Photography: What Is This 'Focal Length' Thing All About?

By Haje Jan Kamps on at

One of the things you hear about a lot when people are talking about lenses is the focal length of a lens. But what is it? And why does it matter?

 

 

What Is Focal Length?

If you've ever wondered what this distance was, you might have looked it up on Wikipedia or similar, and you'll have been met with the extremely unhelpful "When a photographic lens is set to infinity, its rear nodal point is separated from the sensor or film, at the focal plane, by the lens's focal length. Objects far away from the camera then produce sharp images on the sensor or film, which is also at the image plane."

Did you follow that? Yeah, me neither, and I do this stuff for a living. Let's simplify this hugely... When the light travels through your camera, it's flipped upside-down:

focal_1.png

As an aside, this flipping-upside-down is why a SLR camera needs to have a Pentaprism (the bulge at the top of your camera) -- this flips the image rightside-up again:

focal_2.png

Anyway, I digress. As you can see from the drawing, when two lines intersect, they have to cross at some point. In optics, this crossing point is known as a "nodal point". The distance from this point to your imaging sensor is your 'focal length':

focal_3.png

 

Why Is Focal Length Important?

The Focal length of a lens describes how wide its field of view is, and this, in turn, is directly proportional to how 'wide' a lens is. We'll talk more about 'wide' and 'telefocus' in just a moment, but let's take a look at focal lengths first.

Imagine if the place the rays cross is close to the imaging sensor. This would make an object appear small in the photograph:

focal_4.png

Now, imagine that the place the rays cross is further away from the imaging sensor. This would make the object appear much bigger:

focal_5.png

As you can see, a low focal length creates a wide angle of view, which is why lenses with low focal lengths are known as "wide angle" lenses. The opposite is also true: A long focal length creates a narrow angle of view; these lenses are known as "telephoto" lenses.

 

Zoom and Non-Zoom Lenses

We've talked about focal lengths, and I've hinted at the fact that some lenses can have more than one focal length. The very first lenses all had only a single focal length, but then, the clever engineers started thinking "wouldn't it be nifty if a single lens could have several focal lengths?" In other words: Wouldn't it be fantastic if you could take photos of big things that are close to you (such as buildings and landscapes), or perhaps small things that are far away (birds, or people running on a football field) all with the same lens?

The science boffins came up with a way of changing the configuration of photographic lenses so it became possible to move the nodal point (as you'll remember, that's the point where the image flips upside down inside the lens) closer or further away from the imaging sensor. By doing that, they created a variable focal length lens, or a 'zoom' lens, as we like to call it today:

focal_zoom_v2.png

When zoom lenses were invented, they needed another word for a non-zoom lens - so they became known as 'prime' lenses.

Learn more - This article is an except from Haje's epic tome covering just about everything you want to know about lenses.

Haje Jan Kamps is a prolific photography blogger who has written a small stack of books about photography. He also developed the Triggertrap camera trigger and has been known to travel the world a bit. If you’re of the tweeting kind, try him on @Photocritic!

Top Image Credit: Steve Huff