Lens Contrast (Part 9 : Flare And Glare)
Taken with a Nikon D50 and AF Zoom-Nikkor 70-300mm f/4-5.6G lens
So far I've mentioned overall contrast and lens contrast. The final type of contrast we have to deal with is something still different from either of the two definitions above, and this is "local contrast", or tonal differentiation within certain specified tonal ranges. A film/paper combination whose characteristic curves interrelate in a certain way can yield high highlight contrast (i.e., not much tonal discrimination in the highlights, but a greater sense of "snap" in the gradation you do see) and low shadow contrast, or good shadow contrast and low highlight contrast. In lenses, local contrast issues are accounted for mainly by flare and veiling glare, and are affected mainly by lens coatings. A lens can have exactly the same level of overall contrast (i.e., it will transmit the same overall range from light to dark), but it might have much worse shadow contrast, for instance, in certain real-world situations. Meaning, there will not be as much separation between slightly different shades of gray in very dark areas of the picture. (Transmission of color is also very much affected by the efficiency of the coatings and the relative contribution of flare.)
The big question mark where local contrast is concerned is that almost all actual picture-taking situations allow flare and veiling glare (the latter an overall dulling or haze of the image similar to "flashing" an enlargement with a low dose of non-image-forming light, or fog) to contribute in varying amounts and varying ways. Despite lots of scientific research, there still seems to be not much way to quantify it exactly, or predict its contribution exactly with any given system ("system" meaning camera-lens/film/enlarger-lens/paper) in real-world situations. Flare is always present to at least some degree, but it is seldom present in exactly the same way in two different systems encountering two different situations.
Before lens coatings were invented, lens flare was a major determinant of image quality. The best lenses were generally the ones that allowed performance to remain high with the fewest elements, because there were fewer air-to-glass surfaces to create flare. This explains the lifespan of the exceptionally long-lived Tessar-type, despite its speed limitations. Lens coatings are of critical importance to modern lenses; virtually all zoom lenses and many highly-corrected multi-element lenses would be useless for general photography without them. Often, coating is what makes the most difference between an average lens and a very good one.
Have you ever noticed how many early 35mm photographers tried to avoid bright sunlight? You might be forgiven for thinking that the decade of the 1940s was entirely overcast (and not just by the world political situation). With experience as their teacher, many photographers in the '30s and '40s learned various clever ways of avoiding or minimizing high-flare situations. The amateur admonition to "never point the camera in the direction the sunlight is coming from" dates from this era. Such was life with "miniature" cameras before the days of multicoating.
Labels: AF Zoom-Nikkor 70-300mm f/4-5.6G, Animal, Nikon D50, Photography
