photography

STEP 2: CAMERA CONTROLS / Focusing
As we have seen, a fixed position lens on a simple camera is placed so as to give an equally sharp image from about 6.5ft (2m.) to infinity. At closer distances, the nearer you are to your subject the more the image will be out of focus. This can be avoided if you have a lens on your camera which can be focused: that is, moved away from the film plane, as shown in the diagram. Right, the nearer the subject is to a lens; the farther behind the lens its image is formed. Thus for close subjects the lens must move away from the film plane to focus the image.

Subject distance and focus
When a convex lens is focused for very distant subjects it is positioned at its own focal length from the film, for
Example, when the film is 50mmaway from a 50mm lens. If the subject is closer, its ray are much less parallel when they reach the lens , and the " bending power" of the glass will only bring them into focus , to form a larger image , at a greater distance from the lens , as shown in the diagram ,right. On many cameras, the lens moves forward to compensate for this, so that the image still focuses on the film plane. The nearer the subject, the greater the distance required between lens and film.

Being able to focus the lens gives you two important advantages: you can get sharp pictures when you are quite close to your subject, and you can give emphasis to one part of the subject by focusing on it and allowing everything nearer or further away to appear progressively un sharp In both cases it is vital to know which part of the scene is being sharply rendered. An adjustable focusing lens body therefore has a scale (or at least a series of symbols) showing the correct lens positions for focusing on various distances. To set this you have to estimate the subject distance, unless you have a camera with a focusing aid coupled with the viewfinder or through – the – lens focusing, as shown opposite.
Using the focus control
Most lenses are focused by turning the largest control ring on the lens barrel. This will slowly extend or retract the lens. At the same time a scale of subjects distances moves past a fixed mark. On end of this scale is marked “inf.” Or ∞, which denotes infinity, on this setting the lens is at its closest position to the film, and will give sharp images of distant subjects – in practice about 50ft ( 15.2m) or beyond . at the other end of the focusing scale, which may read 3 ft ( 0.9 m ) , the lens will have moved out to its furthest distance from the film. Some lenses have a greater focusing movement (and therefore allow closer photographer) than others quality , since a lens designed for general distance photography may not perform so well when used extremely close up. Attachments for close- up work are discussed on pages 102-3.
Focusing symbols Some simple cameras use symbols on the focusing scale, like those above, which focus the lens for close – up, middle distance.

The effect of focusing

The tow pictures above differ only in the camera focus setting and show how this control can be used to pick out parts of the subject. In the picture, left, the lens setting was about the same as for a fixed focus lens – 30 ft (9m). Almost all the picture, except for the fore- in the picture, right, and the lens was focused for about 3 ft (0.9m) so that the foreground appears sharp while detail in the background is un sharp. In this way we can use the focus to concentrate interest on areas of your subject, in the same way as subconsciously you focus your eyes on selected parts of your field of vision.