Focal Lengths and Digital Cameras

In the early days of digital photography, the camera makers (Nikon and Canon, as they were the first into the mass market) decided to create digital SLRs with a sensor size smaller than 35mm film. The reason was one of cost; a sensor to match the size of a frame of 35mm film was prohibitively expensive at the time. The smaller sensor also changed how photographers thought about lenses.

One characteristic of any lens is called the ‘angle of view’ and is a value stated in degrees. A lens may have a large angle of view, encompassing a wide section of the scene in front of the camera or it may have a small angle of view, taking a relatively narrow slice of the scene. A value somewhere between the wide and narrow angles is considered a ‘normal’ angle of view since it is roughly equivalent to human visual experience. The smaller sensors in the first digital SLRs made every lens designed for a 35mm film camera ‘less wide.’ In other words, the angle of view became narrower because the smaller sensor covered a narrower section of the image being projected inside the camera body by the lens.

Eventually, camera makers introduced digital SLRs with full-frame sensors matching the size of 35mm film. However, cameras with smaller sensors are still made and constitute a sizable part of the market because the smaller sensor translates into a less expensive camera.

Since the first digital cameras were essentially built on 35mm film camera designs, the 35mm lens focal length frame-of-reference stuck with us. You will often see focal lengths for cameras with smaller sensors also stated in 35mm equivalent focal lengths. This is so everyone can match the angle of view with the old 35mm film angles of view.

To further add to the confusion, two camera makers, Olympus and Panasonic, took a different approach and designed an entirely new sensor size and camera platform, called Four Thirds and later, Micro Four Thirds. Sensors for these cameras are smaller than the sensors for other digital cameras. Focal lengths for Four Thirds and Micro Four Thirds are also stated additionally in 35mm film equivalents.

The following tables list common 35mm film focal lengths and the focal lengths for smaller sensors that provide approximately the same angle of view. There are two sets of conversion factors if you want to do the math yourself.

To convert a focal length on a camera with a smaller sensor to the equivalent 35mm film lens, multiply:

• 1.5 for Nikon, Fujifilm X and Pentax cameras using what is usually referred to as an ASP-C size sensor (the smaller sensor first introduced in digital cameras)

• 1.6 for Canon (Canon makes its own sensors and chose a 1.6 factor instead of 1.5 for its ASP-C size sensor cameras. Canon has also made 1.3 factor sensors but those cameras are not covered in the tables.)

• 2.0 for Four Thirds and Micro Four Thirds

To convert a focal length for 35mm film to the equivalent on a camera with a smaller sensor, multiply:

• 0.667 for Nikon, Fujifilm X and Pentax cameras using ASP-C

• 0.625 for Canon ASP-C

• 0.5 for Four Thirds and Micro Four Thirds

The photo at the top of this article shows a ‘normal’ focal length lens for three different sensors sizes. From left to right:

• Olympus M.Zuiko Digital 25mm f/1.8 for Micro Four Thirds (equates to a 50mm lens for 35mm film)

• Fujifilm Fujinon 35mm f/1.4 XF R for Fujifilm X mirrorless cameras with an ASP-C size sensor (equates to a 52.5mm lens for 35mm film)

• Nikon Nikkor 50mm f/1.2 AIS lens designed for 35mm film Nikon F bodies and that also works on Nikon DX (ASP-C sensor size where the angle of view is that of a 75mm lens) and Nikon FX (full frame sensor size where the angle of view is that of a 50mm lens)