The aperture of your camera lens has two critical effects on your photograph. It determines how much light enters your camera – and therefore how well exposed your image is – and how much of your scene is acceptably in focus, an effect known as depth of field.
It’s therefore essential you have a good grasp of it, if you’re going to achieve your creative goals. Fortunately, the basics are straightforward and you’ll find everything you need to know in this article. We’ve also included some more technical info at the end, if you want to find out more.
Table of Contents
What does aperture mean?
The aperture is the opening inside your lens, through which light passes on its way to your camera’s sensor or film. You can see this in the photo at the top of this article.
How is aperture measured?
The aperture is measured in f-numbers. What seems odd to many new photographers is that the lower the number, the larger the aperture – so f/2.8 is larger than f/4, which in turn is larger than f/8 and so on.
This is because the f-number is actually a fraction – it’s the focal length of your lens (represented by the f in f/4, for example) divided by the width of your aperture. If you think about fractions, it should be clear that f/4 is bigger than f/8.
How aperture affects exposure
As you’d expect, a larger aperture means more light entering the lens and a smaller one means less light – just like the pupil in your eye controls the amount of light that hits your retina. This means your aperture is one of the two things you can adjust to achieve a correct exposure in your picture, along with your shutter speed.
You can also change your ISO but in digital photography this is an electronic adjustment to the brightness of your photo, rather than a change in the amount of light captured. As a result, ISO is not strictly a part of exposure, despite the so-called exposure triangle.
The diagram below shows a more accurate way to look at it:
Understanding how much light you’re letting in is slightly less intuitive for aperture than it is for shutter speed. Shutter speed is straightforward: doubling your shutter speed – say from 1/100 to 1/200 of a second – halves the amount of light. Halving your shutter speed has the opposite effect. Each time we double or halve the amount of light, we’ve adjusted the exposure by one stop.
With aperture, the amount of light entering is relative to the area of the circle through which the light passes. To increase or decrease the amount of light by one stop, the aperture needs to be divided or multiplied by roughly 1.4 (see the box at the end of the article if you’re interested in why).
For this reason, there’s a difference of one stop between each of the following apertures:
f/1.4 > f/2.0 > f/2.8 > f/4 > f/5.6 > f/8 > f/11 > f/16 > f/22
That means that if you decrease your aperture from f/5.6 to f/8, for example, you need to halve your shutter speed to maintain the same exposure. Going from f/4 to f/11 is a difference of three stops, so you’d make a three-stop adjustment to your shutter speed to keep the exposure the same.
Note that while we often talk about moving in whole stops when adjusting apertures, your camera will be able to work in smaller increments (usually one-third of a stop), allowing you to fine tune the aperture to get the combination of aperture and shutter speed you want. If your camera is only letting you work in whole stops, you need to adjust your settings via the menu to access one-third stop increments.
How aperture affects depth of field
First, a quick primer on depth of field. Think of a typical composition, which has your subject, the space in front of it (the foreground) and the space behind it (the background).
You want your subject to be in focus, as that’s where the viewer’s attention will fall. You then need to decide how much of the foreground and background you want in focus. This is the depth of field – the distance between the nearest part of the foreground and the furthest part of your background that is acceptably sharp.
In genres such as landscape photography, you’ll want a large depth of field, so your foreground interest and your background are all acceptably sharp. In portrait photography, it’s common to want a shallow depth of field, so you blur the background and only your subject is in focus.
Adjusting the aperture is one of the primary ways to achieve different depths of field. The smaller the aperture – and the higher the f-number – the larger the depth of field. An aperture around f/11 should produce a sharp landscape photograph. A good portrait lens will allow you to use an aperture around f/1.4 for the blurred background effect.
We cover this in a lot more detail in our dedicated article on depth of field.
You’ll often hear ‘bokeh’ mentioned in the context of shallow depth of field and blurry backgrounds. This isn’t about the amount of blur you can achieve but how good the blur looks, in particular whether the lens creates smooth blur with nicely shaped highlights. This will be particularly noticeable if you have lights in the background of your shot.
The quality of the bokeh you get will depend on the how good your lens is – you’re unlikely to get great performance with the kit lens that came with your camera. Bokeh also depends on your aperture, with the highlights becoming more or less circular with even modest changes to your aperture setting.
Balancing depth of field and exposure
You’ll have spotted here that there’s a trade off between depth of field and exposure. As you increase one, you reduce the other. You’ll need to find the right balance for your particular image, so you have creative control over what’s in focus and your shutter is at the right speed, adjusting the ISO if necessary. For small apertures and slow shutter speeds in low light, a tripod will help you to get a sharp image.
How to adjust the aperture of your lens
On modern cameras, the aperture is usually controlled electronically, with the camera telling the lens what the aperture is. This means you need to set your camera to a mode where you’re in charge of the aperture, which is either fully manual or aperture priority mode. Most cameras have a dial on top that allows you to switch modes. Manual mode is represented by M, while aperture priority is usually A or Av (aperture value).
Aperture priority is a semi-auto mode, in which you choose the aperture and the camera automatically adjusts the shutter speed according to the light conditions, to achieve a correct exposure. This can be really handy when the shutter speed is not particularly critical (for example, you don’t need a fast shutter to freeze motion) and you’re more interested in controlling depth of field.
(Conversely, if you choose shutter priority mode, your camera will automatically adjust the aperture to ensure your shutter speed remains constant.)
Changing the aperture itself usually involves using one of the command dials on your camera. You should be able to change the settings in your camera so that each click of the dial moves you on by either 1/3 of a stop or a whole stop, depending on your preference. As noted earlier, 1/3 stop increments give you greater control.
Back in the days of manually operated lenses, an aperture ring on the lens itself was standard. These days only a minority of lenses have aperture rings built in. However, some lenses have programmable rings and you can set those to control the aperture if you like.
What’s the biggest aperture my lens can do?
If you look at your lenses, you’ll see that they’re marked with an f-number. This is the maximum aperture the lens is capable of. The f-number doesn’t always include the ‘f’ on the lens itself but you should be able to work out which one it is.
The lens on the left below has a maximum aperture of f/4. As this is a zoom lens, the fact that there’s only one f-number here is important, as it indicates that the maximum aperture doesn’t change as you zoom in and out.
The lens on the right is also a zoom and it has a range of maximum apertures, from f/4.5 to f/5.6. That means that as you zoom in and the focal length increases, the maximum aperture you can use gets smaller. With this 70-300mm lens, an aperture of f/4.5 is only available at the 70mm end. At 300mm, you’re limited to f/5.6.
It’s important to know the maximum aperture of your particular lens, as that will affect the photographic effects you can achieve with it. We’ll discuss what you might want to look for in a lens later on.
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How does aperture affect the sharpness of my photograph?
Most lenses have an aperture range where they are at their sharpest, with images being softer with the lens wide open, then increasing in sharpness as the aperture reduces, before softening again at small apertures.
Why does this happen?
As light passes through a lens element, it suffers from spherical aberration. As you move away from the centre of the lens, the curvature of the glass increases and the aberration increases with it.
The larger the aperture, the more light you have passing through the outer regions of the lens elements – hence the common complaint that images taken at wide apertures are sharpest in the centre and get softer towards the corners. Lens designers are good at compensating for this, by having multiple elements within the lens to control the aberrations. Even so, no lens is perfect and some aberrations will remain.
As the lens is stopped down (i.e. the aperture is made smaller), a greater proportion of the light entering your camera is channelled through the central regions of the lens elements. This progressively reduces aberrations, making the image sharper.
However, the smaller the aperture becomes, the more diffraction begins to outweigh the benefits of reducing aberrations. Diffraction occurs when a wave – in this case light – reaches an aperture, which causes the wave to bend. The smaller the aperture through which the wave has to pass, the greater the divergence. This results in your image becoming increasingly soft at very small apertures, with loss of fine details.
The sweet spot for sharpness is specific to the lens but around f/5.6 is common. Photographers who value sharpness typically avoid apertures as small as f/16. To learn how your lenses perform, you’ll find sharpness information in the more detailed reviews online or you can conduct your own tests.
Creative effects – the starburst
While diffraction is generally a negative, it does have one upside in creating the starburst effect. This is where a strong light source in your photo, whether the sun or an artificial light, is seen to have light rays emanating from the centre. (Technically, the light rays are known as diffraction spikes).
The starburst effect increases as the aperture gets smaller. While some rays may be visible at larger apertures, stopping down to a narrow aperture around f/16 (as I did with the photo above) will achieve a stronger effect.
Interestingly, the number of spikes depends on the number of blades in the diaphragm. If it’s an odd number, you’ll get twice as many spikes as blades. For an even number, the number of blades and spikes is the same. The seven-blade Nikon lens I used therefore produces 14 spikes.
How else can aperture affect my image quality?
Wide apertures can result in an effect called optical vignetting, which is where the outer edges of your frame are darker than the centre. This results from the lens construction itself shading the outer edges of its elements from light that would otherwise enter the lens off-centre, so not as much light is recorded at the corners of your picture.
In the image below, taken on a Sony RX100M6 at f/2.8, you can see the centre of the photo is brighter than the bottom corners in particular.
Vignetting isn’t necessarily a problem and many photographers add vignettes in post-processing, because darker edges focus the viewer on the centre of the frame. If you don’t want vignetting in your photo, it’s also easily corrected in post.
Smaller apertures can also cause an issue with sensor dust becoming more obvious in your image. This happens because small apertures result in the light hitting your sensor coming from a narrower range of angles, creating a hard-edged shadow on the sensor when the light hits the dust. Larger apertures mean more light coming from more angles and a softer shadow, which can reduce the appearance of dust or render it invisible.
How aperture affects the size, weight and cost of your lens
In photography, as in life, there are always trade offs. Lenses with large apertures are typically bigger, heavier and more expensive, particularly for zoom lenses with a large maximum aperture (such as f/2.8) that stays constant throughout the zoom range.
The size and weight partly reflects the fact that a lens with a large constant aperture simply has to be bigger than one with a smaller maximum aperture, and all that extra glass inside the lens adds up. Lens design also becomes more complicated as apertures increase, as the shallower depth of focus requires lens elements to compensate for a range of different distortions.
Because constant aperture zooms appeal to photographers with specific needs, who are often professionals, they also have a higher build quality and use metal components rather than plastic. Again, this adds to both the weigh and the cost.
When you’re choosing a lens, you’ll need to decide whether the extra cost and inconvenience of a big and pricy lens is outweighed by the creative freedom it will give you.
If you like to walk around with your camera, a day spent with one of these zooms hanging from your neck can get tiring and painful. For landscape photographers, a trio of f/2.8 zooms will be a serious weight to carry. In these circumstances, you’ll mostly be using smaller apertures for more depth of field, you may be better off going for an f/4 lens rather than an f/2.8.
For other uses, the low-light capabilities of a large aperture may be crucial and you’ll have no alternative but to pay up and accept the additional weight. f/2.8 zooms for mirrorless cameras tend to be smaller and lighter than the DSLR equivalents.
Alternatively, if you need a large aperture but can manage without the zoom, a prime lens is the way to go. Primes are ideal for uses such as portraits, where a shallow depth of field looks great and you can easily adjust the framing of your image by moving closer or further away.
What’s the effective aperture of a lens?
Earlier in this article, we said that the f-number was the focal length divided by the aperture. That’s an oversimplification. It’s actually the effective aperture that’s important here.
What does that mean?
The diaphragm in your lens has several lens elements in front of it and behind it (see image below). This means that as you change the focal length in a zoom lens, the lens elements move towards or away from the diaphragm.
When you look through the front of the lens, the size of the aperture will appear to change at the same time. This means the effective aperture has changed, even if the actual aperture hasn’t. The technical term for the effective aperture is the entrance pupil.
Therefore if your lens is at 100mm and you want an aperture of f/4, your aperture will appear to be 25mm wide when you look through the front of the lens, even though the actual size of the aperture may be different.
Why are f-stops not linear for aperture?
The opening in the lens is (more or less) a circle and the size of the opening determines how much light gets through. You’ll probably remember the formula for the area of a circle from school:
Area = π x radius2
In this formula, the radius is half the diameter of your aperture.
If we want to double the amount of light that reaches the camera sensor, we need to double the area of the circle. To double the area, we need to increase the radius by an amount that – when squared – will equal 2. The radius therefore needs to increase by the square root of 2.
The square root of 2 is roughly 1.414. Hence every time you increase or decrease your lens aperture by one stop, you’re multiplying or dividing your aperture by this amount and this is reflected in the change in the f-number.