What is Focal Length Multiplier

If you spend any time reading through photography forums or blogs, or if you actually cracked open the manual that came with your camera, you may have come across the term Focal Length Multiplier. While this term does sound intimidating, the concept is fairly straight-forward.

Before digital cameras came along, 35mm film was a popular format. The majority of consumer-grade digital cameras available today have sensors that are smaller than 35mm film. The majority of these smaller sensors align themselves with the Advanced Photo System type-“Classic” (APS-C) image format, which measures smaller than traditional 35mm film (36×24mm). Therefore, by nature, these digital sensors are only capable of capturing a portion of the image that 35mm film is capable of. This is where the term focal length multiplier (FLM) comes into play.

Some related terms, “crop factor” and “magnification factor,” can be used interchangeably but the latter is generally used in reference to telephoto focal ranges. It’s important to note that the magnification never really changes when switching from standard 35mm film to APS-C sensors. Only the field of view changes.

The focal length multiplier is determined by the diagonal length of a piece of 35mm film divided by the diagonal length of the digital sensor in your camera. Not all digital sensors are the same size, so the FLM may vary from camera to camera.

For example, the camera I use has an APS-C sensor with a FLM of 1.6. This means that if I took a shot with a 50mm lens, the field of view would be the equivalent of an 80mm lens used on a 35mm camera. To determine this, you simply multiply the focal length of the lens by your camera’s FLM; in this case, 50mm x 1.6 = 80mm.

The photo to the right is an example of the field of view you’d get with a 50mm lens on a APS-C sensor versus a traditional 35mm film (or full frame) camera.

There are advantages and disadvantages to having a digital sensor smaller than 35mm film. One advantage is increased depth of field. Since you’re able to achieve a given field of view with a lens of shorter focal length, you’ll typically see greater depth of field with smaller digital sensors.

Another advantage comes on the telephoto end, where a 200mm lens has a field of view equivalent to 320mm with a 1.6 multiplier. This means you can take advantage of telephoto focal lengths without having to buy bigger and more expensive telephoto lenses. Conversely, it’s harder to achieve a wide angle field of view; a 20mm lens would have a focal length of 32mm.

Some high-end digital cameras boast a Full Frame digital sensor, which means that the digital sensor is the same size as 35mm film. Therefore, no FLM is applied.

You can see how this information is important, especially if you’re considering purchasing lenses for your digital camera. If you’re a Canon shooter, don’t forget to check out the Canon 50mm f/1.8 lens (review). This lens is a solid, low-cost portrait lens when paired with a 1.5-1.6 FLM.