Understanding Focal Length On Smartphone Cameras

If you have ever been to a site like GSM Arena and looked up the specs of smartphone cameras, you might have come across a measurement such as 26mm, 52mm, or 12mm next to the lens specifications. This measurement is of the lens’s focal length, which plays a big part in the final image that is produced by your camera.

In this article, we’re going to look at what part focal length plays in photography and how it impacts your photos.

But before we can look into focal length on smartphone cameras, it’s important to know what it is and how it works in traditional photography, first. This background understanding will help you understand how it works on smartphones.

What does focal length mean?

Measured in millimetres (mm), the term ‘focal length’ refers to the distance between the camera’s image sensor and where light converges and focuses the image inside the lens.

Let’s break that down a bit…

When light travels, it travels in parallel lines. If a single convex lens is put in front of it, the parallel light rays will converge and bend towards a single point– the focal point.

In a standard camera lens, however, things aren’t that simple. Instead of passing through just one lens, light has to pass through an array of concave and convex lenses that converge and diverge it before it reaches the focal point.

When the image is in focus, the distance from where the light converges in the lens to the image sensor is the focal length.

Source: expertphotography.com

So, if a lens on a DSLR camera is said to have a 28mm focal length, that means that at its sharpest focus, the light converges in the lens 28 millimetres above the sensor. If the lens has a 50mm focal length, the light converges and focuses the image sharply 50 millimetres away from the sensor.

Why does the focal length matter?

The focal length of a lens is important because from it you are able to know the angle-of-view (also known as field-of-view) and the magnification of the lens.

The longer the focal length, the less of the scene you’ll see, and the more magnified your subject will appear. This is perfect for portrait photography, where the attention is on the person, not the scenery.

A short focal length gives you a wide angle-of-view that works well for capturing shots of landscapes.

So, having looked at what focal length is and how it’s measured in traditional, dedicated cameras and lenses, it’s hard to ignore that something doesn’t quite add up.

If the focal length is a measurement of the distance between the sensor and optic centre of the lens, then it’s impossible that a smartphone camera can have a 24mm lens, for example.

This begs the question…

How is focal length different in smartphones?

Because space is very valuable real estate in smartphones, mobile cameras are very small and have tiny sensors compared to the likes of a full-frame DSLR camera.

As a result, the actual distance between where the image is focused inside the lens to where it’s captured on the sensor is very short. In fact, a smartphone camera marketed as having a 26mm lens actually has a focal length of 4.25mm

Quite frankly, 4.25mm means nothing in true photography terms. The focal length is calculated according to the equivalent of a 35mm full-frame sensor.

For this reason, you’ll find that the focal length of the phone’s lens or lenses is expressed in the equivalent of a 35mm-format sensor instead of the smartphone’s own.

What does 35mm equivalent mean?

The 35mm focal length equivalent of a lens gives you an idea of the focal length of the lens you’d need to use if you were to capture the same image using a camera with a 35mm full-format sensor.

So, when looking at the specifications of a smartphone camera and see that it has a camera with a 24mm lens, it basically means you’d need a 24mm lens on a full-frame DSLR camera, for example, to get the same shot with the same angle-of-view as you got with your phone camera.

Knowing the 35mm equivalent is important particularly to photographers who change lenses between cameras with different size sensors because it affects the focal length.

For mobile photographers, knowing the 35mm equivalent is helpful mostly in giving you an idea of how wide or narrow the angle-of-view of the lens is.

Focal length and angle-of-view

As mentioned earlier, the focal length is closely linked to what is known as the angle-of-view. This basically refers to how much of the scene is seen through the lens. This measurement is given in degrees.

If your lens has a short focal length, that means you will have a wide angle-of-view and therefore see more of the scene. Some ultra-wide-angle lenses have an angle of view as wide as 180 degrees. However, this introduces distortion in the picture known as the fisheye effect.

Source: manfrottoimaginemore.com

A long focal length means that you will have a reduced field of view and your subject will appear magnified. In other words, if a lens with a wide angle-of-view gives you a shot of an entire landscape, a lens with a long focal length will give you a narrower angle-of-view with less of the landscape in the frame and more of the buildings. See the example below.

The longer the focal length, the more magnification you get. When you zoom out, you have a wider angle-of-view and a shorter focal length.

What’s the typical focal length of a smartphone lens?

This is difficult to answer because different devices have different lenses. Also, smartphones with more than one camera tend to have lenses with different focal lengths.

The table below shows the typical focal length of various smartphone cameras.

These measurements are approximate and only for reference.

Single-lens smartphone cameras commonly have a wide-angle lens of roughly between 22mm and 30mm equivalent.

Smartphones with more than one camera can have any combination of a wide-angle main camera with an ultra-wide-angle second camera, or a telephoto lens, or both. Ultra-wide-angle lenses are roughly in the range of 12mm to 18mm equivalent.

A lot of smartphones have “telephoto” lenses that are double the focal length of the wide-angle lens. It is not uncommon these days, however, to find smartphones with cameras that have a telephoto lens with a focal length that is said to be more than double that of the wide-angle lens.

In most of the cases, a smartphone telephoto camera that has more than 2x optical zoom is known as a periscope camera or periscope zoom lens.

Although it is indeed a telephoto camera, I’ve listed it separately in the table above because of how differently it works compared to a traditional telephoto lens on a mobile phone. You can learn more about periscope cameras here.

The longer the focal length of the telephoto lens, the more the phone camera can zoom in optically. This means better picture quality than when using digital zoom.

How does attaching an external lens affect a smartphone camera’s focal length?

You get different types of add-on smartphone camera lenses: telephoto, macro, ultrawide-angle, and others. These are similar to some of the lenses on phones with multiple cameras.

Similarly, these different types of add-on lenses have different focal lengths. They are also within the same range of focal length and angle-of-view as the mobile camera lenses shown in the table above.

Because the primary rear camera is usually the one that has the best mobile camera specs on a particular mobile phone, most external smartphone camera lenses are designed to be attached to the main camera in order to take advantage of its picture quality.

The primary camera of most smartphones has a wide-angle lens with a focal length of anywhere between 24mm and 27mm. From this camera, you can get pretty wide shots that show a decent amount of the scene.

Attaching an add-on lens to that camera will give you a different angle-of-view that matches the external lens’s focal length.

For example, if you attach a 58mm telephoto lens (which has a narrow angle-of-view) to a 26mm wide-angle camera on a smartphone, the mobile camera will not capture a wide-angle shot but rather a zoomed-in shot that matches the telephoto lens’s focal length.

If you’re going to use add-on lenses when taking photos, you need to be careful. Cheap lenses may be easy on the pocket but they often result in poor quality images despite the optical abilities of the phone’s camera.

Two brands I highly recommend for add-on smartphone lenses are Moment and Olloclip. They produce high-quality lenses specifically for mobile photography and even mobile filmmaking. They’re not the only options, of course. There are other options to suit various phone models and budgets.

Conclusion

Attaching an external lens to your smartphone camera will not improve the quality of your photos by increasing the resolution (megapixels) nor will it give your shots a shallow depth-of-field (blurry background).

Add-on lenses only change the angle-of-view of the camera and nothing more. If you want to take photos that are better than just nice with your phone, then you need to learn more about your mobile camera and how to get the most out of it despite its limitations.

This article first appeared on my other blog thesmartphonephotographer.com. Check it out!