Why do some cameras not have to be focused?
Because the lens is focused permanently at the distance at which most snapshots are taken, usually at about 10 to 13 feet (3 to 4 meters), and objects closer or further than this will simply not be critically sharp.
N.B. This informal holiday shot is a suitable type of subject for a fixed focus lens.
Why is it that everything looks quite sharp in the viewfinder of my camera no matter where it is focused?
This is because a viewfinder camera uses a separate optical device which projects a different type of image purely for the convenience of view finding. A camera with a focusing screen such as an SLR or a twin lens reflex will show a distinction between sharp and unsharp planes.
Are modern cameras focused in a different way from early cameras?
The principle of focusing is that the lens is moved further away from the film in a order to focus a sharp image as the subject comes closer to the camera. Whit a 35 mm camera, for example, this is achieved by means of a helical thread which extends the lens as the mount is rotated.
What is the camera obscura?
This was the forerunner of the modern camera and dates from the sixteenth century. It was simply a darkend room in which an image of the scene outside was projected on to a wall by means of a lens or a small hole. later versions were portable which allowed the user to set it up like an ordinary camera and trace the image on to paper.
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