Focus: A Film Review (1948-1949)

142 FOCUS F have too big a picture, which causes loss of brightness, not to mention eye strain. The width of the picture should be about 1/8 of the length of the room: in the average classroom this generally means a picture of about 30" x 22" (or rather less if a translucent screen is used), and with a 2" lens, the width of the picture is 1/5 of the length of the throw, so that the machine should be 12^ feet from the screen. Under certain conditions, for instance with a very bright light or as has already been said with a translucent screen, it can be still smaller. In general a useful working maxim is: the smaller, the brighter. Screens. There are four types of surface, each of which possesses certain advantages. The white screen. Any opaque matte surface serves the purpose — a board or space of wall can be whitewashed, or a piece of cloth, e.g., oilcloth, painted with .white matte paint, and white blotting paper also makes a good surface. Thus a white screen is inexpensive and can be homemade; and it is generally useful as it distributes the light evenly over a wide space, though it is not so bright as the other surfaces. It is the best type for a wide room. The silver screen. This is made with silvered paint, on a board or cloth, and can also be improvised. It gives a brilliant picture to those in front, but the light falls off quickly at the sides: it should therefore be set up in a corner of the room if used in a classroom, as these are generally wider than long. The beaded screen is similar to the silver screen with the same features increased — it gives a very brilliant picture to those directly in front, but a duller picture to the rest of the audience. It is the make employed in the commercial cinema and is more useful in the school hall than in the usual classroom. It cannot be homemade, and is the most expensive type of screen. The translucent screen. This can be made of any semi-transparent material: engineers’ tracing paper or tracing linen, ground glass, or one of the plastics like Celastoid or Bexoid. The picture is projected on to the back of the screen and passes through and is seen on the right side. This screen gives a more brilliant picture than any other, and can easily be improvised, at little expense: it is therefore strongly recommended for classroom use. As there is usually not much space to spare at the back of the screen, the throw can be doubled by placing a mirror behind it, which reflects the picture back on to the screen, doubling it in size. The mirror should be set with its centre at right angles tothe screen to avoid distorting the picture, but the projector may be at any angle. Construction. Screens may be variously constructed — a rigid screen can be made to hang or stand; a folding screen can be fixed on rollers like a map, or made to roll automatically into a box or carrier, like a spring blind — or any flat surface in the room may be . utilised: a wall or space behind the I blackboard, or above it if tilted forward — or the blackboard may be hinged and the surface painted on the back — and many other ideas will suggest themselves. (It is to be noted, however, that the silver and translucent screens must be carefully placed on account of lighting requirements.) Darkening the room. For shows in the school hall, as has been said, a total blackout is needed and a way must be found of darkening it completely, combined iwith some means of ventilation: but the old idea that all light (and j with it all air) must be excluded from I the classroom is now giving way to ! “controlled lighting’’, on the ground I that the more the normal classroom atmosphere (both physical and mental) • can be preserved, the better. The (' purpose of using pictures in classwork I is to instruct, not to entertain: and to I plunge the class into darkness leads to 1 loss of control if not to dissipation, and > makes it difficult to question or discuss, and impossible to take notes: in a word the lesson tends to disintegrate. The room should be partially dark I ened if necessary, and it will be found j that the eye soon grows accustomed to the lighting. Curtains or blinds can be I partly drawn, or, if artificial light is I being used, a very efficient dimming arrangement can easily be made by I shading them on the side of the screen. If a white screen is being used, comparatively little light can be allowed as j it reflects it back directly, so it must be i-i shaded in the same way, but with a silver screen the light can be deflected