Focal Length.—An important and much disputed point is the focal length of the lenses. Roughly speaking the focal length should not be shorter than the diagonal of the plate to be used, and even then, with such a short focal length as this represents, it will be necessary, should it be desired to take a large-scale portrait, to bring the camera so near to the sitter that the peculiarities of faulty perspective will appear in the picture. We need not in this place discuss what those peculiarities are, or explain the reason for their existence. Suffice it to try an example to prove that, though the position may have appeared quite natural to the photographer, the drawing will be so ridiculously exaggerated as to destroy any chance of a satisfactory rendering. At the same time it should be pointed out that the peculiar perspective is only due to the wrong use of the lens, and not to any faulty quality inherent in it. It is produced simply by the lack of distance between the camera and the sitter, in relation to the scale of the portrait. Of course, if the idea is to enlarge the subsequent result, it is quite possible to use a lens of quite a short focus, but where, as will be mostly the case, the resulting negative is intended to be used direct, a lens of not less than double the diagonal of the plate should be used and this will be the length for head and shoulders and half-length portraits ; for large heads the focal length of the lens should be even longer than this.
This faulty drawing caused by the use of short-focus lenses is the reason why so many amateurs' portraits are unsuccessful. From what has been said it will be seen that it is desirable to have at least two lenses of different focal lengths.
The Plate.—The plate used for portraiture must naturally be of a high speed and should be capable of giving a soft or strong negative in response to its treatment. It is particularly necessary that the plate shall be able to render the subtle tones of fine modelling in the human face which can give to the photographic portrait its peculiar charm, and which places in the hands of the pictorial portraitist such tremendous advantages over the painter artist.
Delicate drawing and expression of tones are injuriously affected by the defect known as halation, and this may be shortly described as the double image caused by reflection of the rays from the glass side of the plate. This defect is especially strong if the lens be directed against the light. It frequently happens that it is necessary to use an arrange ment of this character and indeed many charming effects can be so obtained. When it is desired to work at any considerable degree against the light, we must employ " backed " plates in order to absorb the reflected light, and thus preserve the delicate gradation of tones.
Orthochromatic Plates.—The use of the orthochromatic plate for portraiture is to be highly recommended, but at present the longer exposure necessitated by the yellow screen discounts the value of this class of plate to some extent. In theory, of course, the plates are almost in dispensable ; the rendering of the tones and colour values is more correct, and it is one of the chief charms of a picture when these values are correctly rendered. The hair, for instance, has in each case not only a definite colour and tone value according to the personality of the sitter, hut its own tone value is different from the flesh tones.
Only if we are able to correctly translate the relation of the tones, after allowing for the absence of colour, do we get a lifelike effect ; and only thus does the flesh appear as real flesh, and the hair as real hair. The correct representa tion of tone values is the basis upon which all the claims on behalf of photography have been made by artist photo graphers. No graphic art can hope to rival photography in this rspect, and it is important therefore to develop the capacity to its highest expression, and to this end the plate must be capable of giving a long range of gradation. This is not the place to discuss the authentic difference between the ordinary and the orthochromatic plate. It will be sufficient for the purpose to mention that the former is vastly more sensitive to blue and violet than to the warm green, yellow, and red rays, and, as is well known, white light is composed of certain proportions of all the rays. It results therefore that a plate which is only sensitive to a portion of the rays cannot give an entirely true rendering, and the effect would be still more inaccurate were it not that coloured objects reflect not only their own colour but a proportion of white light as well.
The plate manufacturer has great difficulties to overcome before a highly sensitive and harmoniously working ortho chromatic plate of good keeping quality can be placed upon the market. But the time cannot be far distant when the demand for such will produce the supply, and the difficulties presented by the combination of high general sensitiveness with harmoniously working colour sensitiveness will be satisfactorily realised.
There are a few well-known and old-established makers, such as Messrs. Wratten and Wainwright, who even now produce exceedingly valuable material in this direction.