Sunday 12th March 2006 4:03pm
'Contrast' can be confusing because the term is used loosely to consider different things. At its simplest level 'contrast' describes the difference between the lightest white and the darkest black that a medium can produce.
Figure 1. Simple View of Contrast
Figure 1 shows the simple view of contrast. The left hand pair of boxes depicts a high-contrast medium and the right hand pair of boxes depicts a low contrast medium. This absolute contrast range is determined by the properties of the print paper selected, and may be adjusted by dyeing and toning techniques.
In general, in the darkroom, the entire range of tones is desired in the print. This means that, for a given paper, the vast majority of prints will have the same contrast, from the simple view. They will all contain some areas of the lightest-white and some areas of the darkest black.
The look of the print can be radically altered by the chosen contrast grade. 'Grade' is the root of 'gradient', and contrast grade is describing the rate of tonal change that the paper exhibits in response to exposure.
Figure 2 depicts low and high contrast grades. Note how the higher grade reaches the high tones faster, the entire tonal range has been compressed. In comparing the bottom and top strips we can see the effect of a grade shift from a low to high grade. Some of the low tones shift into the low-mid tone. Some of the low-mid tones shift into the high-mid tone. In this example all of the high-mid tones become high tones.
To relate the above illustration to actual prints consider the following 'meanings' of the four tones shown:
The high tone represents highlights without detail.
The high-mid tone represents highlight details.
The low-mid tone represents shadow details.
The low tone represents shadows without detail.
If two prints were made at these illustrative grades the effect on the print of the grade-shift from low to high contrast grade would be:
Overall the print will look lighter, because more of the higher tones are created.
Shadow detail will be easier to discern since some of the low shadow tones (without detail discernable) have shifted into the low-mid tone (where detail is discernable).
Some highlight detail will be lost as the high-mid tones have shifted into the highest tone (where detail is not discernable).
As a rule of thumb, lower contrast grades improve highlight detail and higher contrast grades improve shadow detail.
The above is just one example. The actual effect on a print of changing the contrast grade depends on the nature of the tonal range in the negative and the exposure time used for the print. Figure 2 considered only the tonal properties of the print as they change in relation to the grade of the paper. Plate 1 shows a practical demonstration of the effect on tonal range of contrast grade. This was produced using Ilford Multigrade IV RC Deluxe.
Plate 1. Grade Test Strip
The compression of the tonal range produced at higher contrast grades is clear in Plate 1. Producing such a grade test strip is useful as the process followed has a certain rigour.
Darkroom work can be very intensive. Concentration on the minute detail of a given printing task can have a negative effect upon the printer's ability to step back and see the image as a whole, and how it currently relates to the visualised end print. Achieving balance in the two approaches is critical.
Freedom of expressive thought is attained when the mechanical aspects are controlled by defined, rigorous, methods. Taking time out in the darkroom to experiment with materials in the simplest way to builds experience quickly. The full impact and resonance of the results of any such test may not be immediately apparent. However the memories related to performing a test will assist in recognising the influence of the material's behaviour on later printing tasks.
Theory and printed examples can not achieve this.
To produce a gradetest strip on variable contrast papers:
Set enlargement height to maximum and aperture to minimum
Place low and high grade filters side-by-side on top of the printing paper - so they are in contact with the paper
Expose the entire sheet for 1 second
Cover a portion of the sheet so that the top edges of both filters are covered over. Stop down the lens 1 stop and expose for 1 second
Repeat for each lens stop, progressively covering more and more of the filter-pair.
Repeat at maximum aperture doubling the exposure time for each step until the filter-pair is entirely masked.
Using Multigrade a 3-way grade test strip was achieved by leaving a gap between the low (0) and high (5) grade filters, as Multigrade prints at grade 3 with no filter, as per figure 3
Figure 3. Grade Test Strip Method
This method takes no account of differences in paper speed at different contrast grades, but it is adequate to express the impact on the paper's tonal range (or more accurately, rate-of-change of tone).
Plate 2. Example at Grades 0, 4, and 5
In Plate 2 the middle print, at grade 4, is closest to the visualised image. The water is inky and the main rock tones are rich. In the left-hand print (grade 0) the background in the upper left corner is not separated from the main rock enough also the 'definition' suffers. In the right-hand print (grade 5) the tonal range is clearly over-compressed.
In the Plate 2 example a base print was made at grade 4 and this was compared to prints at grades 0 and 5. Because the effective paper speed changes with the contrast grade filters used the exposures were adjusted for the comparison prints. This is not the case when a variable contrast enlarger head is used because then the contrast grade is adjusted by adjusting the proportion of two light sources whilst keeping the overall illumination constant. When changing grades by filtering a single light source the over all illumination is changed depending on the strength of the filter. By adjusting the exposure time in relation to the filter used the three example prints were effectively given the same exposure.
Yet there is a sense of 'lighter' and 'darker' between the prints.
Both the grade 0 and the grade 5 print require more overall exposure than the grade 4 print to approach the same subjective feel. This clearly demonstrates the complexity of contrast adjustments. There is no simple method to calculate the change in exposure required when changing grade.
Figure 4. Behaviour of MG IV at grades 4 and 5 (refer Ilford)
Figure 4 compares the tonal output (density) achieved for a range of exposure inputs for Multigrade at grades 4 and 5. It shows that grade 4 produces lower densities than grade 5 for high exposures. Grade 4 also produces higher densities than grade 5 at lower exposures, and there is a cross over point. This bears out the findings from the grade test strip. The steeper, grade 5, curve compresses the tonal range. In shifting from grade 4 to grade 5, the grade 4 tones slip up or down the curve (towards the shadows or highlights) depending on which side of the cross-over point they started from.
If the predominant tone of the print lies below the cross-over point then the whole print will appear lighter after the contrast shift (remember, the graph is 'upside down', highlights - low density - are at the bottom). Similarly, if the predominant tone in the grade 4 print is quite dark - above the cross-over point - then the grade 5 print will appear darker. This is why an exposure change is required when a grade change is made.
The amount of exposure change required depends on how far from the cross-over point the predominant tone was to begin with. Because we are dealing with curves, that shift is not linear and so is difficult to compute, even if the starting exposure of the predominant tone is known. Also, each grade (and half grade steps) has its own unique curve and so the exposure shift required is different in all cases. In the darkroom, without image analysis tools and calibrated sensitometry data, a grade shift will usually require a new exposure determination to be made.
Plate 3 shows the earlier example printed at grades 0 and 5 with exposure adjustment.
Plate 3. Grade 0 and 5 prints adjusted for exposure
The grade 0 print is now right in terms of the major tones wanted, however the bright veins on top of the rock and the surf of the water are too muted. The grade 5 print still suffers from too many white highlights.
The final print was made at approximately grade 4.5 with an exposure of 24 seconds. The original grade 4 print had received 30 seconds.
Boosting the contrast by an extra half-grade has improved the separation of tones between the water and the surf, and between the rock and its white veins.
Increasing the contrast though would lower the tonal values of the main rock; since they are already quite dark they will become darker still. Reducing the exposure by 20% kept the overall tonality constant.
In summary, the basic features of contrast are:
The absolute contrast, difference between the lightest and darkest tones, is determined by the paper.
The contrast grade describes the rate at which the paper responds to exposure and delivers its tones.
High contrast grades compress tonal-range and lose detail in highlights, but gives good separation of mid-tones improving shadow detail.
Low contrast grades expand the tonal-range preserving highlight detail and smoothing the mid-tones, but may lack 'definition' or 'sparkle'.
When changing grades the exposure must be recalculated because the paper speed may change and the 'predominant tone' will shift.
If the predominant tone is 'quite dark' increasing the contrast will require a reduction in the exposure, decreasing contrast will require an increase in exposure.
If the predominant tone is 'quite light' increasing the contrast will require an increase in the exposure, decreasing contrast will require a decrease in exposure.