> 4 minute read

Focal Lengths Demystified

The viewing angle we see though our camera’s viewfinder is not just down to the lens you’re using, but also the size of the sensor. All X Series cameras use the same sensor size, so no problem there, but what if you want to compare lenses with photographers using other systems? The answer is equivalence – read on to find out more.

Perhaps the most distinguishing characteristic of any lens is focal length. Technically, focal length is the distance from the lens to the sensor, when the lens is focused on a subject at infinity. This measurement – quoted in millimetres – tell us the type of view a lens will give us. Wide-angle lenses have short focal lengths, such as 16mm or 23mm, and give a wide field of view. Telephoto lenses, like a 90mm or 200mm, have longer focal lengths and provide a narrower field of view. Focal lengths of around 35mm are known as ‘standard’ on X Series cameras, since they offer a view similar to that of the human eye.

Understanding Equivalence

The common standard adopted since the dawn of digital photography is the size of 35mm film – chosen originally because this is the format photographers were migrating from when digital photography came about. Many digital cameras featured sensors that were smaller than the dimensions of 35mm film, so photographers wanted to be able to get a feel for how their lenses would work on these cameras.

The system works like this: 35mm film is 1.5x larger than the sensor in a FUJIFILM X Series camera. So by multiplying the focal length of any X Series lens by 1.5, we can calculate the focal length we’d need to get the same angle of view on 35mm film.

Each film format has its own conversion factor. Here’s a list of some of the most common:

So, no matter what camera type you own, you can confidently compare lenses between systems by multiplying them up to be as if they were on a 35mm film camera using the conversion factor.

While there are many different standards in photography, equivalence is a great way of making sure we are all talking the same language when it comes to lenses.

A lens’s focal length affects more than its angle of view, however. The way in which we use telephoto lenses makes them appear to compress perspective, so objects seem closer together than when we photograph them with wide-angle lenses. Their higher magnification also gives the impression that the area of sharpness in an image (depth-of-field) is shallower than when using standard or wide-angle lenses.

However, focal length is not the only factor in determining viewing angle or the appearance of perspective and depth-of-field. The size of your camera’s sensor also plays a big part.

Why (Sensor) Size Matters!

When you make an image, your lens projects a circular view of the scene in front of you into the camera. The rectangular sensor picks out part of this image circle and turns it into the finished JPEG picture you see on the rear LCD. But a larger sensor will take in more of the lens’s image circle, giving us a wider angle of view. Likewise, a smaller sensor will use less of the circle, giving us a tighter, more telephoto viewing angle.

The same focal length lens can give a wide-angle, standard, or telephoto view depending on the size of the sensor it’s working with. Or, in other words, to get the same angle of view with cameras that have different sizes of sensor, we need to use lenses of different focal lengths.

This makes it hard to compare lenses between, say, the FUJIFILM X Series and cameras with larger or smaller sensors. What we know as a standard lens (35mm) will act as a wide-angle lens on a medium format camera (which has a much larger sensor) and a telephoto lens on a smartphone (which has a tiny sensor).

Your Next Steps

• CHALLENGE Try making an image of the same scene from two different focal lengths and see how they compare. Post the results to social media with the hashtag #learnwithfujifilm and #focallengths. You can also submit your work here for a chance to be featured on our social media channels.
• WATCH Check out our video below to learn more about focal lengths.