Digital Camera Lenses

The lens of a camera is similar to the front part of the eye: it is in charge of forming the image and regulating the intensity of the light. Nonetheless, there are notable difference between vision and photography. The eye has a more precise perception in the center of the visual field and a peripheral vision as reference. The high precision color sensors (cones) are situated on the retina, in a zone ( macula ) very close to the axis of the lens. The macula limit's the clear field of vision of the eye, which is an angle close to 45º. The rest of the retina is covered predominantly by black and white sensors (rods). This is the reason why we loose color sensation and precision when the brightness level is low. The lens of the eye and the lens of a camera can be considered converging lenses. The main identification sign of a lens is the focal length , which determines the medium distance between the lens and the formed images, as the size of them. Unlike the eye, which practically has a single focal length, the lens we use can provide images of an equal, greater or lesser size than what we see with plain view.

The phenomenon which makes possible the formation of images is the refraction of light. The light rays displace through a homogenous medium following a straight line. When a ray passes to another medium of a different density, the line it follows isn't a continuation of the last (the ray is refracted or deviated). The line perpendicular to the separation surface of the two mediums on a point called normal . When passing from a less dense medium to a more dense one, the light ray deviates to normal, and moves away from it if it passes in an inverse direction. Refraction is the explanation of phenomenon such as the curved aspect we see in a rod partially submerged in water, the apparent proximity of the bottom in a mass of water, etc. A converging lens (thicker in the middle than in the border) is a transparent solid limited by

one or two curved surfaces (generally spheres), which has the property of making light rays converge. All the rays coming from a same point and captured by the lens are deviated for them to reunite in a same point on the other side of the lens. If a scene is the infinite sum of points, with a converging lens we can obtain an image of it which will appear vertically inverted on the plane it is formed on.

A diverging lens (thicker on the border than the center) makes the rays that come from one point to diverge. It produces reduced images without an inversion, and these are only observable by the eye, they can't be projected or captured on a plane. The lens of a camera can be considered a converging lens, although it is also made up of diverging lenses which improve the quality of the image and help eliminate defects or aberrations. The axis of a normal lens is it's center. In a zoom lens, it is the line that unites the centers of all it's lenses and represents, as in the eye, the observation direction. The main characteristic of a lens is the focal length , which we define as the distance between the center of the lens and the plane where the rays parallel to the axis converge. The rays coming from an object placed at a large enough distance (infinity), can be considered parallel.