April 03, 2008
This is part of an ongoing series of short articles about lenses and how they are used on Digital SLR cameras. The series currently contains the following articles:
Most digital SLRs use sensors that are slightly smaller than the size of 35mm film. Film has an image diagonal of 43mm, while these smaller sensors have image diagonals of 27mm. The lenses they use are still designed to create an image on a piece of 35mm film. What happens is that the image that is created is effectively a crop of what the image would have been if it were a full frame or film camera. The ratio of full frame to the smaller sensors comes out to 1.6, and many times these cameras are referred to as having a 1.6 crop factor.
The image above should help make this clearer. This is Tunnel View looking east over the Yosemite Valley toward Half Dome. The circle is the image that is projected on the back of a 35mm camera. The red rectangle represents 35mm film, or the area of a full frame sensor. Note that the circle is a bit larger than that area, and in practice probably has even a bit more overlap.
The yellow rectangle represents the area of a 1.6 crop sensor. Notice that nothing about the projected image has changed. The smaller sensor simply records a smaller area of the image, like a croppped area of the image.
The image above shows how each image appears when sized to the same pixel dimensions, e.g. if both the full frame camera and the 1.6 crop camera have a 10 megapixel sensor (or if you printed out both images at 8"×12"). The image on the left, from the full frame sensor, shows a wider view, with much more in the scene. The crop sensor image, on the right, has a smaller field of view, in this case in a ratio of 1.6 : 1, which gives the effect of appearing to be shot with a longer lens. There is less in the image, but what is there appears larger and closer to the camera.
If a 17mm lens is used to create an image with the smaller sensor, then a full frame sensor would need to use a lens with a focal length of 27mm to have an equivalent field of view (17mm × 1.6 = 27.2mm). This is where the term Focal Length Multiplier originates.
It is more accurate to refer to this as the crop factor since the focal length of the lens is not changing. The characteristics of the lens’ focal length, and their effecst on the final image are the same on both the full frame and the cropped camera.