Digital Photography Opinions, Observations, Articles, Insights, and Tutorials The "Multiplier Factor" ... also called the "Crop Factor" Digital Single Lens Reflex cameras (DSLRs) use an electronic sensor instead of film to capture a picture. Most sensors are smaller than a frame of traditional 35 mm film, but all sensors and film planes are mounted in the same relative position. As a result, images projected through the lens onto the back of the camera are physically larger than those smaller-than-full-frame sized digital sensors. When that happens, only the center portion of the total image is captured by the sensor. The illustration at left shows the relative size differences of different sensors on the exposure plane and their associated areas of image capture. The outer portions of the total image are cropped away in varying amounts by the smaller sensors. The cropped capture area has a reduced field of view, creating what is often called a "multiplier factor". The multiplier factor is used to compare the practical focal length of a lens on a Digital SLR to a full frame camera. Because of the cropped view, the listed focal lengths of all lenses used on digital cameras with sensors smaller than a full frame must be adjusted by the "multiplier factor" to calculate the longer practical focal lengths with their associated reduced fields of view. Camera Type Sensor Measurements Horizontal View Multiplier Factor (rounded off) Physical area (mm2 ) of sensor compared to a full frame Kodak, Canon, Nikon, and Sony FX Format Cameras 36 mm x 24 mm N/A (full frame) 100% Nikon DX Format Cameras 23.7 mm x 15.6 mm 1.5 42.8% Canon DX Format Cameras 22.5 mm x 15 mm 1.6 39.1% Olympus E Series Cameras 18 mm x 13.5 mm 2.0 28.1% Nikon CX Format (Nikon 1 System Cameras) 13.2 mm x 8.8 mm 2.7 13.5% Calculating the practical focal length Take the focal length of any given lens and multiply it by the multiplier factor to get the practical (cropped view) focal length for the camera in question. For example, an 18 mm focal length lens produces a field of view like a 24 mm lens on a Nikon D70 (18 mm x 1.5 multiplier), while a 200 mm lens on a Canon 20D has a field of view like a 320 mm lens (200 x 1.6 multiplier). Some call it a telephoto-like effect. Example of the cropped field of view and its associated multiplier factor The picture set below shows the relative differences in the fields of view between a full frame camera and a DSLR camera with a 1.5x multiplier factor. 400 mm telephoto lens view of an airplane in flight as seen through the viewfinder of a full frame camera. The reduced field of view cropped by a 1.5x multiplier sensor is shown inside the red border. The cropped 1.5x view produces a 600 mm effective view (when compared to a full frame camera). Just as "zooming in" is a result of increased real focal length, the cropped area created by the multiplier factor increases the perceived (practical) focal length of the lens. Objects look closer because they are in a cropped viewing area.   Why is the crop factor (multiplier factor) important? Experienced photographers buy and use their lenses according to focal length (wide angle through telephoto), and speed (aperture rating). Speed refers to "fast" lenses (low base aperture numbers) compared to "slow" lenses (higher base aperture numbers). Fast lenses are higher quality (more expensive), and give a broader range of use under different lighting conditions. (Fast is desirable.) The increased practical focal length changes the traditional use of the lens application. A wide angle lens may not be "wide enough" because of the multiplier factor. On the other hand, a huge, heavy telephoto lens might be substituted with a lighter weight and cheaper midrange telephoto because of the increased practical focal length. The speed rating (aperture value) of the lens doesn't change with the increased practical focal length. For example, a 400 mm f/5.6 lens with a 1.5x multiplier becomes a practical 600 mm f/5.6 lens. (Super telephoto, reasonably "fast", and reasonably priced.) Compare that with the several thousand dollar cost of a "real" 600 mm f/5.6 lens.