Resolving power and contrast -partial transcription-
(click on thumbnails to see the largest image)"...The resolving power is not as important a criterion for image quality as is generally believed. This statement will be proved in the following paragraphs. A number of photos were taken with perfectly uniform image quality over the entire field, so that it is not necessary to balance center sharpness against edge sharpness. Neither do these pictures show any visible vignetting nor distortion. We may, therefore, use these photos without any reservation for comparative image quality tests. Let us first consider photos 1 and 2. They are both of poor quality. If you had to choose, however, which of the two would you prefer? At first glance, you would probably select photo 2. At least, that is what everyone did who saw the pictures up to now. Photo 2 appears to have much higher contrast than photo 1, the latter giving the impression of being fuzzy. However, if you take a close look, you will notice that photo 1 has a considerably higher resolution and better definition than photo 2 which, upon close examination, is rather unsharp. We do not know for which of the two pictures you will finally settle. Your decision will largely depend on your personal preference. In any case, however, you will certainly not find photo 1 so much better as the resolution figure would have it. Actually, the lens resolution figure for photo 1 is twice as high as that for photo 2. Let us now turn to photos 3 and 4. Their image quality, or at least the image quality of photo 4, is much better than that of the pictures previously studied. And yet, it is photo 3 which has the higher resolution. This is easily recognizable in the ornaments of scepter, crown and robe. But unless we take a very good look, we do not realize it. The higher resolution, therefore, is of no consequence for the impression created by the picture. Far more striking examples could be presented in photos of really good image quality. Unfortunately, this cannot be done here because the screens employed in the printing process would destroy the fine detail which we need to prove that a poorer photo actually can have the higher resolution. But with the aid of a trick we can create similar conditions to those en- countered in very good pictures. For this purpose, we need only choose a larger viewing distance, in other words, we observe photos 3 and 4 from a distance of, say, 3 or 6 feet instead of from the normal reading distance of approximately 10 in. There will then be absolutely no doubt about which of the pictures is the better one, and we realize of how little avail high resolution can be. The fine detail reproduced in the photo with the higher resolution can no longer be clearly seen. In other words, it does not matter whether it is resolved or not. The above examples have demonstrated that it is possible to have poor photos show- in high resolution, and good pictures with moderate resolution. Two photos of identical resolution may be entirely different in image quality. Just compare photos 1 and 4 from a distance of about 3 feet. Both pictures have the same lens resolution figure - but what a difference in image quality. It is evident that the resolving power - or at least the resolving power alone - is not the decisive criterion in evaluating the quality of photographic lenses, and this is what we have tried to show. At this point the question may be raised whether the poor image quality in photos 1 and 3 may not primarily be the result of softer printing as compared to the printing of photos 2 and 4. The answer is no because all four examples were taken on identical photographic material and treated alike during processing. There is, of course, a possibility of improving photos 1 and 3 to a certain extent during processing (by using high-contrast photographic material with the added control of dodging, redevelopment etc.), but the improvement will be rather insignificant even with a subject of very few intermediate tones, as is the case in our examples. With true half- tone pictures, the above technique would be practically use- less, because any attempt at improving the image through the use of high-contrast photographic material would automatically lead to a decrease of tonal gradation. But, you will ask, what is it that distinguishes photo 4 with its modest resolution from photo 1 which has the same resolution as 4, and photo 3 with high resolution? What then, if not lens resolution, is a valid criterion for image quality? Our comparative photographs have shown that the image quality is not so much deter- mined by the definition of fine detail as by the manner in which the more easily perceptible, larger structural elements in the picture are reproduced. The more faithful the contrast ratio, the better the image quality. It is obvious that the degree of accuracy of contrast rendition in an image depends on how coarse or fine the respective structural elements of the image are...."
Resolving Power and Contrast
Erich Heynacher and Fritz Kober
ZEISS INFORMATION No. 51