Photography THE USE OF FILTERS IN PHOTOGRAPHY
By W. R. AYLING
SOME amateur photographers seem to find it difficult to understand how and when to use a filter over the lens. It
will perhaps simplify matters for them if it is appreciated that - a filter, in photography, does the same work as a filter in chemistry, or indeed in any sphere. The ability of the filter to hold back something, whilst permitting the remainder to pass, is utilised in photography to restrain those colours to which the emulsion is extra sensitive. Used in this manner, other colours which would not impress them- selves sufficiently on the emulsion are given an extra period of exposure. With emulsions which are colour-blind a filter is useless. With orthochromatic emulsions, a yellow filter
will hold back the sky portion, where it is blue, and enable light clouds to be rendered in their correct tones. Correct, that is, as the eye interprets colours, because we are told that all objects which we understand to be coloured in themselves are merely reflecting parts of the spectrum, absorbing the other parts. For general landscape work, therefore, use a yellow filter (termed K.i or K.2, the latter is stronger than the former), with orthochromatic films. In order to allow the emulsion to register the colours to which it is less sensitive, an extra period of exposure must be given with nearly every filter. The yellow K.i requires double the normal exposure, the K.2 four to six times according to the film used. Most manufacturers enclose with their films a list showing the filter factors (or number of times increase in exposure required).
With panchromatic films, sensitive to all colours but in different degree according to the partioilar film, a filter can almost always be used to good effect. Here again, the film manufacturer's instructions for the use of filters should be closely followed. Some panchromatic films are extra sensitive to red and orange. In making a picture of a red rose with such a film, it will be necessary to consider the red, the green of the foliage and any other colour which might be included in the immediate background. The eye accepts blue as a mid- tone or dark tone, but the film does not record it as such unless some assistance is given through the filter of the appropriate colour. In order to lighten a colour on the print, that is darken it on the negative, a filter of the same colour, or colour com- bination, must be used. At this stage the primary colours in photography should be considered. They are blue-violet, green and red, the divisions into which white light is_ separated by the spectroscope. A blue-violet filter will pass light of the same colour when reflected from an object. If that object is itself lighted by white light, it is absorbing the other two colours, green and red. In similar fashion green and red filters will pass their own colour, reflected from objects which are absorbing the two colours and passing on the third. In actual practice it is not as straightforward as this, because there is seldom a case of a simple colour to be considered. Combinations of colours may be split into their primaries, and the correct filter ascertained from the knowledge gained. The study of light and colour is very interesting and I would recommend photographers who would like to specialise in flower photography, or where the monochromatic registration of colours is important, to buy one of the excellent booklets on the subject. Ilford Lttl. publish Panchromatism at 6d. or 8d. post free. Kodak Ltd: publish The Photography of Coloured Objects at 2S. 6d. The latter is a fairly full treatise on the subject. With panchromatic film, whilst there is less need to use a filter for general work, a K.2 filter will help_ considerably to record those cloud effects, which make a landscape so picturesque. A light green filter will also do the same work. So far only filters for correction have been mentioned. There are also uses for filters to increase contrast, and to provide dramatic effects to an otherwise normal scene. An orange filter " G." will slightly over-emphasise clouds in a blue sky, whilst a light red filter will turn a few clouds into an impending thunderstorm. Be careful in the use of filters for _ excessive contrast, for not only will the sky blue be altered but also the colours of grass, tiles on a roof, flowers and foliage. For effects wish filters, I will advise, if letters are addressed c/o The Editor. Please put me in possession of the full facts, and describe exactly -what it is desired to record.
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