What Is an Aperture and How Does It Work?
A Brilliant and Essential Photographic Tool for Controlling Exposures16 min read by
The workings of apertures eluded me for a long time. Though I’ve learned what it is used for fairly quickly — controlling the amount of light that falls onto film — its agency remained a mystery.
A shutter’s function is simple: it creates a precise window of time for film to saturate with light; the longer shutter stays open, the brighter the image becomes. But an aperture is a lot more mysterious. How does it manage to cut parts of the light’s path without cropping the image itself? And how does it change the exposure by doing so?
There are barely any guides online that explain how apertures work fully. Instead, most focus on their application in photography, with little attention to the working principles. This article is written to complete that knowledge gap. Of course, it will still illustrate and explain all the effects an aperture can have on your image in simple terms and introduce the best ways to apply that knowledge in practice.
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What is an aperture?
An aperture is the opening through which the light can enter the camera. The volume of that light can be controlled by altering the size of an aperture, which is done with an aperture diaphragm.
In a typical lens, an aperture could be open, meaning that the lens will be gathering the most light it could. Or it could be stopped down, which means that the aperture diaphragm’s diameter is decreased.
Most lenses’ diaphragms are made of blades that individually look like a black metal flower’s petals, arranged in a circle. When you twist a ring on your lens, the blades move and either increase or decrease the diaphragm’s diameter.
✪ Note 1: Some lenses will have their aperture blades hidden behind a leaf shutter — another kind of a diaphragm.
An aperture diaphragm often looks like an iris in a human eye. This might be because they both perform precisely the same function: control the light’s flow.
✪ Note 2: Aperture diaphragms are often called “aperture” for short.
An aperture’s purpose is to control the volume of light that falls onto the film plane. Though shutters can also control exposure, apertures do it differently:
How apertures differ from shutters.
Imagine that you are next to your kitchen sink with a glass and a stopwatch. If you open up your tap to its fullest, the water will fill the entire glass within five seconds. But if you open your tap halfway, you can safely assume that your glass will take ten seconds to fill — half the flow, double the duration.
If you measure your water flow precisely, you won’t even have to look at the glass to know when it’s about to get full. Your stopwatch will signal you to shut off the tap.
In photography, apertures control the volume of light like your tap’s valve controls the water’s flow. A camera’s shutter, on the other hand, controls the duration of an exposure, like the stopwatch that tells you when to turn your tap off to avoid overfilling the glass.
To put it concisely, an aperture controls the volume of light reaching the film while a shutter controls the duration of an exposure.
See how apertures affect exposure: a practical DIY experiment.
A larger aperture diaphragm’s opening lets more light onto film while a constricted (stopped-down) aperture carries decreased light power. In this section, I’ll show you how you can observe this effect with a loupe and a light bulb.
Before we continue with the experiment, you should understand that the location of aperture blades in a lens system is important. As you can see in Figure 1, they are close to the focusing element. In this position, they effectively decrease the radius of the lens and thus limit the amount of light it can gather. If placed close to the film plane, however, the aperture blades will blackout the edges of the image without affecting its brightness.
To observe the effects of an aperture, grab your loupe and find a dark spot in your house. Turn a single lamp on and place your loupe underneath it so that you can see a small, focused outline of your light on any flat surface. It may look like a ball of light with sharp edges. Or, if you have a chandelier, you should recognize its shape.
Then, make a circle with your thumb and index finger that’s slightly smaller than your loupe in diameter and place it above or below your lens — as close to the glass as you can. Your focused lamp’s image should get a little dimmer. The image will dim even further as you squeeze your fingers to make a smaller circle.
But if you move the circle you’ve made with your fingers close to the surface that has your lamp’s image projected, you’ll notice that you are no longer affecting the image’s brightness.
F-stops: a way to measure apertures.
In photography, where a barely-noticeable change in light can spell the difference between a great photo and a ruined image, setting exposures precisely is important. An aperture, being an instrument that controls exposures, has to be measured and controlled accurately.
However, the diameter of an aperture alone isn’t a convenient way to measure its effectiveness.
Consider that the longer the lens’s focal length is, the smaller its angle of view. That is, longer focal lengths allow your lenses to become more “telephoto.”
However, longer focal length lenses need a larger aperture radius to compensate for the diminished light coming from a smaller angle of view/area.
Turns out we can not ignore the lens’ focal length when calculating the effectiveness of an aperture. The diameter alone can’t tell the whole story. Thankfully, we’ve got 𝒇-stop numbers.
𝒇-stop numbers are ratios of lenses’ focal lengths to their aperture diameters. They are used to measure the effectiveness of a lens’ aperture.
For example, an 𝒇/2 number means that the aperture diameter is 1/2 the size of the lens’ focal length. A smaller fraction, 𝒇/4, means that the aperture diameter is 1/4 of the lens’ focal length.
Let’s also briefly define the “stop” bit in 𝒇-stops.
In photography, a stop of light is a log base 2 value that comparatively measures the light volume or sensitivity. Or, simply: each incremental stop results in twice as bright of an exposure.
With film, stops are used to indicate the difference between emulsion sensitivities. For example, ISO 400 film is twice as sensitive — or one stop faster — than ISO 200 film. An ISO 100 film is four times less sensitive than ISO 400 — or two stops slower.
✪ Note: In photography, a system that yields a brighter exposure is often referred to as “faster.” A “slower” system would similarly yield dimmer exposures.
Shutter speeds can also be compared using stops. For example, a shutter speed of 1/500 lets half the amount — or one stop less — of light onto film than 1/250.
Finally, aperture 𝒇-numbers one stop apart look like this: 𝒇16, 𝒇11, 𝒇5.6, 𝒇4, 𝒇2.8, 𝒇2, 𝒇1.4 — where larger numbers (being fractions) indicate physically smaller aperture diaphragm openings. These numbers don’t follow the easy patterns like shutter times and film speeds that double or half with each stop. There’s a mathematical reason for that.
The mathematics of aperture stops.
The 𝒇-stop numbers are calculated using this formula: 𝒇/N = d, where 𝒇 is the lens’ focal length, N is the 𝒇-number (i.e. 2 or 2.8), and d is the diameter of the lens’ aperture. The area of an aperture is πr² or π(d/2)² — since a radius r is half of the diameter. Substituting the diameter for 𝒇/N, an aperture’s area becomes π((𝒇/N)/2)² or π(𝒇/2N)².
Now, let’s assume we’re working with a lens with a 50mm focal length and a maximum aperture of 𝒇2. To decrease the exposure by half (i.e. by a single stop), we’ll need to half our aperture’s area. Using the formula above, we’ll get:
½(π(50/(2×2))²) = π(50/2𝒙)²
Where 𝒙 is the 𝒇-number for an aperture that would have half an area of 𝒇2 and thus one stop slower. Solving for that, we’ll get:
½(π×12.5²) = π(50/2𝒙)²
245.4369260617 = π(50/2𝒙)²
78.125 = (50/2𝒙)²
8.8388347648 = 50/2𝒙
8.8388347648×2×𝒙 = 50
17.6776695296×𝒙 = 50
𝒙 = 2.8284271248
And so goes the progression: 𝒇2, 𝒇2.8, 𝒇4, 𝒇5.6, 𝒇8, 𝒇11, 𝒇16, etc.
Some manufacturers will mark in-between aperture numbers in either half or ⅓ stops. In some cases, this adds accuracy to the measurements. Other times this is because a fraction of a stop is all they could push their product’s maximum aperture by — as is the case with Olympus H.Zuiko Auto-S 𝒇1.2 lens.
How apertures affect depth of field.
Depth of field describes the amount of background blur you’ll get in your photographs when focusing on up-close objects. More precisely, it’s the distance between the furthest and closest points in space in front of a lens that will appear in focus. And the aperture size has a direct effect on the depth of field.
The reason for such a relationship is a circle of confusion — a cone of unfocused light when it intercepts the film plane. Larger apertures direct more light through their wider cones of light, which yield larger circles of confusion when the focus is off.
Additionally, longer lens’ focal lengths will create shallower depths of field (more blur) at the same 𝒇-stops as shorter focal length lenses. This is because the aperture area of a longer focal length lens needs to be larger than that of a shorter focal length lens to yield the same 𝒇-stop number.
For example, a 100mm 𝒇2 lens will have a shallower depth of field than a 50mm 𝒇2 lens. The 100mm lens will also have a much larger aperture radius.
Of course, the size of your circle of confusion is relative to your film plane’s size. So if you shoot a smaller film format with the same lens’ focal length and aperture, your depth of field will decrease; that is, you’ll have more background blur.
However, unless you change your lens’ focal length, your field of view/angle of view will also change (on a smaller format, your scene will look “zoomed-in”). Matching a smaller format’s field of view to that of the original format requires decreasing the lens’ focal length, resulting in an overall larger depth of field. Hence, less background blur on smaller formats with the same aperture 𝒇-number and angle of view.
The quality of background blur is often referred to as bókèh. Sounded from Japanese “暈け” or “ボケ,” the word is meant to apply specifically to lens blurs rather than motion blurs.
Bókèh balls refer to specular highlights turned into large discs via circles of confusions’ fanning cones.
Some older lenses are known to produce swirly bókèh caused by optical vignetting.
Your aperture blades also play a part in shaping your bókèh balls. For example, you may want to shoot your lens wide-open to get nicely-rounded ovals and circles. Some lenses will even let you customize the shape of your bókèh highlights.
If you want more bókèh, you should increase your lens’ focal length, shoot a larger film format, increase your aperture’s size, or get closer to your subject.
Aberrations at maximum apertures.
Knowing that larger apertures yield brighter exposures and more pronounced bókèh, you may be tempted to shoot your lenses wide-open (at their maximum aperture) as much as possible. However, doing so is going to result in loss of sharpness.
The light rays travel at more extreme angles on the fringes of the lens’ glass which means that designing a precise system to rein them in is difficult. Stopping down the aperture/decreasing its size excludes those difficult edges, thus resulting in an overall sharper rendering of the in-focus elements of your photograph.
Diffraction at minimum apertures.
Stopping down your aperture to 𝒇16 and smaller will get more of your scene in focus. Limiting your lens’ active surface to just a small circle in its middle will also help avoid some aberrations. Unfortunately, doing so may still decrease your image’s overall sharpness due to another optical phenomenon — diffraction.
Diffraction is the manifestation of the famous double-slit experiment that demonstrates the light’s dual properties as particles and waves. In the experiment’s setup, two thin slits are created so that when the light passes through them, it displays a wave-like pattern on the other side. A small-enough aperture can function as one of those slits, causing waves to form around points of light on film.
Those waves are usually too small to see in the resulting image but their overall effect is blurrier photographs.
Though diffraction may cause loss of sharpness, it can also produce interesting effects in your photographs, namely starbursts or sun stars. Most apparent in photographs shot at smaller apertures with the points of strong light surrounded by contrasting shadows or dark objects. Starbursts may have a various number of spikes and come in different sizes.
The smaller the aperture you’re photographing with, the larger your sunburst will appear.
An even number of aperture blades on your lens will produce an identical number of spikes in your starbursts. However, odd-numbered apertures will create double the number of spikes in their starbursts.
The shape of your aperture blades also plays a role in forming starburst effects. Rounded blades will often create less apparent effects.
To understand how starburst effects form, consider the diffraction pattern in the image above. It is a result of light passing through a perfectly circular aperture. However, introducing hard edges that are caused by blades at smaller apertures causes the waves to create interference patterns as they bounce around. Those patterns tend to form peaks that appear as rays of light on film.
Apertures with an even number of blades have the rays overlay with those formed on the other side of the aperture (as the rays run across). Apertures with an odd number of blades don’t have their rays overlay, doubling the number of starburst spikes as the rays run in both directions.
Choosing an optimal lens aperture.
An aperture can do a lot more than control exposure. At its minimums and maximums, it can render beautiful out-of-focus bókèh effects and starburst sparks. But of course, those effects come at the cost of an overall image sharpness which may or may not be a more important objective for your photography.
There are no absolute rights or wrongs when choosing between acuity and pretty extras. Instead, the wisdom comes in controlling your lens and understanding how to create an image that you’ve envisioned.
It is also important to understand that all lenses behave differently. Many will create similar results, while others will render your photographs remarkably distinct. This is why I can only give you general guidelines that you can use as a starting point for whatever lens is in your hand:
For sharpest images, consider choosing an aperture that’s two to three stops smaller than your lens’ maximum but no larger than 𝒇8 or 𝒇11, as everything beyond that will soften your photographs. Although, large-format cameras can still produce sharp results with apertures smaller than 𝒇16 due to their immense resolution.
Your bókèh balls will look their best when shooting with your aperture wide-open, preferably focusing on something close-by. On the other hand, starbursts will become most noticeable at your larger apertures, especially when your points of light are contrasted by something dark right next to them.
Most lenses will have an aperture control ring that you can set to an 𝒇-number of your desire. They will also often include depth of focus calculations that will show you what will appear in focus at a given distance and aperture — very useful when using a rangefinder or a zone-focusing technique. They will also typically “click” in-place when you reach an 𝒇-number to help you set your exposures more precisely.
SLR cameras will also typically have a way for you to preview your depth of field by pressing a button on your lens or your camera’s body. This is to give you a visual idea of what will be in focus on your photograph, since focusing on SLRs is usually done with the aperture set to a max value to let the most light onto your viewfinder for a brighter image.
Zoom lenses (i.e. one on Pentax Espio 140V) are often marked with two maximum aperture numbers. This is because the larger aperture on those lenses usually corresponds to the max aperture at a shorter focal length and vice-versa.
Use your aperture to control flash exposure.
When you are shooting with flash, your shutter’s role diminishes as the ultra-fast bright light emitted by your strobe is often the only source of effective light. In this case, your lens’ aperture becomes paramount in controlling the power of your exposures. To learn more about how this works, check out my “Simple Guide to Using Flash on Manual Film Cameras.”
Aperture as a shutter.
Though most cameras will have a dedicated shutter mechanism in addition to a working aperture, some do away with a single element playing the role of both. Just like a water tap’s valve — see example above — leaf shutters may be used to both hold off the flow of light completely and open to a precise position for a timed stream of photons towards a film plane.
Pinhole: an aperture as a lens.
A pinhole is the smallest aperture possible, typically 𝒇128 or 𝒇256. Due to its size, it will create very small circles of confusion, thus rendering every part of the scene in focus — without relying on glass to converge the light rays into a single point.
Of course, an aperture that small will suffer from diffraction blur and will require long exposure times as it drips the photons slowly onto the film surface. But its remarkable upside is a simple construction giving anyone with a spare box and a pin ability to create own camera at home.