What is focal length? Technically, in terms of camera lenses, it is a complex measurement that you will need to study optical engineering to fully understand. Thankfully, you won’t have to go back to university because there’s a quicker way to get the gist of it. Basically, the focal length is the distance in millimetres from the optical centre (or nodal point) of your lens to the image plane inside the camera when the lens is focused on infinity.
Nifty fifty, ultra-wide, super-zoom. They are terms you may well have heard of in your photographic lives. But what do these terms relate to?
In fact, they are terms used to represent one of the most important elements we have in photography, lens focal length. Focal length is a term given to lenses and defines how wide or narrow our scenes field of view will be.
If you are new to photography you might be happy just shooting away to build your confidence. But eventually, you are going to need to know why your lenses have a number defined in millimetres and what the relevance of that figure is to your photography.
What Is Lens Focal Length?
Put simply the focal length of a lens is the distance from the optical centre of the lens to the film plane or sensor when the lens is focussed on infinity.
Although you might hear that a 50mm lens becomes a 75mm lens when put on a cropped (APS-C) sensor, this is not entirely true. The lens is still a 50mm but the field of view of that lens has changed. The field of view is the angle between the the left/right extremes of the lens.
When you use a 50mm lens on a full frame camera, that lens projects a circle of light onto the sensor. The extremities of that circle will lie just outside the edges of the sensor.
Use that same lens on a cropped sensor and the circle will be exactly the same. However, the sensor is smaller which means the field of view is narrower. So your 50mm lens on a cropped sensor will give you the equivalent field of view of a 75mm on a full frame sensor.
The sensor is actually using a significantly smaller part of the lenses circle.
Full Frame v APS Sensor
We tend to associate lens focal length with full frame 35mm cameras. This is partly from in the days of film, 35mm was by far the most common format and secondly because a standard lens is regarded to be 50mm. The field of view of a 50mm lens is very similar to our eye’s own field of view.
There are now many lenses that are designed specifically for cropped sensor cameras. They still maintain the same numbering system for focal length but are designed so that the circle of light they project covers just the area of the cropped sensor, not a full 35mm sized sensor.
This tends to make them lighter and cheaper.