Understanding Focal Length Definition of a Camera Lens

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Welcome to our Easy to Understand Guide to Understanding Focal Length Definition in Camera Lenses

focal length diagram different lenses

What is Focal Length?

In previous articles, we explained 3 critical principles of photography: Aperture, ISO, and Shutter speed , make sure to read those articles if you haven’t done so yet, to fully understand how these key principles of photography affect your images.  In this article, we will take a look at Focal Length. By the time you finish this article you will have a full grasp on exactly what it means and how to manipulate it like a master to improve your photography.

So what is the definition focal length?

Focal length usually represented in millimeters and is a basic description of a photographic lens. The measurement is not the length of the lens of itself, but actually a calculation of the optical distance from where all light rays converge inside the lens to the image sensor of the camera.

Below you can see a visual representation of the Focal Length Definition

Lens Focal Length-diagram

Let’s take the Focal Length Definition and explain it in plain English

The focal length of a lens is determined when the lens is focused at infinity. The focal length tells us the angle of view—how much of the scene will be captured—and the magnification—how large individual elements will be.

  • The longer the focal length, the narrower the angle of view and the higher the magnification.
  • The shorter the focal length, the wider the angle of view and the lower the magnification.

Here is a visual example of varying focal lengths and the images they produce:

Image Examples with Different Focal Lengths
Here you can see examples of varying focal lengths from Nikon.com

So as you can see above the further or longer the focal length used the more narrow the angle of view and the more magnified the image will be.

  • So Use shorter focal length to capture a wider angle of view & and Longer focal length to capture a more narrow angle of view with greater maginfication

A lens with a focal length of 24 millimeters, for example, would have light range coverage 24 millimeters above the image sensor. The focal length number tells us the angle of view, how much of the scene will be captured, and the magnification, how large a subject will appear.

The lower the number, the wider the field of view, and lower the magnification increasing the amount of the scene that our camera can see. The larger the number, the narrower the field of view and greater the magnification decreasing the amount of the scene our camera can see.

It’s also worth noting that often times photographers will refer to larger focal lengths as longer and smaller focal lengths as shorter.

While the lens you attach to your camera comes with the focal length depending on the type of camera you attach to it, its focal length can actually vary.

Back in the days of film, the 35-millimeter film was the standard sensor size for all types of photography. Once film cameras left the scene, digital cameras replaced them with many different sized sensors. The majority of them smaller. 50-millimeter lens regarded as a standard focal length for film became a short telephoto on the new smaller sensor cameras. This is due to something called, crop factor. Full frame cameras do not have any crop factor since they operate on a standard 35-millimeter format used in the days of film. APS-C sensors, however, have a crop factor of 1.5 for Nikon or 1.6 for Cannon. What this means is any lens attached to an APS-C camera will need to have its focal length multiplied by either 1.5 or 1.6 to get its effective focal length. The actual focal length of the lens remains unchanged but switching camera bodies that have different sensor sizes will change what the picture will look like.

For example, if you have a 50-millimeter lens on a full frame camera its effective focal length will be 50 millimeters, but if I swap it to a crop sensor camera with a crop factor of 1.5 the effective focal length suddenly becomes 75 millimeters. There are other camera form factors as well, but full frame APSC cameras are most common. When considering a lens, there are a number of categories defined by ranges of focal lengths.

  • The widest lens are the fisheye or ultra wide lens. These lens have focal ranges from 8 to 24 millimeters. They are limited in use and typically used for landscape or architecture. They create such a wide angle of view that images can sometimes appear distorted. They are not suitable for all portraits since they make facial features look distorted.Example of Fish Eye Lens anything under 16MM
  • Next comes, regular wide angle lenses with focal lengths typically between 24 millimeters to 35 millimeters. This is where distortion stops appearing unnatural.
  • Standard lens with focal lengths between 35 millimeters to 70 millimeters produce images that most closely reproduce what our eyes see. They are great walkaround lens and very popular.
  • Telephoto lenses typically start around 70 millimeters and end around 300 millimeters. Portrait lenses fall in this range. As you increase focal length telephoto compression will cause foreground and background objects to appear close together.

Extreme telephoto lens round out the bunch with focal lengths exceeding 300 millimeters. These lenses are used for wildlife and situations where extreme magnification is required.

Within these categories lie prime and zoom lens. Prime lenses are lenses that have a fixed focal length. They are often simpler lens making them easier to manufacture and in return costs less and are usually are of higher quality than the zoom.  Zoom lenses, on the other hand, cover a range of focal lengths and because of their versatility allow you to carry, say, one lens instead of two or three to cover the same focal range. The only drawbacks are the increased cost and slightly reduced quality of manufacturing something so complex.


Focal length is a simple concept, but an important one. By the end of this article, you will have learned the principles of focal length and the various categories of lenses that together will help you make an informed decision on which lens to purchase, and how focal length will affect your picture.

Reminder if you want to read about other important photography concepts – read our guides on Aperture, ISO, and Shutter speed . 

If you want even more information on the different types of camera lenses then you have to read our epic post here.

or our Guide to the best lens to learn photography post.

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