What is the Focal Length Multiplier of Camera Lenses?

Converting 35mm Focal Lengths to APS-C Digital Cameras

Certain digital cameras require a focal length multiplier in order to ensure the photographer is getting the angle of view they are expecting. 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 or not a focal length multiplier needs to be considered.

It could dramatically affect the lens that you purchase because you could be buying a lens that does not meet your specific needs.

What is the Focal Length Multiplier?

Many DSLR cameras are APS-C, also called crop frame cameras. This means that 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 of us are accustomed to using. For instance, a 50mm is considered to be 'normal,' a 24mm is wide-angle, and 200mm is telephoto.

Since the APS-C camera has a smaller image sensor, the focal lengths of these lenses have to be 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 though most manufacturers like Canon require you to multiply the lens' focal length by x1.6.

Nikon and Fuji tend to use x1.5 and Olympus uses x2. This means that the image will capture a frame that is 1.6 times smaller than what would be captured with 35mm film.

The focal length multiplier does not have an effect on 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. In fact, the equation looks something like this:

Full Frame Focal Length ÷ Focal Length Magnifier = APS-C Focal Length

In the case of a Canon APS-C with x1.6 it would look like this:

50mm ÷ 1.6 = 31.25mm

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 will give you your 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 our minds 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
(degrees)
35mm
'Full-Frame'
Canon x1.6
APS-C 'Crop'
Nikon x1.5
APS-C 'Crop'
Super Telephoto2.1600mm375mm400mm
Long Telephoto4.3300mm187.5mm200mm
Telephoto9.5135mm84.3mm90mm
Normal39.650mm31.3mm33.3mm
Normal-Wide54.435mm21.8mm23.3mm
Wide65.528mm17.5mm18.7mm
Very Wide73.724mm15mm16mm
Super Wide8420mm12.5mm13.3mm
Ultra Wide96.716mm10mm10.7mm

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.