Function of Camera Lens
Function of Camera Lens
When we place a lens in front of any ray of light, the lens will simply refract (bend) that ray. A camera lens uses refraction to focus light on the film, or in a digital camera on the charge coupled device (CCD) or other light-sensitive array to capture the image. From an object in front of the lens light is reflected in all directions. Some of this light is gathered in by the lens and transmitted to the plate, to form an image of that object. The rays of light which travel from the object toward the lens are bearers of the image. The function of the lens is to assume control of these image bearing rays and convey them, steer them so to say, to the plate or film plane, in such a manner that the image formed by these rays will be a true representation of the object.
Incoming light rays that are parallel to the optical axis and parallel to each other converge (refraction) to a common focal point after going through the lens. The distance between the lens and the object will greatly influence the image size (height and width of the image formed on the screen). When a ray of light passes from air to glass, it passes from a rarer medium to a denser medium. In such a case, the refracted ray bends towards the normal drawn at the point of incidence.
From each and every point of the part of the object facing the lens, a set of rays reach the lens. These are transmitted to the plate, where they form an image of that point. Each image point is a representation of its corresponding object point.
The first and simplest optical rule is that light travels in a straight line as long as it continues in the same medium, or even if it is passing from one medium into another of the same density. However, when light passes from one medium into another of a different density, then different rules apply. If it enters the new medium at right angles to its surface, it will continue in a straight line; if it enters a new medium at an oblique angle, it is bent out of its original course and proceeds in a different direction. Remember that a ray of light refracts or deviates from its original path as it passes from one optical medium to another because the speed of light changes.
Following are the Elementary principles of optics involved in lens construction, and the properties of photographic lenses as based on those principles.
Refraction takes place whenever a ray of light passes at an oblique angle from one medium into another of different density. The surface at which the refraction takes place is called the refractive surface. The angle which the entering (incident) ray forms with the normal at the point of entry (incidence) is termed the angle of incidence; the angle formed by the refracted ray at the same point is termed the angle of refraction.
The degree of refraction depends upon two factors: (a) The angle at which the incident ray meets the refractive surface: (b) The relative density and consequent relative refractive power of the two media.
This refractive power of glass is of immense value in the construction of photographic lenses. This makes it possible to control the image bearing rays, to make each set of rays originating from a point (a point in the object) converge to a point (the corresponding point in the image) and form a correct image. Light passing through the lens forms an image, which is laterally reversed.
Another case would be when the light rays are parallel to each other but not parallel to the axis through the centre of the lens. The lens will focus these, but to a point above, below or beside the focal point for rays along the axis, and all these points of focus of parallel rays will form a plane, called the focal plane of the lens. So, you put the film at this focal plane, and you’ve now used the lens to concentrate the light on the film and form an image.
This will only work for objects that are so far from the lens that the light rays that fall from the object to the lens are roughly parallel to each other. For closer objects, you need to move the film a little closer to the lens. That’s what happens when you focus a camera (or autofocus does this for you). The bigger the lens, the more concentrated the light, but the more critical focusing becomes.
So, being in focus by focusing the lens means moving the lens elements forward and backward until all the rays coming toward the camera from the subject meet at one point.
This is all called principal focus. It is defined as either of the two points on the axis of a lens, or other optical system. One point is such that rays diverging from it are deviated parallel to the axis upon refraction by the system and the other point is such that rays parallel to the axis of the system converge to the point upon refraction by the system.
The focal plane is the flat plane onto which a lens projects the image of the focused image subject. The focal plane sits at a 90-degree angle to the optical axis. The film’s or digital sensor’s light sensitive surface should be identical with the focal plane.
Light and the Eye
The retina is covered with specialized cells called rod cells (black and white) and cone cells (colors). These cells convert light that hit them into electrical impulses/nerve impulses that are interpreted by the brain. The locations of the millions of cells are transmitted in-time and upside-down. The brain ‘flips’ the image to right side up.