In photography, we use focal length as a primary descriptor of lenses. It’s the measure (in millimeters) of the distance between the rear nodal point and the focal point of the lens, while the lens is focused to infinity. Yep, that’s quite a mouthful, so let’s break it down.
How Focal Length Works
The purpose of a photographic lens is to take parallel rays of light and converge them into a single point of focus so they can be recorded, either on a piece of film or, far more commonly, with a digital sensor. The point where the lens causes the light rays to converge is called the focal point. In the image at the top of the article, the parallel rays of light entering the lens are represented by the two red arrows. The focal point at which they converge after passing through the lens is marked with an “F.”
Without a lens to focus the light for your camera, all you get is a blurry mess. Here’s a selfie I took without a lens on my camera as an example. Aren’t I pretty?
There isn’t just one set lens size or shape that will converge rays of light. Any convex lens (that’s one that curves outwards) will work, but the focal point will be different. The focal length of a convex lens is the distance between the center of the lens and the focal point.
Unfortunately, the lenses we use for photography are a lot more complex than a single convex lens. They generally have multiple different lens elements that work together to converge light with as few optical aberrations as possible. This means there isn’t really a true center we can measure from. Instead, focal length is measured from the rear nodal point—which, along with the focal point, is one of the cardinal points in Gaussian optics—to the focal point while the lens is focused at infinity.
What the Focal Length Tells Us About a Lens
If all this still sounds like it’s a bit complex, don’t worry. You really don’t need a very deep understanding of how focal length is measured to take good photos; you just need to know what it means for your photos.
The reason we use focal length to describe lenses is because it tells us one very important thing: what the field of view of that lens will be. And since the sensor remains the same size regardless of what lens you use, the field of view tells us how much a lens can magnify distant subjects.
Photographic lenses generally have a focal length of between 14mm and 600mm, although there some expensive exceptions that have shorter or longer focal lengths. The shorter the focal length, the wider the field of view. The longer the focal length, the narrower the field of view.
Here’s a photo taken with a focal length of 18mm on my Canon 650D.
And here’s a photo taken from the exact same spot a few seconds later with a focal length of 135mm.
As you can see, the 135mm photo has a much narrower field and so it appears like I’ve zoomed in on far away objects.
The human eye has a focal length of somewhere between 40mm and 58mm, with 50mm being the usual compromise. This is referred to as the “normal” focal length. It’s hard to measure because a camera lens is not a perfect analog of our eyes. Any lens with a focal length shorter than that normal focal length is a wide angle lens and things in the image will appear smaller than they look to your eyes. Any lens with a focal length longer than the normal focal length is a telephoto lens and things in the image will appear larger.
What Focal Length Should You Use?
There is a place for lenses of every focal length in photography and choosing the right one for the image you’re trying to take is often a very important decision.
If you want to take landscape photos, for example, you are much more likely to want a wide angle lens than if you’re shooting sports, in which case you’ll want a telephoto lens to get close to the action. Normal lenses are great for casual photography and portraits.
Focal length is the most important measurement of a photographic lens. It, along with the aperture, are what tell us how a lens will affect our images.
Image Credits: Henrik via Wikipedia.