Focusing your Digital Camera

Roughly, a camera is a light-proof box with two parallel sides, on one you put the film, and on the other is a hole which can open and close with a converging lens. We also have a viewfinder which lets us have an idea of the image captured by the camera.

There are two technical functions of a camera: clearly forming an image on film or a digital sensor (focusing), and produce an exposure with the information provided by the photometer. The camera has three essential controls: a focusing mechanism, the shutter and diaphragm.

Focusing
The focusing mechanism is used to from the image clearly on the film or digital sensor, according to the distance from the object being photographed to the camera. (Many people mistakenly use the word focusing meaning framing.)

We have seen that light rays emerging from a same origin converge in the same point or image, when passing through a converging lens. Every point of a scene causes it's corresponding image, and the cluster of points in the image is proportionately accurate to the original. A lens projects a three-dimensional image: the images of two points which are at different distances from the lens are formed on different levels of the image. The images of points that are very far away (infinity) are formed closer to the lens than those at closer distances, which are formed farther from the lens; the closer to the lens a point is, the further away from it the image is formed.

Now, we can capture an image by projecting it onto a plane. A photograph is the result of projecting and registering a scene, generally three-dimensional, on a photosensitive plane. The lens of the eye projects an exterior image onto the retina, which is a curved but also bi-dimensional surface.

The image from any point of the scene is formed at a certain distance from the lens. If we put the film at that distance it's as if we cut the three-dimensional image projected by the lens to capture the images of objects a certain distance away . Therefore images corresponding points on other planes of the scene will be found on other planes of the image projected by the lens and it's image will appear unfocussed . The distance which is focused on the plane is called the focus distance .

It's necessary to place the lens at the right distance from the film according to the position of the object to be photographed. The focusing mechanism moves the lenses regarding the film, to obtain a focused image of what is at the distance marked on the mechanism (the focusing distance ring). .