What You Need to Know About Focal Length on a DSLR

Convert 35mm focal lengths to APS-C digital cameras

Certain digital cameras require a focal length multiplier to ensure the photographer gets the angle of view they expect. This only became a factor when photography transitioned from film to digital, and changes were made to many DSLR cameras that affected the focal length of common lens sizes.

When pairing a digital camera with a lens, it is important to know whether a focal length multiplier needs to be considered. This information could dramatically affect the lens that you buy because you won't buy a lens that doesn't meet your specific needs.

Chasing the Northern Lights in Sweden
Matthew Micah Wright / Getty Images

What Is the Focal Length Multiplier?

Many DSLR cameras are APS-C, also called crop-frame cameras. they have a smaller sensor (15mm x 22.5mm) than the area of 35mm film (36mm x 24mm). This difference comes into play when referring to the focal length of lenses.

The 35mm film format has long been used as a gauge in photography to determine the focal length of the lenses that many photographers are accustomed to. For example, a 50mm lens is considered to be normal, a 24mm is wide-angle, and 200mm is telephoto.

Because the APS-C camera has a smaller image sensor, the focal lengths of these lenses are altered using a focal length multiplier.

Calculating Focal Length Magnifier

The focal-length multiplier varies between manufacturers. This can vary by camera body as well, although most manufacturers such as Canon require you to multiply the lens' focal length by x1.6. Some manufacturers differ: Nikon and Fuji tend to use x1.5 and Olympus uses x2. This indicates that the image, in the Canon ecosystem, captures a frame that is 1.6 times smaller than that captured with 35mm film.

The focal-length multiplier does not apply to the focal length of lenses used with a full-frame DSLR because these cameras use the same format as 35mm film.

All of this does not necessarily mean that you are multiplying the full frame lens by the focal length magnifier.

Conversely, if you are putting an APS-C lens on a full-frame camera body (not advised because you will get vignetting), then you would multiply the lens by the focal length magnifier. This gives you the full-frame focal length.

Think Angle of View

It is more about the angle of view in relation to the capture size than the actual focal length of the lens, and so that 50mm lens is actually a wide angle lens on an APS-C.

This is the challenging part for photographers who have been using 35mm film for years, and it takes some time to wrap your mind around this new way of thinking. Concern yourself with the angle of view of a lens rather than the focal length.

Here are some of the common lens sizes to visually help with the conversion:

Angle of View
Canon x1.6
APS-C 'Crop'
Nikon x1.5
APS-C 'Crop'
Super Telephoto 2.1 600mm 375mm 400mm
Long Telephoto 4.3 300mm 187.5mm 200mm
Telephoto 9.5 135mm 84.3mm 90mm
Normal 39.6 50mm 31.3mm 33.3mm
Normal-Wide 54.4 35mm 21.8mm 23.3mm
Wide 65.5 28mm 17.5mm 18.7mm
Very Wide 73.7 24mm 15mm 16mm
Super Wide 84 20mm 12.5mm 13.3mm
Ultra Wide 96.7 16mm 10mm 10.7mm

The Digital Lens Fix

To avoid this problem, many camera manufacturers now produce specific "digital" lenses, which only work with APS-C cameras.

These lenses still display regular focal lengths, and they still require focal length multiplication to be applied to them, but they are designed to only cover the area of the sensor used by crop frame cameras.

They are usually a great deal lighter and more compact than normal camera lenses.

Was this page helpful?