What is aperture and depth-of-field?

You might have heard the terms ‘aperture’ and ‘depth of field’ floating around in the photography world, and you might kind of have an idea of what they are. But perhaps not entirely. And not enough to really know how to use them in your photography.

So, if you want to start getting your head around these concepts, keep reading.

(Before we get started, I have to say that the world of how cameras and lenses work is one filled with physics. I’m not going to explain the physics-y stuff. It’s in-depth and complicated. So, minimal physics ahoy!)

What is aperture?

To put it at its most simplest, an aperture is a hole. Yup. A hole. A roundish hole. In the lens of your camera.

The function of a lens is to guide and focus the light hitting the film or sensor in your camera, and the aperture is part of that focusing system. In modern cameras, lenses have a diaphragm, made up of a number of blades. In the centre is the aperture – the hole. You can adjust the diaphragm to increase or decrease the size of the aperture.

The mechanism’s very similar to our eyes. The iris is the diaphragm, and the aperture is the pupil. The iris can adjust the size of the pupil to let in more or less light.

But why have an aperture in a camera? As far back as the 1500s, scientists understood that an aperture could improve the image quality in telescopes. Having that diaphragm blocked some of the peripheral light coming through the lens. Peripheral light can cause (mini physics alert!) optical aberrations, which can cause blurring or distortion. So, the effect of the aperture is to help control how the light travels through the lens to sharpen and focus your image.

Variable apertures were first produced for cameras in the 1850s. They were brass plates with a different-sized hole in the centre of each one, and you had to swap them in and out of your camera to change the aperture. Sounds clunky, but cameras were clunky and slow to use back then anyway.

Adjustable apertures appeared in the 1880s, and became a standard part of cameras in the early 1900s.

Every camera lens these days has an aperture in them – the majority of them adjustable – and it has a duality in how it affects your photographs.

How does aperture affect your photography?

Your exposure

The first and most obvious way in which you can use the aperture is to adjust your exposure.

Exposure is all about how bright or dark your photo is, and you do this by adjusting how much light hits your sensor or film.

Just as the pupils of your eye dilate when it’s dark to let in more light, so you can widen the aperture on your camera to let in more light – great for when you’re in low light situations. And vice versa.

There’s really nothing much more to it than that.

Your depth of field

The other way your aperture setting affects your photo is your depth-of-field.

Depth-of-field is a term used to describe how much of your photo is in focus in a linear plane from the front of your camera (i.e. front to back – between your camera and the horizon), relative to where your lens has focused.

When you change your aperture, you change how deep that depth-of-field is.

A wide-open aperture causes your field of focus to fall away more quickly than a narrow aperture, so a wider aperture causes more of your photo to be blurry.

This blurriness might be something you want to have in your photo, or something you don’t. This is an aspect of photography that’s a more artistic choice. Think about the difference you see between a portrait of someone, where their face is in focus, but the background is pleasantly blurry (shallow depth-of-field), and a landscape scene, where everything from the clouds in the sky to that big rock in the foreground is in focus (deep depth-of-field).

This photo has a shallow depth-of-field – the statue itself is in focus, but the background is out of focus.

This photo has a deep depth-of-field – everything from the tall grasses in the foreground through to the tower and clouds on the horizon, are all in focus.

When it comes to artistic choices, there’s no right or wrong. You might see most landscape photos use a deep depth-of-field, but you choose to mess around with that and do your own thing. Great! That’s the fun of art; you can make of it what you like.

I often like using shallow depth-of-field. You can use it to draw your viewer’s eyes straight to your subject.

In the photo above, I chose to make the pot and Anna’s hands the focus, not her face, because it was her conservation work I was documenting.

And again, in this photo, I used a shallower depth-of-field to help separate the statue in the foreground from the others behind it, giving the image a much more 3D feel.

Now, it’s worth bearing in mind that aperture isn’t the only thing that affects your depth of field. You also have:

• Focal length (how zoomed in or out your lens is): wide-angle lenses (zoomed out) have a much deeper depth-of-field than longer lenses. So, if you zoom in on your subject, you’ll get a shallower depth of field.
• The size of your camera’s sensor: the smaller the sensor, the deeper the depth of field. So, those with a full-frame sensor (DSLRs and some mirrorless cameras) will have a naturally shallower depth of field than those with cropped sensors (compact cameras, mobile phones and other mirrorless cameras).
• How close you are to your subject: the closer you are to your subject, the shallower your depth of field.

In this photo, I used a longer lens – 80mm – which gives you that lovely blurry background.

In this photo, I used a much wider lens – 16mm. But, because I was much closer, I still got that shallow depth-of-field.

An extreme example of your depth of field being affected by distance is that of macro photography.

With a macro lens, you can get really, really close to your subject – think of those photos you see of flowers or insects – and you get very, very little of your photo in focus.

It does mean that you can get a very beautiful picture, where having very little in focus can be quite pleasing to the eye.

However, it can be too much sometimes. With the macro photography I do of ancient Egyptian amulets, I’m so close to them that a sliver of literally a millimetre or two is all that’s in focus, and that’s where I have to use focus stacking, a method whereby you take multiple photos of your subject, adjusting the focus a little each time, then use special software to smoosh them all together into a composite image.

This photo is of an Egyptian amulet of Horus. It’s around 4cm tall. With my macro lens as close as it is, only the tip of Horus’s beak is in focus.

It took over 80 shots to get every part of him in focus. The resulting composite photo is much better:

The aperture scale

The scale on which you set your aperture is something that seems a bit crazy and upside down. Shutter speed is simple enough – it’s seconds, fractions of a second, or even minutes or hours. ISO is a nice decimal scale that doubles each time you jump (100, 200, 400 etc).

But, the aperture scale? Ooof. They’re called ‘f-stops’, and the scale runs in what seems to be a completely unfathomable … nay, unreasonable … way:

• f/1.4
• f/2
• f/2.8
• f/4
• f/5.6
• f/8
• f/11
• f/16
• f/22 etc.

It becomes all the more confusing, when you find out that the smaller the number, the wider the aperture.

What you’re actually looking at with these f-stop numbers is a maths equation, which gives you the diameter of the aperture.

The ‘f’ stands for the focal length of your lens (measured in millimetres), and the / is ‘divided by’. So, f/2 is the focal length divided by 2. If, for example, your lens has a focal length of 50mm, f/2 gives you an aperture diameter of 25mm (50 ÷ 2). And f/4 would give you a diameter of 12.5mm. Therefore, a smaller number = a wider aperture

So there is method to the madness.

But, that still doesn’t explain the strange way in which the numbers jump. Simply put, each f-stop value will let in twice or half as much light as the one before or after it: f/2 lets in twice as much light as f/2.8; f/5.6 lets in half as much light as f/4. It’s all horribly mathsy, and you don’t have to remember it (don’t worry – no tests at the end!), but I explain it to try to put the f-stop scale and its seeming backwardness into perspective.

A final note on aperture values and terminology: lenses with a maximum aperture of f/2.8 or wider are known as ‘fast’ lenses. Because they have particularly wide apertures, you can use faster shutter speeds with them, which makes them great for low light when you can’t use a tripod.

Aperture and portrait mode in mobile phones

Most mobile phones have a fixed aperture – you can’t change it. Although the apertures tend to be quite wide, you still get a deep depth-of-field with mobile phone cameras, because the sensors are really very small, and the lenses are quite wide.

You may have a portrait mode or aperture mode (or both) on your phone camera, which gives you those lovely photos with the blurry background, but that’s not achieved by altering the aperture. Your phone uses a secondary lens and what’s known as ‘computational photography’ to simulate the shallow depth-of-field.

Starting with the Galaxy S9, S9+ and Note9, Samsung have added a true, adjustable aperture, although you only get to choose between f/1.5 and f/2.4. The technology is quite new, but I’m sure we’ll see it filter through to more phones in the future.

For the most part, a portrait mode’s faux depth-of-field isn’t a bad thing. I mention it only to give you the heads up that you can’t use the portrait mode on your phone in the same way you’d use a wide aperture on a camera to help you out in low light situations. From a purely artistic perspective, however, go for it!

tl;dr rules

If all of this has given you a bit of a headache, then here’s it all in a nutshell:

• The aperture in your lens is like the pupil of your eye – it’s a hole designed to let in and focus light. It can be made wider and narrower to control how much light hits your camera’s sensor
• Depth-of-field refers to how much of your photo is in focus
• Shallow depth-of-field has a smaller amount of the photo in focus
• Deep depth-of-field has lots of the photo in focus
• Your aperture setting affects your depth of field: wide aperture = shallow depth-of-field, narrow aperture = deep depth-of-field
• Your depth-of-field is also affected by your focal length (wide angle/zoomed out = deeper depth-of-field), the size of your sensor (smaller = deeper depth-of-field) and how close you are to your subject (closer = shallower depth-of-field)
• The aperture values are called ‘f-stops’ and the lower the number, the wider the aperture
• Many mobile phones have a fixed aperture – you can’t change it, so portrait mode simulates shallow depth-of-field in your photo using computer power

If it’s all still a bit fuzzy …

… the best thing you can do is have a play. Set up something (or someone) at home to be your subject. It can be anything: a houseplant, an ornament, some books, or even just a coffee cup. Make sure there’s something behind them with some detail, too – a bookshelf or a garden wall, for example – with a bit of space between them. This will make it easier to judge the depth-of-field.

Then just set your camera to aperture priority and take the same photo a few times with different aperture values to see how the depth of field changes.

Try zooming in and out, and moving closer to or further away from your subject.

If you’re on a mobile phone, look to see if you have portrait mode or aperture mode. Take photos with them switched on and off and compare the two.

The more you practice and play with settings, the more your photography will improve.

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Julia

Unless otherwise credited, all photos in this post are © Julia Thorne. If you’d like to use any of my photos in a lecture, presentation or blog post, please don’t just take them; drop me an email via my contact page. If you share them on social media, please link back to this site or to one of my social media accounts. Thanks!