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"Print So Fine" paper developer
"UC" wrote in message oups.com... All films have slopes that vary over the scale of the negative. 'Separation' is just another term for 'steep gradation'. The slope at any point of a fil'm's curve is an indication of the contrast in that part of the scale. If the slope in the mid-tones is low (e.g., TMY) mid-tone separation will suffer. The slope of TMY is higher in the upper end of the density scale than in the mid-tones. Theerfore, highlight separation with TMY is greater than mid-tone separation. With Tri-X, it is just the reverse. TMY: http://www.kodak.com/global/en/profe...002_0507ac.gif Tri-X: http://www.kodak.com/global/en/profe...009_0490ac.gif In general I have to agree with this. The relation between the tones of the original scene and the tone reproduction on the print are controlled by the response of both emulsions. The response is what is shown in the characteristic curve, sometimes called the H&D curve. These curves vary from paper to paper and from film to film. Note that overall contrast is shown by the slope of the curve. High contrast materials range from their minimum to maximum densities with small changes in exposing light, low contrast materials require a large change of exposure for the same density variation. However, the _relative_ values depend on the distribution of sensitivity of the halide particles, which is what the _shape_ of the curve reveals. A demonstration can be found by comparing two Kodak films: 400T-Max and ISO 320 Tri-X. T-Max is a relatively "straight line" film with a short toe (definition of toe is below). The ISO-320 version of Tri-X has a very long toe, in fact the curve is upward deflected all along its usable length. The difference in tone rendition will evident by thinking about the effect of this in relation to the original scene brightnesses. If you match the two curves for equal contrast index and for the same shadow and highlight densities, you will find that the Tri-X has less density for the mid-grays, so it will reproduce them darker than T-Max. This is sometimes desirable. One can find similar differences in paper curves except that a long toe on paper will result in brighter mid tones. In general, developers have little effect on the shape of the curve. However, additives like Potassium bromide does affect them. Bromide tends to suppress the very lowest exposures, so it has the effect of reducing the usable toe area. Another way of stating this is that it increases the relative contrast of the toe of either film or paper. The effect on film is to suppress fog and, if much is used, to lower speed. On paper the effect is also to reduce fog and to brighten the highlights. Specific developing agents do not have much effect but overall contrast of the paper can be varied a little (for some papers). The variation possible is not nearly so great as for film because film for pictorial use (as opposed to graphic arts use) is developed far short of its maximum possible contrast. Prints, OTOH, are usually developed to their maximum contrast, or at least, to reach their maximum densities. Some so called low contrast developers are merely slow and do not reach this density in normal development times (or sometimes ever). Some developers are selective in a way that does affect overall contrast but the range is limited, certainly less than one paper grade. Again, except for adding bromide or Benzotriazole, there is little effect on curve _shape_. BTW, neither variable contrast or graded papers have consistent curve shape throughout their contrast ranges. In most cases one grade different negatives can be printed to have identical tone rendition by changing the paper grade one step. The same with printing the same negative on a condenser and a diffusion enlarger, but larger variations may show some differences in mid-tone reproduction. Contrast and image color. Image color of prints (and negatives too) depends largely on the scattering of light in the emulsion. This is in turn controlled by the particle size of the silver making up the image. Extremely finely devided silver looks bright yellow (its used as the filter layer in Kodachrome). As it become coarser it shift toward blue. Since the grain size of the image is somewhat due to the developer very active developers that tend to produce coarse grains also tend to produce bluer colored images. Less active developers, for instance Kodak Selectol Soft and Ansco 120 (nearly identical) tend to produce finer grains and warmer images. They also tend toward lower contrast and slower development. Tone Seperation. I am never quite sure what is meant by this. presumably the ability to detect small variations in brightness. This is partly affected by the resolution of the image. Blurry images do not have good separation of anything including tone values since they tend to blend into one another. Partly, it is due to so called local contrast. At the highlight and shadow ends of a print the contrast is lower than in the mid gray area regardless of the paper, film, and developers used. This is because the toe of the film is affecting the shadow area contrast and the toe of the paper is affecting the highlight area. Often, when photographers complain of lack of tone separation in highlights (blocked highlights) its because they are trying to print a greater range of brightness than the paper is capable of reproducing. The only solution to this is burning in, either by hand or by means of a contrast mask of some sort. If variable contrast paper is used the blocked areas can be printed in using a lower contrast filter (with masking). The eye wants to see contrast similar to the original scene so simply lowering the contrast of the reproduction won't look right. As far as any developer claiming to modify the paper curve shape (or film for that matter) the proof is in properly done sensitometric testing. If there _is_ an effect it will show up plainly on an H&D curve. There is a lot of room for errors to creep in this sort of testing so one must be wary tests which are not very carefully designed and controlled. -- --- Richard Knoppow Los Angeles, CA, USA |
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