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The coverage of a 
One common misconception about lenses is that focal length determines the angle of view for a lens, when in reality it is a combination of focal length, lens coverage, and sensor size. A lens of a certain focal length on a large sensor will yield a wider angle of view than the same lens used with a smaller sensor.
lens
A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements ...
is the size of the image it can produce, measured as the diameter of the image circle
The image circle is the cross section of the cone of light transmitted by a lens or series of lenses onto the image plane. When this light strikes a perpendicular target such as photographic film or a digital camera sensor, it forms a circle of ...
produced by the optics. In a camera
A camera is an optical instrument that can capture an image. Most cameras can capture 2D images, with some more advanced models being able to capture 3D images. At a basic level, most cameras consist of sealed boxes (the camera body), with a ...
system the coverage must be large enough to cover the sensor or film used. In a view camera
A view camera is a large-format camera in which the lens forms an inverted image on a ground-glass screen directly at the film plane. The image is viewed and then the glass screen is replaced with the film, and thus the film is exposed to exactly ...
system, because the film plane can be moved independently of the lens plane, the lens must have enough coverage to cover the film while it is offset from the center of the coverage area. Inadequate coverage can lead to vignetting
In photography and optics, vignetting is a reduction of an image's brightness or saturation toward the periphery compared to the image center. The word '' vignette'', from the same root as ''vine'', originally referred to a decorative bord ...
.
Sensors in modern digital cameras come in several sizes. Some popular sizes include " full-frame" or 35mm 35 mm may refer to:
* 135 film, a type of still photography format commonly referred to as 35 mm film
* 35 mm movie film, a type of motion picture film stock
* 35MM 35 mm may refer to:
* 135 film, a type of still photography format ...
, APS-C, and 1-inch. Lenses are often intended to work with one of these sensor formats, but in some cases a lens can be used with multiple formats. A “full-frame” type lens produces an image circle that is large enough to cover a 35mm sensor. Because APS-C sensors are smaller than 35mm sensors a full frame lens should be compatible with an APS-C sensor. An APS-C type lens often does not have the coverage to cast light on the entirety of a 35mm sensor, causing dark corners in the images.
One common misconception about lenses is that focal length determines the angle of view for a lens, when in reality it is a combination of focal length, lens coverage, and sensor size. A lens of a certain focal length on a large sensor will yield a wider angle of view than the same lens used with a smaller sensor.
References
Science of photography {{Photo-stub