A Beginner's Guide to What the Focal Length of a Lens Really Is

We all have a fairly intuitive grasp of what focal length is in relation to taking photos: higher numbers mean more zoom. But what actually is the focal length of lens? This great video will show you what it is and what consequences it has on the lenses you choose and the photos you take.

Coming to you from Apalapse, this helpful video explores the concept of focal length and what it means for your photography. In terms of physics, it's the distance it takes an optical system to bring initially collimated rays of light to a point of convergence. Thus, you might think of wide angle lenses as "more powerful" in the sense that they bend the light rays more severely to make them converge over a shorter distance. However, you may also notice that some modern lenses are actually shorter than their focal length. They aren't breaking the laws of physics; rather, through the miracle of multi-element lenses and some clever design (specifically, a telephoto group), they effectively lengthen the light's path by reducing the angle of convergence after the rays have already converged partially. It's a pretty neat trick. Check out the video above for more on the topic. 

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Dave F's picture

Definitely a good reference for beginners, although this is one of those times I wish the term "field of view" was used more often to replace "focal length" (or "effective focal length") for parts of the explanation.

From the video:

"...switching camera bodies that have different sensor sizes will change what the picture will look like. For example if I have a 50mm lens on a full frame camera, it's effective focal length will be 50mm. But if I swap it to a crop sensor camera with a crop factor of 1.5, the effective focal length suddenly becomes 75mm."

The problem is that focal length affects other things as well, like depth of field. While putting a 50mm lens on a crop sensor camera may give you the field of view equivalent to 75mm on a full frame camera, you still won't get exactly the same picture, only the same field of view. When people say "effective focal length", I think it's generally assumed that most people know what they're talking about, but it makes it harder to explain certain concepts to people who are still learning.

I think there's a similar thing going on right now between Thunderbolt 3, USB 3 and USB-C. The number of times I see people use USB3 and USB-C interchangeably makes me cringe, especially when I see it in marketing materials and on product descriptions for online retailers (haha forget discussion forums, they're hopeless :p ).

Jon Rolfson's picture

Useful knowledge, particularly given that a lens' actual aperture is defined by and expressed as the diameter of the aperture as a ratio of the focal length. That is to say that f/1 of a 50mm lens would be an aperture with a diameter of 50mm, f/2 of a 50mm lens would describe an aperture of with a diameter of 25mm, f/4 of that 50mm lens would describe an aperture of with a diameter of 12.7mm.