Camera Lens - Theory of Operation

Theory of Operation

Typical rectilinear lenses can be thought of as "improved" 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 would be excellent except for a few serious 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 (below left). 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.
• Diffraction limits the effectiveness of shrinking the hole, so at a point, making the hole smaller makes the image blurrier as well as darker (below center).

Practical lenses can be thought of as an answer to the question "how can we modify a pinhole lens 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 ). This allows us to open up the pinhole significantly (below right) 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" of light rays.

Standing in front of the camera, you would see the small hole, the aperture. 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 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: pencils of rays are collected at the entrance pupil and focused down from the exit pupil onto the image plane.