Archived posting to the Leica Users Group, 2000/11/12
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]> DOF is perceived sharpness. The key word is perceived. There is only a > single plane which is sharp in a picture and that is the plane which the > lens was focussed on. Everything else is more or less unsharp. Martin, Your comments are the best in this thread yet IMO, and without contradicting you I have one amplification to contribute. "There is only a single plane which is sharp in a picture" may not always be true simply because the plane of best focus (p.o.b.f.) itself may not be rendered sharply. The "perceived unsharpness" of objects farther from the plane of best focus is also dependent on the perceived _comparative_ sharpness of those objects to the perceived sharpness of the p.o.b.f.--i.e., how much differentiation the mind of the viewer registers. For instance, when the plane of best focus is not particularly sharp, say because of camera shake or because there are no image objects on the plane, perceived d.o.f. actually appears to be _greater_, because more of the unsharp area comes closer to the standard of the plane of best focus. This helps explain a seeming conundrum, which is that perceived d.o.f. becomes _worse_ as the plane of focus gets _better_. When there is some visual cue linking the p.o.b.f. to the o.o.f. areas--such as graininess--perceived d.o.f. is also better. To demonstrate this, print a picture straight and then print the same picture through some sort of texture screen. The "sharpness" in the print made with with texture screen suffers, but the perceived d.o.f. seems to improve. You really have to have this demonstrated to believe it, but it's an impressive demonstration. Just as d.o.f. is entirely a matter of perception, so is "sharpness," and this has been proved repeatedly by empirical means. Various contributory technical properties of "sharpness" can be described rigorously, but viewers will still sometimes describe an image as "very sharp" that does not have these technical properties, and as "unsharp" certain images that have the given technical property in abundance. One specific example of this is that in pictures with lots of front-to-back image objects, viewers decisively label as "sharper" those prints with more d.o.f. even if the p.o.b.f. is much worse, to prints where the p.o.b.f. is much better but the d.o.f. much worse. Another specific example I can give is this: the Leica 35mm Summicron-R has very high large-structure (i.e., 5 lp/mm) contrast but not terribly good fine-detail resolution. The Pentax 43mm has the opposite--something which can be made obvious by simply putting a loupe on the fine detail. Yet in comparing prints made with the two lenses, people consistently prefer the images of the Leica lens because perceived "sharpness" is higher, even though actual recorded fine detail is worse. Another example is that people will consistently prefer photographs in which the objects are what is called "in drawing" (presented arranged so that their structure is comprehensible from their surfaces) and well lit, to pictures in which the optical imaging properties are superb but the structure of the objects is confusing because of poor arrangement or confusing lighting. This is what Ted's referring to when he says "Quite often I do nothing more than walk around the room looking at my hand to see how the light appears when I hold it so it imitates the shape of the head, as in I make a fist, then I determine where the subject should be placed and away we go! :-)." Pictorial photography begins and ends with perception. This is what Ctein means when he says "if you can't see it in the print, it doesn't count," and the same thing Ted is referring to when he says "I'd much rather folks were looking at my pictures for the content and impact" and many other of his comments along the same lines. KISS yes, but "it's the picture, stupid," is also true. If it looks good, it is good. If it doesn't, it ain't. Perception is the whole ball game, not only in the meaning of the subject, but in pictorial photographic technique, too. I'll give a specific example in the next post. I've unpacked thousands of books since my move, but maddeningly there are still far too many in boxes. I simply can't find the references on "image gestalt" I need to tell anyone where to go to get more information about d.o.f. and perception. However, the person I know of who's done the most work in this area is Dick Zakia, professor emeritus of the School of Photographic Arts and Sciences at Rochester Institute of Technology. My memory is that he's written several good, understandable books for laymen on the subject, although it's been years since I've read them. - --Mike