The greater the magnification, the more clearly the sharp boundary between black and white is outlined at their meeting point or "flowing” these values into the gray intermediate zone.
However, if the assessment of resolving power (although very imperfect) is possible even under average laboratory conditions using test charts, the measurement of the degree of contour transfer requires complicated and very expensive apparatus. Under average laboratory conditions, it is only possible to find a more or less marked separation of white from black with a sharp line, but it is impossible to measure the degree of this contrast.
Factories have such equipment, but they generally do not publish the results of the study, and they do not announce the results of their resolving power measurements, because these data are of little help in practice.
Both measurements are made on tables, on which the resolving power is indicated by the number of lines in millimeters on the abscissae, and the ordinate indicates the contrast transfer capacity in percent. 100% would mean the maximum, perfect contrast, at which black reflects absolutely no rays, white is reflective 100% rays, co, Of course, is not achievable in practice.
The measuring device automatically plots both curves on such a table, from which the relationship between resolving power and contrast transfer power can be read.
Of course, even the best lenses of the same series show minimal differences in the curves of both sizes and only the measurement of a dozen or even several dozen lenses of the same type will determine their average quality. Also metering at different apertures and at different locations (in the middle of the field of view, on the banks) sometimes gives very different results, because everyone, even the best, the lens gives maximum sharpness in the center of the field of view and with a smaller aperture. Generally accepted, that the best results are given by lenses with a resolving power of approx. 40 line on 1 mm and a contrast transfer capacity of approx. 40—50%. Increasing one of these values causes the other to decrease somewhat. Test charts usually give four numbers, namely for: 1) the full aperture of the lens, 2) aperture reduced by two stops, 3) rays passing through the center of the field of view, 4) rays passing at the edge of the field of view.
What does this mean in practice? For example, if we take photos. steel engraving with very thin strokes, a lens with a higher contrast transfer capacity produces an image, which seems sharper than with a lens with higher resolving power. The difference to the disadvantage of the first image will occur only in the case of a significant enlargement of a small part of the photo, because fine lines will turn into thick strokes. Higher contrast generally produces better results, just don't mix the contrast produced by the lens with the contrast produced by the use of hard paper, because there the subtle lines will simply stay ,,eaten”, which will also be revealed when magnified significantly.