Focal Length Explained

What do those numbers on our lenses mean? How does sensor size affect focal length? Do lenses designed for different sensor sizes mean they have alternative focal lengths?

Whilst answering these questions won’t have any great impact on the photographs you head out to shoot today, the knowledge can alleviate some of the confusion that arises when the topic of focal length comes up as, often when it comes to focal length, the conversation gets blurry when sensor sizes get discussed.

In reality focal length is a really simple concept to understand. It is a measured point at which light converges in front of the sensor….simple right? Well perhaps it sounds far more complicated than it needs to be so let me explain a little more. Focal length is a property of a lens, not a camera body. And this explains why focal length is not changeable dependant on the sensor size within any given camera body. In laymans terms, when lenses are designed, the focus is set to infinity and the lens optics are set to converge light rays at a set point in front of the sensor. This point is measured in mm: 16mm, 40mm, 135mm etc. This is then the focal length of the lens. So it doesn’t matter the size of the sensor as the measurement is in front of the sensor.

As an aside, I’ve often wondered about the sensor equivilence conversation and whether it has any use whatsoever. It’s always seemed to me to be little more than a marketing ploy designed to subtly remind us all that ‘Full Frame’ sensors are the standard and any other sensor needs to be ‘made right’…..but I digress.

Lenses are always the focal length designated on the side of the lens. Whichever camera mount or sensor size those lenses are designed for, that remains constant. What actually changes with sensor size is magnification, how big the image is magnified on the camera sensor, and angle of view, how wide or narrow the view is through the lens.

If this makes no sense to you whatsoever, not to worry. In this short video tutorial, I’ll show you, in as simple a way I could come up with, exactly how it all works. If, like me, you enjoy a good visualisation of a concept in graph form, then this one is for you.