Understanding Lens Focal Length: A Beginners Guide

lens focal length
Image by tookapic

Nifty fifty, ultra-wide, super-zoom. They are terms you may well have heard of in your photographic lives. But what do these terms relate to?

In fact, they are terms used to represent one of the most important elements we have in photography, lens focal length. Focal length is a term given to lenses and defines how wide or narrow our scenes field of view will be.

If you are new to photography you might be happy just shooting away to build your confidence. But eventually, you are going to need to know why your lenses have a number defined in millimetres and what the relevance of that figure is to your photography.


What exactly is a Nifty Fifty? By Chris Hsia

What Is Lens Focal Length?

Put simply the focal length of a lens is the distance from the optical centre of the lens to the film plane or sensor when the lens is focussed on infinity.

Although you might hear that a 50mm lens becomes a 75mm lens when put on a cropped (APS-C) sensor, this is not entirely true. The lens is still a 50mm but the field of view of that lens has changed. The field of view is the angle between the the left/right extremes of the lens.

When you use a 50mm lens on a full frame camera, that lens projects a circle of light onto the sensor. The extremities of that circle will lie just outside the edges of the sensor.

Use that same lens on a cropped sensor and the circle will be exactly the same. However, the sensor is smaller which means the field of view is narrower. So your 50mm lens on a cropped sensor will give you the equivalent field of view of a 75mm on a full frame sensor.

The sensor is actually using a significantly smaller part of the lenses circle.

Full Frame v APS Sensor
Full Frame v APS Sensor

We tend to associate lens focal length with full frame 35mm cameras. This is partly from in the days of film, 35mm was by far the most common format and secondly because a standard lens is regarded to be 50mm. The field of view of a 50mm lens is very similar to our eye’s own field of view.

There are now many lenses that are designed specifically for cropped sensor cameras. They still maintain the same numbering system for focal length but are designed so that the circle of light they project covers just the area of the cropped sensor, not a full 35mm sized sensor.

This tends to make them lighter and cheaper.

The Effect of Different Focal Lengths

As photographers, we tend to break down focal lengths into three sub groups:

  1. Wide Angle,
  2. Standard,
  3. Telephoto.

Wide angles are lenses that give us an image that is wider than a human’s field of view whilst telephotos give us a narrower field of view. In 35mm equivalents, wide angles will be anything from 35mm down to 8mm.

A lens at 8mm would be considered a fisheye lens and would give a field of view of over 130 degrees. Telephotos are generally regarded as anything over 70mm.

Of course many people these days use zoom lenses – these enable us to change the focal length of the lens whist shooting.

They can be broken into sub-sections too. Wide angle zooms will be in the range of 12-35mm, standard zooms from 35-70mm and telephoto zooms 70-300. A sub-genre within this is the super zoom. These typically go from wide-angle through standard to telephoto, for example 28-300mm.


An 18-300 Super Zoom. By slgckgc

Wider lenses give the visual appearance of stretching out the distance between the foreground and background. Telephoto lenses do the reverse, making it appear that the background is much closer to the the foreground. This is called perspective and is a vital part of composition. Changing perspective can radically change the look of an image.


A super wide angle stretches out the foreground from the background. By Jason Row Photography


One misconception is that perspective changes when you change lens. If you are in the same place, this is not true, only the field of view changes. However, if for example you change from a telephoto lens to a wide angle lens then move closer or further from your subject, your perspective will change.

Understanding the basics of lens focal length widens the creativity of your composition. By knowing how lens focal length will affect the look of an image, you can choose the lens to suit that shot.

Whatever lens you're using, whether you have the kit lens, a prime, a mid-range zoom or all 3, you'll like to want to unleash the power of understanding how to create better B&W images. We have a fantastic course here by none other than LightStlking contributor and professional photographer Kent DuFault.

Further Resources

About Author

Jason has more than 35 years of experience as a professional photographer, videographer and stock shooter. You can get to know him better here.

This a discussion that only appeared with the APSC sensors
The same applied in the past to medium format 6×6, 6×7 or9, etc and there was no discussion about this
Carlos Carvalho

I think Carlos, to be honest, anyone that bought a medium format camera would already have a pretty deep understanding of photography and hence the discussion was not needed. Also we did not have the internet in those days and so there was not the medium to have these discussions on a global scale.

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