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.

Tuesday, 6 March 2007

Border Control

The above image of Arches at Rievaulx Abbey is printed at 16”x8” on 16”x12” paper. The wide left and right borders, and the lack of borders at top and bottom, greatly emphasise the tall slim format of the image. This arrangement reinforces the image subject. Any other border arrangement considerably weakens the impact of the image. The arrangement of the image on the printing paper is a key concern in the image management stage of the printing process.

Fixed-border easels are cheap and convenient, but the regularity they introduce to the finished work is quickly tiresome. Also, some images require cropping in order that the visualised shape of the image can be achieved given the shape of the negative that the camera produces. A bladed easel provides an important degree of artistic freedom.

In a darkroom it is natural to start the printing process by selecting the image size, crop, and bordering. The boundaries of the image on the paper have to be set before any other printing activities are executed. In a digital workflow the opposite is often true. Images are cropped on-screen in a space that is anonymous with regards their ultimate hard-copy use; if there ever will be hard-copy produced.

In the darkroom, a rough crop is established. The borders are then balanced, and the fine crop selected. When balancing the borders the image shape may be adjusted, so that the border widths are not arbitrary. In the digital workflow selecting the fine crop is the first step, with little regard to the relationship between the shape of the image and the shape of the paper.

In the darkroom, the first crop is determined by the internal composition of the image and the fine crop is determined by the shape of the image on the paper. In the digital workflow, the crop is determined by the internal composition of the image only.

In fact, the very first crop occurs in-camera, and there are some schools of thought that say this should be the only crop required. That viewpoint, though, is somewhat restrictive for medium and small format photography. Certainly the starting point should be to achieve full-frame crop in-camera – but circumstances may not permit this. For example, the photographer may choose to include more foreground than is desired in the end-image in order to prevent the need to tilt the camera.

In most cases the image management process in the darkroom is a matter of finding best fit between the image shape as set by the negative format and the shape of the chosen paper. The shape of an area (such as the negative, the image, or the paper areas) is described as the ratio of the area’s longest side to its shortest side. The shape (ratio) of popular negative formats is given in Table 1.

Width Height Ratio Film
6cm 6cm 1 120/220
7cm 6cm 1.167 120/220
5" 4" 1.25 Sheet
10" 8" 1.25 Sheet
4.6" 3.5" 1.3 Polaroid 55
6cm 4.5cm 1.333 120/220
7" 5" 1.4 Sheet
9cm 6cm 1.5 120/220
36mm 24mm 1.5 35mm
        Table 1. Popular Negative Shapes

Table 2 shows how well each negative format fits popular paper sizes. Where the shapes are the same the table indicates ‘Fit’. Otherwise the table shows by how much the image is over-sized in one dimension
when the enlargement is set to fill the other.

        Figure 1. 35mm versus 10"x8", oversized image

As an example consider figure 1, which shows the resulting image area when a 35mm negative is enlarged so that the full height of the image fills the height of the paper. Some portion of the image then doesn’t fit the paper, in this case 1” left and 1” right – 20% of the image area.

        Figure 2. 35mm versus 10"x8", oversized paper

Figure 2 shows the resulting image area when a 35mm negative is enlarged so that the full width of the image fills the width of the paper. Some portion of the paper then doesn’t fit the image, in this case 2/3” top and 2/3” bottom – 17% of the paper area.

Width Height Ration Matches
Negative Format
6x6 cm 7x6 cm 5"x4"
7"x5" 9x6cm 35mm
10 10 1 Fit 17% 25% 33% 40% 50%
12 10 1.2 17% 3% 4% 11% 17% 25%
24 20
10 8 1.25 20% 7% Fit 7% 12% 20%
20 16
12 9.5 1.26 21% 7% 1% 6% 11% 19%
14 11 1.27 21% 8% 2% 5% 10% 18%
8.5 6.5 1.31 24% 11% 4% 2% 7% 15%
8 6 1.33 25% 12% 6% Fit 5% 13%
16 12
40 30
9.5 7 1.36 26% 14% 8% 2% 3% 11%
7 5 1.4 29% 17% 11% 5% Fit 7%
5.875 4.125 1.42 30% 18% 12% 6% 2% 5%
11.75 8.25
5 3.5 1.43 30% 18% 13% 7% 2% 5%
6 4 1.5 33% 22% 17% 11% 7% Fit
30 20
5.5 3.5 1.57 36% 26% 20% 15% 11% 5%
Table 2. Negative versus Paper shape

The green shading in table 2 indicates paper sizes that it is reasonable to use for a given negative format, in most cases.

Essentially, if the paper and negative shapes require less than 10% of the image to be cropped to achieve a fit then the combination will be workable in the main. It is possible to allow for up to a 10% darkroom fine-crop when shooting – considering most SLR viewfinders do not give 100% coverage.

The foregoing may seem overly prescriptive, but the point isn’t to define the ‘right’ shape of paper. Rather, the point is to understand the relationship between the negative and paper shapes. To ensure that enough allowance is made in-camera for darkroom cropping. This is not just a question of providing enough space around the subject. The in-camera composition is impacted by the shape of the desired end-image. There is little point arranging elements at the thirds of a 35mm negative (ratio=1.5) if the end print is to be cropped to fit 10”x8” paper (ratio=1.25) as those elements will no longer be at the thirds when the end-image crop is applied.

The act of producing an end image should follow a technical process that is comprised of methods and materials that support the artistic process. When choosing paper shapes this means minimizing the variance between shapes that are used for a given negative format, so that the amount of in-darkroom crop will be roughly the same irrespective of the actual size of print wanted. A negative crop that works on 10”x8” paper will not work at 16”x12”. In the least the borders will be balanced differently between the small and large print, and that can be enough to change the impact of the print. In the worse case, the borders will be kept relatively identical but the image shape, and thus crop, will vary. The likelihood of finding two different satisfactory crops from a single negative is low when the image exhibits high compositional cohesion; although this is more likely for loosely composed or abstract subjects.

For a preferred printing size of 16”x12” it is more useful to produce smaller working prints at 9.5”x7” than at 10”x8”. The scale-up will be easier as the ratios are closer. Similarly for a preferred print size of 30”x20” a working print size of 11.75”x8.25” would scale up most easily. If the preferred print size is 20”x16” then 10”x8” working prints are ideal, 20% leeway in one dimension will be required of the in-camera crop.

All of the foregoing disappears as a concern if paper is available in an infinite range of sizes, or rather if oversized paper is cut to the required size. There are several disadvantages with such an approach:
  • Paper sizes are not arbitrary. They have developed over time, some shapes have become familiar. Familiarity is an emotive force; to evoke familiarity (or its counterpart) is an artistic choice. The paper can not then simply be cut to the size of the image, an appropriate paper shape should be selected.
  • Within a portfolio images belong together as a set. They need, at some level, to express coherency. Arbitrary changes in print shape can work against this.
  • Additional handling is required to crop the print, which gives further opportunity for flaws.
  • Cropping the paper in order to maintain image ratio when scaling up or down does not resolve the issue. At some point the images may be mounted or framed in a consistent manner, at which point the relationship between the image and its mount or frame becomes the concern.
A bespoke paper size can be the right decision in some cases, but generally is not.

There are six basic means of laying an image onto a surface, as per figure 3.

    Figure 3. Basic Border Types

The simple border type (Figure 3a) provides a symmetric, equally spaced border on each side. The bordering does not interact with the subject. Effectively it serves only to constrain the image. The macro example left suits a simple border because in the subject we are deliberately cutting out the rest of the word in order to concentrate on the flower cluster, this is shown also by the sharply cut flower heads at the top
left and right-hand sides.

If one of the borders is unequal (figure 3b), and this would usually occur along the bottom edge, then the bordering has ‘weight’. This can complement subjects that are weighted, such as a landscapes, as per the example on the right.

If the borders are arranged in a bilaterally  symmetrical" fashion (figure 3 c) then the borders do not only constrain the image, they also add a sense of balance. In the example on the right a sense of balance in the borders helps to prevent any sense that the statue will fall over.

Partial borders (figure 3d) constrain the image on two sides only. This arrangement emphasises height or width. In the example on the right the long sweep of the cove is not constrained left or right, which allows the image of the cove to  have a sense of ‘continuance’.

A borderless print (figure 3e) is constrained by the edges of the paper, but the lack of a formal border gives a sense of expansiveness – the story within the picture continues beyond the edge of the picture, and by inference, the moment of exposure. In the example on the right the sense of expansiveness relates to time and the age of the subject.

An offset print will likely have two adjacent borders of equal size (figure 3f), the other two borders being of a different, but equal, size. This arrangement can complement the image’s arrangement of passive and active space. For example if the image in figure 3f were to show a cyclist heading down a steep hill, the hill running from top left to bottom right, then the bordering effectively gives some space into which the action in the picture can extend. In the example on the left the offset is in the opposite direction. The tombstone is heavily constrained by its thin adjacent borders, whereas the tree has space into which it can grow, or reach.

Of course an arbitrary arrangement where all four borders are of different widths is also possible. This however can probably not be generalised about and must be considered on a case by case basis.

The basic border types can be created with the use of an easel. Masks of various types can be used for more intricate borders. One example being the vignette, which is currently seen as out-of-date. However it is still useful in just the right conditions. The vignette gives a sense of removal from space, time, or reality; it truly allows a subject to stand on its own, as opposed to standing as a reference to some greater thing or idea. There are no rules of thumb in photography that must not be broken. If the vignette is right, then use it.

The final word must be, to experiment. There are many more options for bordering prints, black borders, double borders, masks. Borders are not the inconvenience of inappropriate paper versus negative shapes. They are a key control in the story that the image tells.