question: If a camera lens is round, why is the picture square or rectangular? Is it automatically cropped or is something else going on there?
comment: The camera's digital sensor (or piece of film), the thing that picks up the light and converts it to a photograph is rectangular, it's more efficient to make and store images this way
passages: A lens to be used on a camera that provides movements must have an image circle larger than the size of the image format (Adams 1980, 54). To avoid vignetting, a photographer using a view camera must ensure that the area remains within the image circle (Adams 1980, 56–57; 151–52; 157–61); a tilt/shift lens or perspective-control lens used on a small- or medium-format camera usually has mechanical limitations that keep the frame area within the image circle.
  
  Section::::See also.
  
  BULLET::::- Film format
  
  BULLET::::- Image sensor format
  
  BULLET::::- Format factor
  
  Section::::References.
   Post-processing can be done with image editing software. This normally involves cropping the image to a square and straightening it. The cropping is necessary because in most cases the (normally square) viewfinder image covers only about 25- 50% of the area of the (normally rectangular) digital picture.
  
  Additionally some photographers prefer to 'flip' the TtV image so that it is no longer a mirror image, especially if the image contains lettering.
   Photographic optics generally project a circular image behind the lens. On smaller format cameras the image circle generally covers only the intended film size with little room to spare. Large format lenses are an exception. For large format use the circular patch of image light usually extends beyond the minimum size circle needed to fully cover the rectangle of the film. The extra image offers room to spare to make use of camera movements that re-align the lens away from dead center on the film.
  
  Section::::Lens designs.
   The lens is designed to split into two pieces, the front and rear elements screwed, usually by a trained technician, into the front and back of the shutter assembly, and the whole fitted in a lensboard.
  
  View camera lenses are designed with both focal length and coverage in mind. A 300 mm lens may give a different angle of view (either over 31° or over 57°), depending on whether it was designed to cover a 4×5 or 8×10 image area. Most lenses are designed to cover more than just the image area to accommodate camera movements.
   Sensors in modern digital cameras come in several sizes. Some popular sizes include "full-frame" or 35mm, 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.
   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 light – the image circle. Various sensor aspect ratios may be used which all fit inside the same image circle, 3:2, 4:3, 16:9, etc.
   This requires that the image circle of the lens be larger than is required to cover the film without use of movements. If the lens can produce a circular image just large enough to cover the film, it can't cover the bottom of the film as it rises. Consequently, lens coverage must be larger to accommodate rises (and falls, and shifts).
   BULLET::::- As the viewfinder of a TLR camera requires the photographer to look down toward the camera, it is inconvenient to frame a photo with a subject that requires the camera to be positioned above the photographer's chest unless a tripod is used. In these cases, the camera may be positioned with the lenses oriented horizontally. Due to the TLR's square format, the composition need not be altered.
   Special film holders and accessories can fit in place of standard film holders for specific purposes. A Grafmatic, for example, can fit six sheets of film in the space of an ordinary two-sheet holder, and some light meters have an attachment that inserts into the film holder slot on the camera back so the photographer can measure light that falls at a specific point on the film plane. The entire film holder/back assembly is often an industry standard Graflex back, removable so accessories like roll-film holders and digital imagers can be used without altering focus.
   The exact shape of this region varies depending on what kind of camera lens is being simulated, but typically it is a frustum of a rectangular pyramid (hence the name). The planes that cut the frustum perpendicular to the viewing direction are called the "near plane" and the "far plane". Objects closer to the camera than the near plane or beyond the far plane are not drawn. Sometimes, the far plane is placed infinitely far away from the camera so all objects within the frustum are drawn regardless of their distance from the camera.
   The geometry of a lens hood is dependent on three things:
  
  BULLET::::- focal length of the lens,
  
  BULLET::::- size of the front lens element,
  
  BULLET::::- dimensions of the image sensor or film in the camera.
   Most SLR camera and lens manufacturers have addressed the concerns of wide-angle lens users by designing lenses with shorter focal lengths, optimized for the DSLR formats. In most cases, these lenses are designed to cast a smaller image circle that would not cover a 24×36 mm frame, but is large enough to cover the smaller 16×24 mm (or smaller) sensor in most DSLRs. Because they cast a smaller image circle, the lenses can be optimized to use less glass and are sometimes physically smaller and lighter than those designed for full-frame cameras.
   BULLET::::- 3 CCDs or CMOS active pixel sensors are used, one for each of the primary colors
  
  BULLET::::- They have interchangeable lenses.
  
  BULLET::::- The lens is focused manually and directly, without intermediate servo controls. However the lens zoom and focus can be operated with remote controls with a television studio configuration operated by a camera control unit (CCU).
  
  BULLET::::- A rotating behind-the-lens filter wheel, for selecting an 85A and neutral density filters.
   This calculation could be a horizontal or a vertical FOV, depending on how the target and image are measured.
  
  Section::::Lens types and effects.
  
  Section::::Lens types and effects.:Focal length.
  
  Lenses are often referred to by terms that express their angle of view:
  
  BULLET::::- Fisheye lenses, typical focal lengths are between 8 mm and 10 mm for circular images, and 15–16 mm for full-frame images. Up to 180° and beyond.
   A camera's angle of view depends not only on the lens, but also on the sensor. Digital sensors are usually smaller than 35 mm film, and this causes the lens to have a narrower angle of view than with 35 mm film, by a constant factor for each sensor (called the crop factor). In everyday digital cameras, the crop factor can range from around 1 (professional digital SLRs), to 1.6 (consumer SLR), to 2 (Micro Four Thirds ILC) to 4 (enthusiast compact cameras) to 6 (most compact cameras). So a standard 50 mm lens for 35 mm photography acts like a 50 mm standard "film" lens on a professional digital SLR, but would act closer to an 80 mm lens (1.6 x 50mm) on many mid-market DSLRs, and the 40 degree angle of view of a standard 50 mm lens on a film camera is equivalent to a 28–35 mm lens on many digital SLRs.
   BULLET::::- In some digital cameras for aerial photogrammetry images from several imaging elements, sometimes with separate lenses, are geometrically corrected and combined to one image in the camera.
  
  Section::::Types.:Orthophotos.
   BULLET::::- Camera facing away: This position automatically starts the camera application to take pictures or create videos. If the phone's video player application is being used, twisting to this position automatically changes the video to horizontal full screen mode, and it is possible to stand the 5700 on its side in this position so it is easier to watch videos.
  
  BULLET::::- Camera facing forward: This position automatically starts the camera application and is intended for self-portraits and for videophone calls. This position lets the phone be laid down horizontally on its side for hands-free use of the video camera.
   The MFT design standard also specifies multiple aspect ratios: 4:3, 3:2, 16:9 (the native HD video format specification), and 1:1 (a square format). With the exception of a few MFT cameras, most MFT cameras record in a native 4:3 format image aspect ratio, and through cropping of the 4:3 image, can record in 16:9, 3:2 and 1:1 formats.
  
  In addition, all current Micro Four Thirds cameras have sensor dust removal technologies.
  
  Section::::Lens mount.
   BULLET::::- Sony Clie PDA (various PEG-XXXX models)
  
  BULLET::::- Sony PlayStation Portable camera
  
  BULLET::::- Game Boy Camera which swivels horizontally for self-portraits
  
  BULLET::::- Veo SD camera for Toshiba e805 pocket pc
  
  BULLET::::- Notion Ink Adam
  
  BULLET::::- Asus M50VM laptop computer
  
  Section::::List of other devices with swivel lens.:Cellphones.
  
  BULLET::::- LG CU500
  
  BULLET::::- LG VX7000
  
  BULLET::::- Nokia N90
  
  BULLET::::- Samsung V200, SCH-U740
  
  Section::::List of other devices with swivel lens.:Cellphones with pseudo swivel lens.
  
  Some cellphones have lens that rotate and allow users to take self-portraits but are limited in the angles where the camera can take pictures. 
  
  BULLET::::- Samsung SPH-A900
   Most lens elements are made with curved surfaces with a spherical profile. That is, the curved shape would fit on the surface of a sphere. This is partly to do with the history of lens making but also because grinding and manufacturing of spherical surface lenses is relatively simple and cheap. However, spherical surfaces also give rise to lens aberrations and can lead to complicated lens designs of great size. Higher-quality lenses with fewer elements and lower size can be achieved by using aspheric lenses in which the curved surfaces are not spherical, giving more degrees of freedom to correct aberrations.
   BULLET::::- "The foundation for the high picture quality of the Four Thirds system is the lens mount, which is about twice the diameter of the image circle."
   Other Tessar lenses, for example for Nokia mobile phones, have only the name "Tessar" in common with the original Tessar. They are for example a 5-elements-in-1-group, aperture-less all-aspherical lens, as in the Nokia 808 Pureview and Nokia Lumia 800 camera.
   Square filter systems often have lens shades available to attach to the filter holders.
  
  Section::::Filter sizes and mountings.:Rectangular filters.
  
  Graduated filters of a given width (67 mm, 84 mm, 100 mm, etc.) are often made oblong, rather than square, in order to allow the position of the gradation to be moved up or down in the picture. This allows, for example, the red part of a sunset filter to be placed at the horizon. These are used with the "system" holders described above.
  
  Section::::Filter sizes and mountings.:Bayonet round filters.
   In 2013-2014 Sony and other manufacturers announced add-on camera modules for smartphones called lens-style cameras. They have larger sensors and lenses than those in a camera phone but lack a viewfinder, display and most controls. They can be mounted to an Android or iOS phone or tablet and use its display and controls. Lens-style cameras include:
  
  BULLET::::- Sony SmartShot QX series, announced and released in mid 2013. They include the DSC-QX100/B, the large Sony ILCE-QX1, and the small Sony DSC-QX30.
  
  BULLET::::- Kodak PixPro smart lens camera series, announced in 2014.
   Coverage (lens)
  
  The coverage of a lens is the size of the image it can produce, measured as the diameter of the image circle produced by the optics. In a camera system the coverage must be large enough to cover the sensor or film used. In a view camera 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.
  
presuppositions: The shape of the camera lens determines the shape of the picture. Round lens should cause a round picture.
corrections: The share of the sensor or piece of film determines the shape of the pic The sensor that captures the light is rectangular so the image is to.
raw_presuppositions: The shape of the camera lens determines the shape of the picture. Round lens should cause a round picture.
raw_corrections: The share of the sensor or piece of film determines the shape of the pic The sensor that captures the light is rectangular so the image is to.