Short Description

Crop factor applies only to 35mm SLRs, other camera segments may have crop factors but their lenses are rated as 35mm equivalents. The crop factor applies to SLRs who's sensors are smaller than a full frame 35mm sensor(36x24mm) but still use 35mm lenses. The crop factor illustrates how much larger a 35mm film is versus the smaller sensor along a single dimension. The ratio is also known as the focal length multiplier which illustrates the relationship between focal lengths of a lens when used with different sensor sizes.

For instance a 35mm sensor diagonal is 1.5X bigger than an APS-C sensor diagonal (the most common dSLR sensor) - thus its focal length multiplier is also 1.5X. Because the sensor is smaller than a full frame sensor the lens projects more image than the sensor can cover - hence the projected image is cropped and thus the name - crop factor. The focal length multiplier allows you to figure out how much is cropped by providing the equivalent focal length of magnification. For instance an image taken on a APS-C camera at 50mm will look like the same image taken on a full frame camera at 75mm - 1.5X magnification. This means smaller sensor SLRs get built-in zoom with a given lens relative to a full frame camera but they also loose the ability to go as wide angle as the full frame camera.

Visualization of Crop Factor

The following diagram shows the image projected by a lens and the relative differences between how much a full frame sensor is capturing and how much an APS-C frame captures. As you can see the smaller sensor captures less, but if it has the same number of pixels than on a relative basis the effect that is caused is basically zoom. If the full frame sensor was to capture the exact same image as the APS-C sensor you can see that the projected image would need to grow - in fact it would need to grow by exactly 1.5X - the same as the crop factor.

Illustration of Crop Factor on a Full Frame vs APS-C
illustration of a projected image being captured by full frame vs aps-cThe photo illustrates the difference between what a standard 35mm full frame sensor captures and a smaller APS-C sensor. It also illustrates the relative zoom afforded by the APS-C sensor and the wider angle afforded by the full frame sensor.

Top Full Frame Cameras

The following SLRs are some of the best full frame digital 35mm cameras available.

Green arrow See more of the top DSLRs with full-frame sensors

Top APS-C Cameras

The following APS-C cameras are some of the best available.

Green arrow See more of the top DSLRs with APS-C or APS-H sensors


Showing 3 comments

Johnsmith (7:34 AM, April 05, 2012)
Canon APS-C have a 1.6x crop factor (smaller sensor) so your 18mm = 28.8mm 
Roy Amrullah (9:16 PM, September 09, 2011)
550d seems to have 1.5x cropfactor then 18mm x 1.5 = 27mm. CMIIW
Avatar for Justin Ji Justin Ji (4:32 AM, June 15, 2011)
Nice! I was wondering why a 18mm lens on a 550d (APS-C) had less wide angle ability than a 26mm equivalent Nikon P100! : )