Saturday, 11 August 2007

Basic Contrast 2

The previous article on ‘Basic Contrast’ concentrated on fundamental definitions and concepts and considered the effect of a single contrast grade upon the tones of an image. This article explores contrast in greater detail so that it can be used as a creative tool rather than a strict technical requirement.
At a technical level the correct grade for a print is that which most closely matches the contrast gradient of the negative. Knowing the negative contrast gradient however requires an expensive densitometer. It is such equipment that allows automated lab processing to produce acceptable prints. But this is not a route to creating a fine print. It is just another piece of equipment that can help, but has similar limitations to the more humble exposure meter. It is probably a good thing that densitometers are expensive, as this means they are rarely used and so at some point all printers have to come to terms with the principles of contrast grade.
Photography is typically taught as a set of rules, or more usually adages, with the diktat that all rules can – and by inference must – be broken. It is possible to print acceptably by following the ‘expose for shadows, print for highlights’ adage, but this doesn’t provide a basis for understanding. To understand contrast grade it has to be seen in action. Variable contrast (VC) papers are an excellent medium for this, they allow for contrast experimentation while allowing all other factors (negative, exposure, chemicals, temperature, paper batch etc) to be kept absolutely constant.

VC papers are often described as containing two emulsion layers, one being of a soft grade and one of a hard grade. This is slightly confusing as it creates a vision of there being an upper and a lower layer, whereas the two emulsions are in fact an homogenous, single layer, mixture. Soft and hard grade silver-salt grains lying side-by-side throughout. The two grain types are sensitised by different wavelengths (colours) of light, typically blue and green. A high contrast print is achieved by exposure to blue light and a low contrast print by exposure to green light. Intermediate grades are produced by balancing blue and green light in different proportions. Because the emulsions are mixed (not layered) it doesn’t matter if the two exposures are delivered together or sequentially. If the exposures are delivered sequentially it doesn’t matter in what order they are applied. Typically the two exposures are delivered together, through a single filter say. It is possible though to expose only the hard contrast salts with a grade 5 filter and then to expose only the soft contrast salts through a grade 00 filter. The practical impact of this is that a print can be made from two (or more) sequential exposures at different grades. This is the basis of the split-grade printing process.
To build-up an understanding of how contrast grades work the analysis tools within Adobe Photoshop™ will be used. In plate 1 the histograms for two single grade prints are shown.

Plate 1: Print Analysis

For the purpose of comparison the two prints have been made at the same relative exposure (allowing for the different filter factors). Clearly from an artistic viewpoint the grade 0 print is too dark, even though the average (mean) density of the two prints is roughly equivalent – as indicated in the histogram statistics. The median densities are significantly different, due to the change in contrast. The standard deviation (SD) is an expression of how the histogram spreads out from the mean. The high SD at grade 5 states that most of the tones lay a long way from the mean-point, as might be expected from the tonal scales shown in the basic contrast article. Median and SD are convenient measures of contrast and will help in confirming the fine-scale adjustments that split-grade printing can introduce.
Such strict analytical methods are of limited value in day-to-day darkroom work, but do provide a very compelling view of the effect of contrast shifts. It is a worthwhile exercise to produce such a comparison as it clearly reveals the nature of differing contrasts within a particular darkroom process. The method is relatively straightforward:
  • Produce the comparison prints at the same exposures, allowing for any difference in effective paper speed if using filtration for contrast shifts.
  • Scan each print ensuring that any ‘auto tone’ is disabled during the scan and that the black, mid, and white points are set to the same value for both scans.
  • Call up the histogram view for the first print and take a screenshot (Ctrl+PrtScr on a Windows™ machine)
  • Put the screenshot into an image file by selecting File->New in Photoshop, the image shape will default to match the screenshot which is on the clipboard.
  • Select Edit->Paste to place the screen shot into the new image.
  • Crop the screenshot image to include just the histogram. Repeat for the second print so that the two histograms are available in two image windows. These can now be saved as files.
  • To overlay the histograms for better comparison drag the base layer from one histogram image into the second, lining the two histograms up. Set the blend mode for the (new) second layer to ‘Linear Dodge’ and the opacity to 50% so that the two histograms merge in a way that reveals the shape of both.
Now that a means to compare the contrast of two prints has been established it is possible to look at the split-grade method in detail and to demonstrate the effect in a non-subjective way.
The opening image in this article (Snowy Owl) is a good example for showing the application of split-grade printing as it exhibits clear contrast difficulties. Very fine highlight detail exists in the ‘white’ face feathers, but good tonal separation is wanted to emphasise the patterns on the back and wings. The face suggests a very low contrast grade whilst the rest of the image suggests a much higher grade.

The final image was produced with grades 1 and 5. The exposure at grade 5 was chosen such that this grade would not interfere with effect created by the grade 1 print. The grade 5 test strip was made across the face and the last strip at which little or no detail could be discerned was selected for the grade 5 time. The grade 1 test strip was also made across the face, this time the strip selected was that which gave the desired detail without introducing too many greys into the face. Plate 2 shows these two opening exposures.

Plate 2: The Initial Low & High  Grade Exposures

These two exposures were then combined into a single print, as at the head of this article. In this application the split-grade printing method has been used in order to quickly derive the required contrast range.

Instead of deriving the grade by trial and error we have considered how the properties of two, extreme, grades can be mixed to achieve the visualised result. Firstly we asked, what is the maximum exposure at a very low grade that will give the desired highlight exposure? Secondly we asked, what is the minimum exposure at a very high grade that does not interfere with the highlights? Plate 3 shows the histograms for the two exposures overlaid and the minimum interference between the exposures is evident. In effect instead of trying to resolve one complex question (which grade to use) the split-grade method applied in this way has allowed us to approach the print in terms of two simple questions (the exposure questions just stated).

Plate 3: Overlay of low & high exposure histograms

The two exposures were both made for 16 seconds at f/5.6. Due to the difference in the paper speed at the different, filtered, grade levels this means that the print received effective exposures of 16 and 8 seconds for grade 1 and grade 5 – the paper is half the speed when filtered for grade 5 than grade 1. So two-thirds of the exposure was given at grade 1 and one-third of the exposure was given at grade 5. The contrast ranges at these two grades are 130 and 40 respectively (from the manufacturer’s data). The contrast range of the final print then is:

Contrast Range 100 is equivalent to grade 2.5 (again, from the manufacturers data). The final split-grade print then is similar to a single grade print made at grade 2.5 with a 24 second exposure. It is unlikely that the differences between the split and single grade print will be apparent in reproduction, so Plate 4 shows the histograms arising from such prints. The single grade print is to the left and the split-grade print to the right.

Plate 4: Single vs. Split-Grade Histograms

The two prints are remarkably similar, but they are not identical. The peaks in the histogram of the split grade print are slightly more pronounced, indicating that the tonal separation is slightly sharper. It may very well be extremely difficult to look at a print and spot this effect, but that in itself is not reason to discount the method. The finest prints are built up from a series of steps that individually cause difficult to discern differences, but when all the tools at the printer’s disposal are used together the cumulative effect is significant.
It has been shown that the split-grade technique can help to find the appropriate grade by working with the way in which extreme grades can be mixed without interference. It has also been shown that the split-grade technique can yield equivalently graded prints to those made from single step exposures, but the split grade variants can exhibit greater ‘definition’, achieved through enhanced tonal separation.

Other applications of the split-grade printing technique are:
  • Add punch to the very lowest tones of any print by making 10 to 20% of the
    overall exposure at the maximum grade.
  • Add detail to very delicate highlights, without forcing those highlights to weak
    grey, by making 10 to 20% of the print at the minimum grade.
  • If necessary use three exposures to achieve both of the above.
  • Apply an effect selectively. E.g. Diffusing shadows only – adding a print
    diffusion filter during short high-grade exposure will provide focus
    softened shadows with little or no effect on mid and high tones.