Introduction
Aperture is the lens mechanism that regulates the amount of light hitting the sensor, but there's much more. What does an "open aperture" actually mean? And why do photographers talk about f/numbers? In this guide, we'll clarify these concepts and help you understand how aperture really works.
What is the aperture?
When buying a lens for a DSLR camera, one of the main parameters to look at is the maximum aperture, expressed in an f value. Let's take as an example a Canon EF 50 mm f/1.4 USM, a good lens produced by Canon, which happens to be our favorite lens.
- Canon = the manufacturer, such as Nikon, Canon, Zeiss, Leica, and so on.
- EF = the type of mount and its compatibility with the camera body.
- 50 mm = focal length, expressed in millimeters
- f/1.4 = maximum aperture of the lens, meaning its brightness
- USM = with ultrasonic autofocus
If you want to understand in detail how to decode lens names, which are often far from intuitive, we invite you to visit the page we wrote on the topic:
On this page, the value we want to focus on is the aperture, indicated by the lowercase letter f, usually followed by a slash and a number, here f/1.4. Some vintage lenses may use the colon, so f:1.4, but it's absolutely the same.
Building really bright lenses is difficult; in this case the focal length of 50 mm is peculiar, since it's relatively easy to built. In fact, this lens is rather cheap, but values such as f/1.4 or lower are definitely uncommon and expensive.
The value specified on lenses always refers to the maximum aperture, but this parameter can be reduced by adjusting the diaphragm. The diaphragm is made up of a series of blades that close down to control how much light is allowed to pass through the lens. You can see how it works below:
We've understood that the aperture value is the maximum amount of light allowed to pass through the lens to the sensor, and we also know that a lower value is preferable. We might say that a 50 mm f/1.4 is brighter than a 50 mm f/3.5, but a generic "brighter" is very inaccurate.
To understand exactly the difference, you need to know the following aperture scale: f/1.4 - f/2 - f/2.8 - f/4 - f/5.6 - f/8 - f/11 - f/16 - f/22 - f/32 - f/64.
Every respectable photographer knows it by heart, and if you want to take this hobby seriously, we recommend doing the same. You could start by learning just the central portion, from f/2.8 to f/11, as it's the one you'll use the most.
In this scale, the difference between one value and the next is called an f‑stop. The change in light between one f‑stop and the next always doubles, so it increases exponentially.
Going back to the previous example, what is the difference in brightness between a 50 mm f/1.4 and a 50 mm f/5.6? We halve the light entering the camera from f/1.4 and get to f/2, halve it again and get to f/2.8, halve it again and get to f/4, and finally halve it one last time to reach f/5.6.
This is why we said the progression is exponential: an f/1.4 lens is sixteen times (2×2×2×2) brighter than an f/5.6 lens. This means that at its widest aperture, the brighter lens allows you to use shutter speeds that are literally sixteen times faster.
A photo that would require 1/50 of a second at f/5.6 can be taken at 1/800 of a second at f/1.4, it's a huge difference!
When we take a photo, we are not always forced to work at the maximum possible aperture. But why would we want to change the aperture value? There are several reasons; let's list them in order of importance:
- Modify the shutter speed.
- Modify the depth of field.
- Increase sharpness.
- Make the lens perform at its best.
In the next chapters we'll see all the previous elements in detail, starting with the one that most affects the results: the shutter speed. Also because in photography, you can be sure of one thing: whenever you gain something on one side, you lose something on the other.
Shutter Speeds
The first and most obvious consequence of having more or less light reaching the sensor is that, under the same conditions, the less light we let in, the longer it will take to obtain a correctly exposed photo.
If we need to photograph a fast-moving subject, we need to use fast shutter speeds; if the light is not excessive, you'll end up using the aperture as wide as possible.
When the subject is really fast, as in the next photo taken during a hockey game in Cortina, the subject is blurred even using 1/640 shutter speed, which may seem really fast at first glance.
On the other hand, when we are photographing a still subject and the camera is on a tripod, it's a good practice to close the
aperture and use longer shutter speeds.
Finding the best balance between shutter speed and aperture is a matter of experience. It's not easy to transfer these skills without a good amount of practice, but we tried in our page about shutter speeds in photography, which you can find by clicking here:
At this point you might wonder: why not always shoot with the aperture at its maximum opening? The answer is simple: your lens performs much better when stopped down by at least a couple of stops, both in terms of sharpness and focus.
Depth of Field
With depth of field is meant the amount of space in which a subject appears perfectly in focus in a photograph. It's not a fixed value: it's a three-dimensional area in which details are perceived as sharp, while outside of it they gradually become blurred.
The depth of field, meaning the portion of the scene perceived as sharp, increases as we stop down the aperture. The distance at which we obtain all the elements in the frame in focus is also known as the hyperfocal distance. There are tables and complex formulas to calculate it, but let's keep it easy with a practical example.
If we take a photo with a 16 mm wide-angle lens and stop down to f/11, anything more than one meter away from the lens will be perfectly in focus, all the way to infinity. If we're shooting a landscape from a tripod, we're at the hyperfocal distance, because every subject will surely be at least one meter from the lens.
The exact value of the hyperfocal distance depends mainly on the focal length and the aperture. In practice, the hyperfocal distance is truly useful only with wide-angle lenses: with telephoto lenses, the required distance becomes so large that it's impossible to stop down enough to keep both the foreground and the background in focus.
On the other hand, if we want a subject in focus that stands out against a blurred background, it's better to set the largest possible aperture and choose a telephoto or a normal lens. This minimizes the depth of field, creating a nice blur. However, you must pay close attention to focusing, because if the subject moves forward even by a few centimeters, it may end up out of focus.
Now let's look at the same photograph with two different apertures. The first one is taken to achieve the maximum possible depth of field, setting the aperture to f/16. You can notice that all elements are in focus, with only a slight blur on the side of the last element.
The second one was taken at f/1.8; the depth of field is very shallow, and the blur is the main subject. Keep in mind that portrait photos taken with such extreme aperture values may end up with the eyes in focus while the tip of the nose is blurred.
Does all this also apply when shooting with our smartphone? In reality, no. Phone cameras have very small sensors, and in most cases they don't have an adjustable aperture, except for a few rare models.
Such a small sensor has virtually no focusing issues and almost always shoots at hyperfocal distance. So how do they achieve a blurred background? Simple: smartphones use artificial intelligence to simulate bokeh in post-processing.
Sharpness
When working at its maximum aperture, a photographic lens operates close to the limits of its design. The photo is less sharp, especially near the borders. To achieve the best possible sharpness, it's generally advisable to stop down the aperture by at least two or three f‑stops.
The lack of sharpness is more noticeable with budget lenses and zoom lenses, which tend to be less sharp at their widest aperture compared to prime lenses. One exception to this rule is ultra-fast primes, which can be a bit too soft when used at their maximum aperture.
One way to find the optimal aperture value for your specific lens is to mount the camera on a tripod, take one photo for each aperture setting, and carefully analyze the results, paying particular attention to the edges. Alternatively, you can check specialized websites where someone has already tested the lens for you.
Below you can see a test made with our Canon 50mm f/1.4. The sample is a crop of the upper-right corner of the frame. The left side was taken at f/1.4, while the one on the right was taken at f/5.6.
Once you've found the optimal value, we recommend memorizing it. At an event, we once saw a professional photographer with a sticker reading "f/5.6" stuck on his lens... besides looking awful, it wasn't very professional either. Can you imagine a race car with a sticker on the dashboard saying, "Remember to shift at 6,000 rpm"?
Diffraction
The maximum aperture isn't the only cause of loss of sharpness. A similar issue can also occur when shooting with an aperture that is too small. In this case, the light rays are forced to pass through a tiny opening, causing a phenomenon known as diffraction.
It could be explained using complex terms from the bending of light as it passes through a very small hole, but let's keep it simple and say that there's a loss of sharpness when using extremely small aperture values.
To notice this loss of sharpness, a high resolution is required; a camera with hundreds of megapixels is unforgiving, while in an image taken at just a few megapixels, it's far less likely that diffraction-related softness will be noticeable.
In light of these facts, we can very roughly summarize that the ideal aperture value for achieving the sharpest possible image with a DSLR generally falls between f/5.6 and f/11, except in the case of cheap zoom lenses mounted on high-megapixel cameras, where we recommend shooting at f/8.
Diffraction can also be used to create artistic effects, the "stars" around light sources. Let's see what they are.
Starbursts on Lights
When we shoot very bright light sources with a closed-down aperture, small "stars" form around the lights, also known as starbursts. This is an artistic effect that can dramatically enhance a photograph, especially in night scenes.
When the aperture is stopped down significantly, light passes through the lens opening; being made of blades, it's never perfectly round. Each edge of the blades generates diffraction and contributes to the final shape of the starburst. The number of rays is determined by the construction of the lens. For example, an aperture with 8 blades produces 16 rays.
Since these aberrations tend to appear in a fairly predictable way, they can usually be removed in post-production with an automatic filter, without the need for manual adjustments. Whenever possible, we recommend using the software provided by the camera manufacturer to correct this issue, as long as the lens is from the same brand, because no one knows their optics better than the manufacturer.
Chromatic Aberration
The last optical issue that can be reduced by slightly stopping down the aperture is chromatic aberration. These are small colored fringes, usually blue or purple, that appear in areas where very bright light meets dark subjects.
They are known as purple fringing, and they are more noticeable when shooting with cheaper lenses at maximum aperture. Below is an example of chromatic aberration, highlighted by the red arrow.
Since these aberrations tend to appear in a fairly predictable way, most photo-editing software includes an automatic filter to correct the defect without the need for manual adjustments.
Conclusion
We hope you liked our guide about aperture in photography. Before saying goodbye, we would suggest you read all the articles we wrote about photographic technique.
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