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Understanding aperture: What is it and how does it affect image quality?
I’m sure many of you use your smartphone as your primary shooter. Even those who own a DSLR or mirrorless camera can’t argue with the convenience of having a good pocket cam. Truth be told, flagship smartphone camera quality is more than good enough for capturing day-to-day moments. To make matters more exciting, manufacturers show no signs of slowing down improvements. In addition to the dual, triple, and quad-camera trends, the latest smartphones are also pushing the envelope in wider camera apertures. But what is aperture, exactly?
While numbers are nice for spec sheets, does this aperture number really make pictures look better? That’s exactly what we aim to answer.
Before reading: Make sure you learn these essential photography terms
It’s all about capturing light efficiently
Photography is all about getting the right amount of light exposure. A pretty good rule of thumb for judging camera quality is figuring out how good it is at capturing light. A top-notch sensor paired with a quality lens is the sought-after combination. The same applies to smartphones, albeit with some limitations.
The small smartphone form factor means lenses and sensors are smaller. Therefore, less light reaches them. This has an impact on final image quality. We’ve seen smartphone manufacturers use larger 1.2µm to 1.55 µm sensor pixel sizes to combat this, with excellent results. The other half of the light capture equation is how much light makes it through the lens to reach these pixels. This is where aperture comes in.
What is aperture? Learning about f-stops
Okay, so what is aperture? Aperture is the opening size through which light enters a camera. Have you ever seen the blades that open and close on a traditional camera lens? That hole in the middle is the aperture. You can appreciate it better on this post’s main image. In turn, aperture size refers to how opened or closed that hole is.
The aperture parameter is measured in f-stops, a ratio of the focal length divided by the opening size. So the smaller the f-stop, the wider the opening, and therefore more light can reach the sensor, resulting in better low light pictures. As you narrow the aperture by a full “stop” — or power of the square root of 2 (ƒ/2 to ƒ/2.8, ƒ/4 to ƒ/5.8, etc.)— you’ll be halving the light gathering area.
The smaller the f-stop, the wider the opening and therefore more light can reach the sensor. This means better low light performance.
A wider aperture allows you to reduce the shutter speed required for a set amount of light capture. The result is diminished blurring in action shots or with shaky hands, making its use with OIS even more powerful. If you’re looking to capture that perfect still frame, a wider aperture will help you achieve that. In addition, a wider aperture makes it possible to lower the ISO, as it can capture more light. This means your images will come out with less noise.
Smartphone sensors are very close to the lens; much closer than in DSLR cameras. A camera’s focal point is the distance between the light convergences in the lens and the sensor. Smartphone cameras have shorter focal lengths than a DSLR. We know that the aperture equation is the focal length divided by the opening size. This helps explain why phone cameras have a wider aperture than most DSLR lenses, even though they aren’t necessarily better at light capture.
Speaking of camera lenses, photography enthusiasts will often associate wider apertures with a shallower depth of field, allowing for nice, soft bokeh. However, with smartphones, we’re usually stuck with a fixed aperture, a smaller image sensor positioned close up to the lens, and a reasonably wide field of view. A phone camera’s depth of field will never be that shallow.
Smartphone sensors are positioned much closer to the lens than in a DSLR, hence why today's smartphones have a wider aperture ratio even though the opening is smaller.
An f/2.2 smartphone camera actually only provides a depth of field equivalent to an f/13 or f/14 aperture on a full-frame camera. It only produces a small amount of blur. Modern phones with enhanced bokeh effects rely on software for a more dramatic look. We usually see the effect artificially added in shooting modes like Portrait Mode.
While a wide aperture is no guarantee of camera quality, a smaller f-stop value allows more light into the sensor, which equates to better images. You should always consider this value in conjunction with pixel size, as larger pixels won’t necessarily require as wide an aperture to capture enough light to expose an image correctly. However, small pixels and a small aperture signify that low light performance will be an issue.
The lens is an equally important but often neglected component in all smartphone camera stacks. Just like everything else, these vary considerably in quality. After all, a dirty lens takes poor pictures, and lens glass with poor clarity or transparency will do the same. This will reduce the amount of light reaching the sensor, and therefore, reduce image quality.
Smartphones using very wide apertures require extra special attention to lens design. A lousy design could worsen aberration distortion and lens-flare effects that have haunted some devices. Consider it like this, it’s trickier to focus light accurately when it’s coming through a wider hole, so these lenses have to be more meticulously crafted. Aberration distortion covers a range of issues that appear when a lens cannot perfectly focus a point of light. Phones with wide aperture cameras are less focused on a specific part of the scene than one with a more closed-in aperture, and therefore more prone to problems.
Aberration distortion comes in a variety of effects. These include spherical aberration (reduced clarity and sharpness), coma (blurring or tailing), field curvature (loss of focus at edges), distortion (image convexing or concaving), and chromatic aberration (unfocused colors and split white light), among others. See some examples below (source).
Camera lenses are built from multiple “correcting groups” designed to focus the light properly and reduce aberrations. Cheaper lenses tend to feature fewer groups and are therefore more prone to issues. Lens materials also play an essential part here, with higher quality glass and multiple coatings offering better correction and less distortion.
Lens quality is harder to judge from numbers or a spec sheet. Many phone manufacturers don’t mention it at all. Unfortunately, this matter complicates talk about aperture and pixel sizes, as cheapening out on the lenses could render these developments useless. Fortunately, renowned companies like ZEISS, Leica, and others have joined the smartphone market. These companies have more expertise in the field.
To conclude, lens quality is just as important as the other factors we’ve discussed, if not more so. A poor lens can undo sound engineering done elsewhere. Unfortunately, it’s challenging to understand and almost impossible to appreciate without testing the camera out.
Putting it all together
As you’ve probably figured out, aperture isn’t the be-all-end-all of a good smartphone camera setup. Just like most other areas of photography, it’s not a very useful number to go by when making a purchasing decision. It’s not an indicator of quality on its own. However, it offers several advantages, including the possibility of better low light capture and faster shutter speeds.
The small size of the sensors means that you’re never going to see much bokeh, except for very close-up shots. These days, most phone cameras that offer bokeh effects are doing so via software, sometimes using data from a secondary camera. If you ask us, we think other specs and features could help more. Wide-angle and zoom cameras are more exciting options if you’re looking for unique shots.
With that said, small smartphone sensors are susceptible to low light. A wider aperture, combined with an excellent lens and sensor, should theoretically help to reduce noise and produce better-looking pictures.
Aperture is but one of the many concepts a proficient photographer needs to learn. We’ve put together more content for you to learn. Here are some important ones you should check out.