When the filter is placed between the subject and the lens, it is sufficient to consider the point P' (Fig. 89) as one of the points on the object, and it will be clear that, after passing through the filter, the rays will seem to come from the virtual point P. which is nearer to the lens than the actual point on the object F. Thus, the whole object is brought virtually nearer the objective, and the amount of displacement is equal to about one-third of the thickness of the filter.
The image is formed farther from the lens than before interposition of the filter. If an object in the plane focussed on is reproduced on a scale of Tint, the increase in extension of the camera will be equal to the thickness of the filter 2 divided by a displacement which is negligible even for the thickest screens, when the object being photographed is not very near the lens.
In addition to the displacement of the image resulting from the interposition of a filter, the image is affected even if the filter is optically perfect ; these various aberrations, however, are fortunately small enough to have practically no disturbing effect.' Cheap commercial filters are mounted be tween ordinary plate glass, the faces of which are usually neither plane nor parallel, and their use instead of optically perfect filters means that the image may suffer considerably in quality, especially when such filters are used with a lens of great focal length and relatively large aperture. When this is the case, plain gelatine screens are to be preferred to filters of mediocre quality.
If the filter faces are parallel only one image is seen when looking at the reflection on the filter of distant objects, the line of vision being preferably glancing along the surface of the filter. If the faces are not parallel two separate reflected images will be seen.
121. The Best Position for Mounting Light filters. A misconception which is frequently entertained is to suppose that the efficacy of a light-filter can vary according to its position in the beam of light. A filter always absorbs a definite proportion of each incident radiation, whatever the intensity of the light or its area of cross-section at the point where the filter is placed. For instance, if a light-filter, used near the lens, cuts the beam of incident light at a cross-section twenty times less than would a filter placed in contact with the sensitive plate, and if the quantity of light incident on the filter is therefore twenty times greater, from each of these radiations which pass through it, it will absorb a quantity twenty times greater per square centimetre. However, the total quantity
of the whole beam of light absorbed will be the same, and the selective effect will be exactly the same.
A light-filter may be placed (a) between the source of light and the obj ect to be photographed; (b) between the object and the lens of the camera, and in this case it is usually mounted on the lens ; (c) between the lenses of the objective ; (d) between the lens and the sensitive plate, adjacent to the lens ; (e) in front of the sensitive plate, almost in contact with it.
Position (a) , generally used in micrography, has sometimes been used for the three-colour reproduction of Autochromes. But it is difficult to imagine a studio glazed entirely with light filters.
Position (c) should be rejected on principle, except when using gelatine screens of negligible thickness, which can be placed against the iris diaphragm after unscrewing one of the com ponents of the objective. Every filter of appre ciable thickness, being equivalent to two-thirds of its thickness of air, would produce very nearly the same effect as if the separation of the com ponents of the lens had been reduced by a third of the thickness of the filter. This would seriously interfere with the definition unless the filter formed an integral part of the lens and was placed in position, with due regard to its effect, by the optician.
In addition to the fanciful argument to which we have done justice at the beginning of this paragraph, the fact that a filter, when used close to the sensitive plate, can be of mediocre optical quality without disadvantage, has been put forward in favour of position (e). Unfor tunately, any local defect in such a screen manifests itself on the image by a spot. Further, a " focal-plane filter " of indifferent quality is at least as expensive and immeasurably less workable than a " lens filter " of satisfactory quality, or, better still, a plain gelatine screen.