13 - Flat copy

Summary

The photography of such things as wall and easel paintings, drawings, plans and even stained-glass windows, has much in common with that of inscriptions, except that the depth of field is rarely of concern, and evenness of illumination, flatness of field, and the recording of subtle tonal differences, becomes of more importance.

Camera lenses, whether for 35 mm or for larger formats, are normally constructed to give an optimum image in terms of sharpness of detail and evenness of resolution and illumination when focussed at or near infinity. For any sort of copying, however, the focussed distance is very much less than this, and many taking lenses will not give an even image of a flat copy at such short distances. Usually the quality is improved if the lens is stopped down to fl6 or less, at the cost of long exposures and some loss of overall resolution. Lenses specially designed for photo-mechanical reproduction (process lenses) are of relatively long focal length, small aperture (often they have a focussing aperture of about f 11 but a maximum taking aperture of f22 or less) and are highly corrected for distortion and curvature of the field. Those designed for colour reproduction (apochromatic) are also corrected to give a minimal difference in the points of focus of the spectral colours. Such lenses are expensive, and work within a limited range of magnification, and they could hardly be justified unless a great deal of flat-copy work were to be undertaken. Copying lenses are less completely corrected, especially for colour, and are often, in fact, enlarging lenses in different mounts.