All about aperture

The aperture of a lens, quite simply, is the opening through which light passes into the camera. The wider the opening, the more light can reach the camera sensor, which in turn affects the exposure of the image.

Just as the pupil in the human eye contracts in bright conditions and expands in low-light environments, the aperture needs to decrease or increase to achieve correct exposure – that is, a clear image that's not too dark or too washed-out – in different lighting conditions. This narrowing and expanding is done by an array of aperture blades in the lens that move synchronously to adjust the size of the aperture, up to the maximum that the lens is mechanically capable of attaining (which is the number in the name of the lens).

The aperture blades contribute to the bokeh – the aesthetic quality of the out-of-focus areas in an image. The number of blades can dramatically affect how smooth or "creamy" the bokeh appears. Typically, lenses with a greater number of blades tend to render rounder and more pleasing bokeh. The number of blades will be included in the specs of the lens (for example on its web page on the Canon site).

• Large aperture vs small aperture
• Balancing aperture with other camera settings: the Exposure Triangle
• Fixed-aperture lenses vs variable-aperture lenses
• Aperture and image sharpness
• Understanding f-stops and t-stops

Aperture is expressed in f-stops. Confusingly, the f-number is not the actual size of the aperture, but has an inverse relationship to it:

          aperture diameter = focal length (f) divided by f-number

This is why f-stops are written with a slash, as in f/1.4 and f/16 – they are actually fractions. It also explains why the lower the f-number is, the larger the aperture – the f-number is the denominator of the fraction, and 1/4 is bigger than 1/16.

Each increment on the standard f-stop scale halves the amount of light that reaches the sensor – f/4 lets through half as much light as f/2.8, f/5.6 half as much as f/4, and so on. Each of these steps, halving the amount of light each time or doubling it going the other way, is described as one stop of light. In turn, this is why the f-stop scale comprises the numbers it does, such as f/2.8 and f/5.6, instead of whole numbers, which would not correspond to whole stops of light.

When we say large aperture, we mean a small f-number and we're talking about an opening that allows a flood of light to reach the camera's sensor, akin to throwing open the curtains to let in as much light as possible. This means wide apertures such as f/1.4 are perfect for shooting in low-light conditions – more on this shortly.

Conversely, a small aperture – indicated by a higher f-number, such as f/22 – limits the amount of light striking the sensor, akin to peering through a narrow slit in the curtains. This setting is ideal on a sunny day or whenever your scene is flooded in light, preventing overexposure (an image that looks washed-out).

Aperture also plays a key role in defining the depth of field in your shots – that is, how much of the image is in sharp focus. A larger aperture, such as f/1.8, results in a shallow depth of field, making your subject stand out against a beautifully blurred background. This is ideal for portrait or macro photography. A smaller aperture, such as f/16, keeps more of the scene in focus, which is ideal for landscapes and architecture photography where you want foreground-to-background sharpness.

Aperture is one of the three sides of the Exposure Triangle, along with shutter speed and ISO. It is the interaction of all three that determines the exposure of an image.

• Aperture controls the amount of light entering through the lens.
• Shutter speed dictates the duration of light hitting the sensor. A fast shutter freezes motion, while a slow one may introduce blur if the subject (or the camera) moves while the shutter is open. If you use a wide aperture (small f-number), then more light is coming in, which means a faster shutter speed will achieve the correct exposure.
• ISO sets the camera sensor's sensitivity to light. A low ISO yields a cleaner image, while a high ISO may introduce noise but may be necessary for shooting in low-light conditions where the risk of blur means you can't simply use a very slow shutter speed.

Mastering the Exposure Triangle means balancing all three elements, while being aware of the implications of tweaking any one of them. Here are some examples.

Portrait photography often demands that the subject should stand out against a creamy, blurred backdrop. To achieve this, you would use a larger aperture (smaller f-number), such as f/1.8 or f/2.8, for a shallow depth of field. This allows more light to reach the sensor, meaning you can use a faster shutter speed to freeze the moment, and keep ISO low for a clean image.

In landscape photography, on the other hand, you might want everything from the foreground to the horizon in sharp focus. This requires a small aperture (large f-number), such as f/16 or f/22, which produces a greater depth of field. However, it lets in less light. To compensate, you may need a slower shutter speed or a higher ISO. But a slower shutter speed could introduce blur from camera shake, and a higher ISO might add noise.

Capturing action, such as a racing car or a frolicking pet, requires fast shutter speeds to freeze motion. This means less light hitting the sensor, so you'll need to counterbalance by widening the aperture, increasing ISO, or both.

What about shooting in low-light conditions, such as a candle-lit dinner or a twilight cityscape? A large aperture (small f-number) will let in as much light as possible but also reduce depth of field. To keep enough of the scene in focus, you might need to use a slower shutter speed instead – but this risks blur, while increasing the ISO exacerbates noise.

The beauty of the Exposure Triangle lies in its flexibility. There's no one-size-fits-all setting; it's about finding the right balance for your subject, your scene or your story – see our useful table for more about how the different settings work together.

You can partially automate this, of course – if you set your camera to Aperture priority (Av) mode, you choose the f-number and the camera sets the correct shutter speed to match; you can select Auto ISO as well or opt to set the ISO yourself. In Manual (M) mode, you can set any combination of aperture, shutter speed and ISO.

These terms apply to different zoom lenses – that is, lenses with variable focal lengths. Fixed-aperture lenses, also known as constant aperture lenses, maintain the same maximum aperture size throughout the entire zoom range. With the Canon RF 24-70mm F2.8L IS USM lens, for example, whether you're shooting wide at 24mm or zoomed in at 70mm, you can still open your aperture to f/2.8. This consistency is an advantage when you're shooting in varying light conditions or when you need to maintain a certain depth of field across different focal lengths. It's particularly valuable in sports or wildlife photography, where light conditions can change quickly and the ability to maintain fast shutter speeds at all focal lengths is essential. Another fixed-aperture lens is the Canon RF 28-70mm F2L USM. This lens is widely used for its fast f/2 aperture across the entire zoom range, making it a powerhouse for low-light photography and providing stunning bokeh.

Variable-aperture lenses offer different maximum apertures as you zoom. For example, on the Canon RF 24-105mm F4-7.1 IS STM lens, at the widest end (24mm), the maximum aperture is f/4. But when you zoom in to 105mm, the largest aperture you can achieve is f/7.1. While this means you may need to adjust your exposure settings when zooming, variable-aperture lenses are often more compact and affordable, making them an appealing option for travel photography or for those on a budget. Another notable variable-aperture lens is the Canon RF-S 18-150mm F3.5-6.3 IS STM. This lens offers an extremely broad, and therefore very versatile, zoom range and image stabilisation, making it a solid all-rounder and a favourite among travel and street photographers.

The choice between fixed- and variable-aperture lenses ultimately depends on your shooting needs and style. If consistent exposure and depth of field across a zoom range are paramount, then a fixed-aperture lens would be your best bet. But if portability and budget are key considerations, then a variable-aperture lens could serve you well.

Your aperture setting also has an effect on general image sharpness. Extremely wide apertures (such as f/1 or f/1.8) might slightly compromise overall sharpness because of lens aberrations – physical imperfections, which are unavoidable simply because of the limitations of physics and materials. These also vary between individual lenses, even if they are the same model of lens and nominally identical. This is one reason why cameras cannot completely eliminate them, despite performing a whole suite of in-camera lens corrections.

Very small apertures (such as f/22) cause diffraction, which can also reduce image sharpness. Think of the light entering the camera as a column of marching soldiers with those at the sides jostling each other and bumping against the walls as they pass through a narrow opening, leaving them less perfectly aligned than when they pass through a wide opening. This is also an unavoidable physical phenomenon, although many cameras mitigate it using Canon's Diffraction Correction feature.

These effects might not be very noticeable in every image, and are typically less severe in the centre of the frame than in the corners. However, there is usually what's called the "sweet spot" of a lens, the aperture setting where it can deliver its sharpest performance.

You could take a set of test shots to determine this but, as a rule of thumb, the optimum aperture is typically 2-stops or so narrower than the maximum aperture of the lens. With an f/2 lens, for example, an aperture around f/4 should produce the sharpest results across the frame. For an f/2.8 lens, try around f/5.6. In the case of a variable-aperture lens, the sweet spot is somewhere in the middle of its aperture range.

The term f-stop derives from focal-stop, and tells you the physical size of the aperture relative to the focal length of the lens. However, as light travels through the lens, some of it may get absorbed, diffracted or reflected by the optical elements in the lens before it hits the camera's sensor. This is where a different measure can be useful: t-stops, or transmission-stops, relate to the actual amount of light that successfully passes through the lens and reaches the camera's sensor.

This distinction is important because not all lenses are equally efficient at transmitting light. For example, two lenses may both have an aperture of f/2.8 but, because of differences in lens design and coatings, one lens might transmit more light than the other. Knowing the t-stop value would allow for a more precise measurement of exposure, ensuring consistency especially when you're interchanging lenses or shooting in a controlled environment such as a movie set.

In practice, however, f-stops are the standard measure of aperture for photographers and are the units displayed on stills camera lenses. They offer a convenient and practical way to control exposure and depth of field, and most modern cameras automatically compensate for the slight variations in light transmission between different lenses. T-stop values are not usually given, or even readily available.

On the other hand, t-stops are typically used in professional cinematography and video production, where maintaining absolutely consistent exposure is paramount, especially when using different lenses or cameras. In practice, working with t-stop values guarantees that every lens used will provide the same exposure for a given setting, ensuring consistency across shots.

Armed with an understanding of aperture and how it works in photography, you're one step closer to capturing the world through your lens just the way you envision it.