Photography 101: Aperture – how much of your photo should be in focus?

This is the second in my ‘Photography 101’ series. We learn how to limit (or increase) the amount of light going through the diaphragm by adjusting the aperture. 

This is the second in my ‘Photography 101’ series, aimed at helping people feel more confident with their camera. You don’t need to read the post on shutter speed first, as long as you read them both. For best results, follow the ‘Homework assignments’ and try the exercises. You’ll get to know your camera a lot quicker if you do!

Under shutter speed, we talked about opening a window in the front of your camera so that light hits the film/sensor. Technically it’s a little more than that, but that’s the basic principle. 

The lens diaphragm

However, there is something in front of that window, inside the camera lens itself, that the light has to get through first before it gets to the sensor. That thing is called the diaphragm. I estimate that probably 62% of you immediately thought, “contraceptive diaphragm”, and you’re not far away – except, the lens diaphragm isn’t designed to stop the sensor getting impregnated with light, it just limits the amount of light going through. Let’s look at that:

Light comes in the lens >>>

It passes through the diaphragm >>>

It  hits the shutter >>>

We open the shutter and the photo is taken.

We limit (or increase) the amount of light going through the diaphragm by adjusting the aperture. 

F numbers

The size of the opening in the lens diaphragm – the aperture – is described in F numbers. You can read why we use F numbers on Wikipedia, but it’s largely scientific and of little practical use to beginners. 

The smaller the F number, the larger the aperture, and F1.4 seems to be the largest aperture on a lot of cameras and lenses, so we’ll start there. Your camera might start somewhere else, say at F1.8, which is also popular. It’s not a problem!

F numbers continue in a particular sequence – technically, because the F isn’t a number, it’s actually describing a ratio. The next stop after F1.4 is F2. At F2, the aperture is exactly half the size of the F1.4 aperture. After that, it’s F2.8, which is half the size of F2. Then F4, F5.6, F8, F11, all the time decreasing in that same ratio – always half the size of the previous aperture.

Stops and stopping down

A terminology point – with aperture, F numbers are described as ‘stops’, and making the aperture smaller is described as ‘stopping down’. So you don’t “decrease the aperture from F1.4 to F2”, you “stop down from F1.4 to F2”. It’s not super important to get right, but if you go on to read more about photography you’ll come across that a lot. So, file it under ‘good to know’. 

Half stops and one-third stops

If you’ve been fiddling with your camera at this point, and having a fiddle is always strongly recommended, you probably noticed that you have F numbers in between the ones in that sequence (unless you have a very old camera). Let’s talk about these other numbers.

Changing from F1.4 to F2 – ‘stopping down’ – is a whole stop. From F2 to F2.8 is another whole stop, and so on. But on some cameras you can also change down in half stops, or one-third stops. This is really just fine-tuning the amount of light that gets through.

So, the sequence F1.4 > F1.7 > F2 > F2.8 > F3.3 > F4 and so on – they’re all half stops.

And the sequence F1.4 > F1.6 > F1.8 > F2 > F2.2 > F2.5 > F2.8 – all one-third stops.

I would say that the majority of modern cameras will be using one-third stops. That gives you an amazing amount of fine-tuning. If yours uses half stops or whole stops, it doesn’t matter because you’ll compensate in other ways and we’ll discuss those in the future. It doesn’t make your camera any better or worse because once you’ve completed Photography 101, you’ll know how to balance aperture with other things.

But that’s not all!

The aperture doesn’t just control the amount of light – it also controls depth of field. That refers to the parts of the photo that are in focus. The depth part comes because we measure focus going forwards from the camera. For example, everything between 5 metres and 10 metres might be in focus on your photo, but everything closer than 5m and further away than 10m would be out of focus.

That might seem a little abstract, so let’s have a look at the same scene, with different apertures and different focal points. Here are some CD cases on the kitchen table. There’s about 3ft between the front and back case.

Starting with an aperture of F2, focusing on the first CD case. F2 is very wide. You can’t tell what the second CD is and you can’t even tell what the last few shapes are, or the ominous black thing on the right.

Aperture of F2, focused around the 3rd and 4th CD. Now we can tell what the last thing is but we’ve lost all detail on the first CD.

Aperture of F2 focusing on the last CD. Look how the foreground is now completely blurred. The wider the aperture, the more this effect is pronounced. Let’s compare these with one shot at a very narrow aperture.

Here is a comparison using the same set-up, with an aperture of F16. As you can guess F16 is a very narrow aperture – it’s 6 full stops less than F2, meaning that the aperture has halved in size six times. Compare this with the first F2 picture above. Focusing here on the first CD, look how much more detail is available on the last CD. You can’t read the writing, but you can at least tell what it is.

Changing the aperture is usually a creative decision. Your camera’s default setting is to make as much of the photo in focus as possible. But if you’re taking a photo of someone in the garden you might want the bushes in the background to have that dreamy, blurry effect, which we call ‘bokeh’ – pronounced boh-kay or boe-kay (bokeh is one of those things that get camera neckbeards into an indecent state of arousal, trust me). To get that effect on demand, you need to know about aperture.

Homework assignment

As with shutter speed, get to know what’s possible with your camera.

  • How do you change the aperture?
  • What is the widest aperture on your camera?
  • What is the narrowest aperture?
  • Put the camera on manual, set the ISO to 400 and the shutter speed to 1/250th of a second (it might just be marked ‘250’ on your camera). Take one picture of exactly the same scene at every aperture on your camera, focussing on exactly the same point every time, and then compare them. You might find this more instructive to do outside, aiming down the garden or along the street – the greater distance will help.

Quick snaps

  • Before light hits the sensor, it has to pass through the lens diaphragm.
  • The size of the opening in the lens diaphragm is called the aperture.
  • The aperture size is measured in F numbers.
  • The smaller the F number, the wider the aperture, so the more light gets in.

Okay – one more key concept to go. Next we learn about ISO numbers!

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