does now mean though, that you have to remember 2 things about the f-number:

  • The lower the number the less things will be in focus
  • The lower the number the more light will be let in and therefore the brighter the image

But armed with this final knowledge of Aperture control, and combined with your knowledge of ISO and Shutter Speed control, you should now start to understand how all 3 can be combined to achieve the right exposure for the correct photograph.

The Rules of Exposure

There are no rules to photography, only the rules you make for yourself. Different photographers find different ways to work out and apply the 3 factors of exposure.

  • Some photographers apply maths, and calculate exactly the right settings. This approach is more helpful when shooting on film as you can’t see the results until you’ve developed the photograph.
  • Some photographers go on memory and experience and just apply changes to the settings based on what they know has worked before – this is often something I personally do
  • Some photographers, and this is a result of the digital age, just feel their way through to the right settings by taking multiple images, adjusting the settings each time. This trial and error approach is brilliant when you’re learning, or trying out something new, but of course as time goes on, can be quite time consuming. Hopefully you’ll move over to one of the other approaches as you gain experience.

The reason why there’s no strict rules as to what settings to use when, is because photography is an art. One photographer might want the scene to be in a shallow DoF, the other deep. One photographer might want the photograph to be dark, the other might want to deliberately blow out the highlights for effect. One photographer might hate grain, the other might love it and want it in every image they take.

If someone tells you you’re doing it wrong, just tell them “no, I’m just not doing it your way” and carry on with what you’re doing.

Sunny 16

There’s some great rule of thumbs out there though, for quickly finding a basic setting. One of my favourites is “Sunny 16″. This great little rule of thumb used to serve me well when shooting on film, and still continues to help me today when I’m shooting on digital. Basically it means that if you’re outside on a sunny day and set the Aperture to f/16, if you then set your shutter speed to the same as your ISO, you’ll get a good exposure of the scene. For example if your Aperture is f/16 and you set your ISO to 200 (or you’re using ISO 200 film) then setting your shutter speed to 200 will give you a nice exposure.

Of course you need to move away from that rule of thumb if it’s not a sunny day, or you’re not outside, or you want a shallow depth of field, etc. But it’s a good one to have in your head.