Amateur Cine World (July 1935)

‘DIFFUSION By P. C. SMETHURST : ARES: of criss-cross lines and the plate held in front of the lens. The effect here is not exactly the same as that of the gauze, for the chief effect is a slight softening of the outlines and a pronounced loss of contrast. For work in artificial light, where shadows are apt to get blocked up, the criss-cross lines are very useful. Naturally the more lines are crowded into the small space through which the light must pass, the more pronounced is the effect. Still another method of diffusion is by buying a “ diffusion disc’? made by a photographic firm. These are eminently satisfactory, though they cost money and (as shown below) it is quite possible to imitate their effect at home. All these discs are flat pieces of glass with raised lines arranged in patterns on their surfaces, so that the raised lines act as supplementary lenses on parts of the image and throw it out of focus. To imitate the action of a diffusion disc a piece of glass from an old negative and a small quantity of vaseline are all that are required. The vaseline is spread thinly on the glass with one finger and the little irregularities of the skin will produce a series of ridges, which will have a refracting effect (like a lens) on light passing through them. If all the lines are smeared in one direction, the image is distorted that way alone, so that a window examined through horizontal smears would have the vertical bar fairly sharp, but the horizontal bar so blurred ini Taken with diffuser. Note how the outlines are softened and the spread of light into the darker parts of the image. that it was completely devoid of detail. For ordinary purposes such a great degree of distortion is not wanted, so that the smears form from the centre. Medium diffusion in all directions then results and the effect is almost identical with that of the glass disc. A very amusing effect can be got by smearing half the glass in one direction and the other half at right angles. The glass is then placed in front of the lens so that both horizontal and vertical smears cover half the lens and the result is a wonderful cross-shape distortion in two directions only. Such an effect would be very effective in a fantastic film, if used in the right place. As regards exposure, the diffusion disc and vaseline smear need no extra on_ that given by the meter. With scored lines on glass, or a gauze, it is useful to increase the exposure a little, though only by about half more than normal. With three gauzes on top of each other the extra exposure needed will be considerable, but can only be found by trial. If we carry diffusion to its logical conclusion we reach the point where the outlines are so blurred that the effect of fog (natural, not photographic) results. It is not easy to score enough lines on a small piece of glass to get this effect, but advantage may be taken of the scattering of light by small particles of pigment. The action of these is something similar to genuine fog, where suspended particles in the atmosphere scatter the light reaching our eyes. A fog filter may be made with gelatine and Chinese white quite easily. Get from the kitchen two or three sheets of gelatine leaf and melt them down in about three fluid ounces of warm water in a medicine bottle. It will be necessary to heat the bottle to get the entire solution properly fluid. When completely dissolved, stir in enough Chinese white to make the solution opaque when held up against a north window on a fine day. (Moist Chinese white from a tube is very satisfactory). Still keeping the mixture warm, clean a piece of glass and warm it to the same temperature (it is not easy to coat evenly if the glass is cold) and have it really flat on a table. Coat the glass with the mixture as thinly and evenly as possible and then put a box-lid over it until it is dry. This will take perhaps 36 hours and at the end the layer should be thin enough to see through, (Continued on page 192) f Bilateral distortion at right angles. All the above are vaseline smears