The Crop Factor

The crop factor is a number by which a lens’ focal length is multiplied to arrive at the equivalent focal length in 35mm film. Although popularized by the sub full-frame Digital SLR, its origins date back to the days of film and Kodak innovation.

In 1996, Kodak developed the Advanced Photo System (APS), a new film format with smaller film cartridges and a frame size of 25.1 x 16.7mm; about half the area of the full 36 x 24mm frame size of 35mm film. The smaller frame size had the same effect as “cropping” the width and height of the image by about 70%. In order to restore the familiar picture “angle” of 35mm film, lenses with a shorter focal length (and thus wider angle) must be employed.

1996 Nikon Pronea 600i, a full-featured APS film SLR with a 1.43x crop factor.

For interchangeable lens cameras such as SLRs, this meant that using a normal 50mm lens would result in the equivalent picture angle of 71.5mm or 1.43x its focal length. This then is the crop factor for APS film cameras. In order to develop new lenses for these cameras, the reciprocal of the formula is used. So to design a “normal” 50mm focal length lens for an APS camera, the focal length must be 0.698x or approximately 34.9mm.

1999 Nikon D1, an APS digital SLR with a 1.5x crop factor.

When digital SLRs became widely available in 1999, they employed sub full-frame sensors and so leveraged off the legacy of this new format. As a result, the APS format has become the new standard in Digital SLRs. APS-compatible lenses are smaller, lighter and less expensive than their full-frame counterparts, yet the APS system remains backwardly-compatible with full-frame “legacy” lenses. This is good news for users of telephoto lenses as their reach is increased by a factor of 1.5 times with no penalty of additional light loss.

APS lenses are designated with their actual focal lengths, not their APS “equivalent” focal lengths. This requires you to know to apply the crop factor to determine the picture angle of the lens when used with your particular body.

Nikon settled on a standard with a slightly smaller frame size than APS film frame, 23.6 x 15.7mm. This gives their cameras a 1.5x crop factor, a relatively easy number to work with. A 50mm normal lens therefore becomes 75mm. 35mm, the traditional length for photo reportage becomes “normal” at 52.5mm. A 24mm lens becomes the new wide angle standard at 36mm. A 300mm telephoto now become 450mm. Nikon calls this new standard “DX”, and in generic terms, it’s known as APS-C.

Canon settled on the slightly smaller frame size of 22.2 x 14.8mm for their APS-C format. This carries a crop factor of 1.6x, a little more difficult to work with, but still in the general ballpark. A 35mm lens on a Canon APS-C camera is therefore equivalent to 56mm. To complicate things further, Canon also has a second sub full-frame format known as APS-H, which carries a frame size of 28.7 x 19mm, or a crop factor of approximately 1.3x. A 35mm lens on a Canon APS-H camera now becomes 45.5, which is actually closer to the “true” normal focal length of 43.3mm.

Most other brands employ the 1.5x crop factor.

APS lenses have many advantages. In addition to the size, weight and cost advantage mentioned above, shorter focal length lenses have greater depth of field. So for any given focal length, there is a greater depth of field to work with than on a full-frame camera.

Here are some popular focal lengths and their APS-C, 1.5x equivalents:

8mm = 12mm (full-frame fisheye)
16mm = 24mm (super wide angle)
20mm = 30mm (extra wide angle)
24mm = 36mm (standard wide angle)
28mm = 42mm (“true” normal)
35mm = 52.5mm (“standard” normal)
50mm = 75mm (portrait)
85mm = 127.5mm (portrait, short telephoto)
105mm = 157.5mm (moderate telephoto)
135mm = 202.5mm (standard telephoto)
200mm = 300mm (super telephoto)
300mm = 450mm (super telephoto)
500mm = 750mm (super telephoto)

The 1.5x crop factor results in legacy lenses having a longer, but still useful range. On the other hand, APS lenses will still mount on full-frame cameras, but because their “coverage” or projected image circle is smaller, the corners will be vignetted to varying degrees.

Nikon AF-S DX (APS-C) NIKKOR 35mm f/1.8G lens used on a full-frame camera body.

Nikon cameras have a special mode for this called “FX” crop, which uses a portion of the sensor to simulate the exposure on an APS-C camera, generating an image with a reduced resolution. However, even without this feature, the images can be cropped in post-processing.

The same image cropped to show how it would appear on an APS-C camera.

Another approach is to crop the frame as a square composition:

The same image, cropped to a square format which eliminates the vignetting.

This works particularly well if you prefer shooting in a square format. For example, the AF-S DX 16-85mm f3.5-5.6G works perfectly on a full-frame body and will capture excellent square format images at  the extremely wide angle of 16mm.

Still another option would be to crop the photo to a panoramic format such as an aspect ratio of 9:6:

A “panoramic” crop with an aspect ratio of 9:6 will also reduce or eliminate vignetting.

Nikon’s literature suggests that DX lenses cannot be used with 35mm film cameras. As you can see from the above images, this is not entirely true. When post-processed accordingly, they can produce rather nice images.

Compact digital cameras also have a crop factor, but it’s much higher due to their significantly reduced sensor size. However, it’s not essential to know as the lenses in these cameras are fixed. What is important however, is knowing the 35mm equivalent of each of its zoom steps, and this can usually be determined through metadata.