Editing Camera lens (section)

==Theory of operation==
Typical [[rectilinear lens]]es can be thought of as "improved" [[pinhole camera|pinhole "lenses"]]. As shown, a pinhole "lens" is simply a small aperture that blocks most rays of light, ideally selecting one ray to the object for each point on the image sensor. Pinhole lenses have a few severe limitations:

* A pinhole camera with a large [[aperture]] is blurry because each pixel is essentially the shadow of the aperture stop, so its size is no smaller than the size of the aperture (third image). Here a pixel is the area of the detector exposed to light from a point on the object.
* Making the pinhole smaller improves resolution (up to a limit), but reduces the amount of light captured.
* At a certain point, shrinking the hole does not improve the resolution because of the [[diffraction limit]]. Beyond this limit, making the hole smaller makes the image blurrier as well as darker.

Practical lenses can be thought of as an answer to the question: "how can a pinhole lens be modified to admit more light and give a smaller spot size?".  A first step is to put a simple convex lens at the pinhole with a focal length equal to the distance to the film plane (assuming the camera will take pictures of distant objects<ref>If the object is at a distance, one can assume the light rays will arrive perpendicular to the plane of the lens, and thus converge at the focal point.</ref>). This allows the pinhole to be opened up significantly (fourth image) because a thin convex lens bends light rays in proportion to their distance to the axis of the lens, with rays striking the center of the lens passing straight through. The geometry is almost the same as with a simple pinhole lens, but rather than being illuminated by single rays of light, each image point is illuminated by a focused [[pencil (optics)|"pencil" of light rays]].

{{Gallery
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|Image:Pinhole-camera.svg|Principle of a pinhole camera. [[Light]] rays from an object pass through a small hole to form an image.
|Image:Big pinhole.svg|With a large pinhole, the image spot is large, resulting in a blurry image.
|Image:tiny pinhole-diffraction.svg|With a small pinhole light is reduced, but diffraction prevents the image spot from getting arbitrarily small.
|Image:Big pinhole with lens.svg|With a simple lens, much more light can be brought into sharp focus.
}}

From the front of the camera, the small hole (the aperture), would be seen. The [[virtual image]] of the aperture as seen from the world is known as the lens's [[entrance pupil]]; ideally, all rays of light leaving a point on the object that enter the entrance pupil will be focused to the same point on the image sensor/film (provided the object point is in the field of view). If one were inside the camera, one would see the lens acting as a [[Image projector|projector]]. The virtual image of the aperture from inside the camera is the lens's [[exit pupil]]. In this simple case, the aperture, entrance pupil, and exit pupil are all in the same place because the only optical element is in the plane of the aperture, but in general these three will be in different places. Practical photographic lenses include more lens elements. The additional elements allow lens designers to reduce various aberrations, but the principle of operation remains the same: [[pencil beam|pencils of rays]] are collected at the entrance pupil and focused down from the exit pupil onto the image plane.