Image formation with a lens
The type and quality of a camera's lens has a major impact on the creation of an image. In this chapter you will discover how a lens works and how to influence the picture quality by using its different components.
The structure of a compound lens
The principles of image formation.
A compound lens, as used in practically every camera, consists of a number of different pieces of glass or plastic, each one being a single lens element, cemented together. The main task of a camera lens is to focus the light reflected from an object in order to produce a sharp image of the object on the film or CCD plane. Glass lenses can gather a greater amount of light than plastic producing an image with shorter exposure times.
Strictly speaking, a lens can only achieve a perfectly sharp image (perfect focus) for only one plane in front of the camera at a given distance. If the distance between film plane and subject plane changes a refocusing needs to take place. This is usually accomplished in manual focus lenses by rotating a ring on the lens barrel, the lens’s focussing device, which moves the different lens components inside the camera (compound) lens until the image is correctly focused.
The lenses inside a compound lens
As a child, you might remember having played with a magnifying glass with the intention of setting dry leaves or paper on fire. You moved the magnifying glass up and down until its lens formed the smallest possible patch of light.With a magnifying glass rays of light from the sun can be concentrated on one tiny spot on a piece of paper to catch fire
You might not have been aware of it, but by doing so you actually produced the image of the sun on a piece of paper, just like a camera produces an image on the film plane. In short, the sun represents the main subject, the magnifying glass represents the camera’s lens, the up and down movement with the magnifying glass in your hand represents the focussing and the little patch of light on the paper plane represents the camera’s final image.
The individual glass elements inside a compound lens, like a magnifying glass, have curved surfaces.
The curved surfaces are classified into two categories:
1. Convex lenses
Convex lenses have a spherical surface that bulges outward at the centre. Light beams, which pass through a convex lens are refracted (their path is altered) and the beams of light converge. In theory a single convex lens could be used as a camera lens, but this is not done due to its imaging imperfections. These imperfections, or aberrations, will be explained later.
2. Concave lenses
Concave lenses have a spherical surface that is hollow, i.e. thinner at the centre than at the edges. Light beams passing through a concave lens are refracted so that they diverge. A single concave lens cannot produce a real image. Nonetheless, concave lenses can be quite useful, as we’ll soon find out.
* This content is taken from the konicaminoltaphotoworld.com website that was unfortunately taken offline. I take full responsibility for offering this here on dyxum but I honestly believe that this contests SHOULD be available to all Konica Minolta customers; this is the least they can do !