The Copy-Print Process

How to get silver-gelatin prints from inkjet positives

By Chris Woodhouse Back to

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Consumer inkjet printers have become consistently acceptable for photographic colour proofs, but their lack of simultaneous performance in tonal purity, permanence, bronzing, compatibility with gloss paper surfaces, and metermerism is significant enough to deter the discerning monochrome worker. These factors, together with a desire to have complete control over the reproduction process, have prompted many to consider using consumer inkjet technology on translucent media to produce large contact negatives. The limitations of inkjet technology are of little consequence, when their output is used as an intermediate step, on the way to a photographic print.

Comparing Digital Negative Processes

Halftone Negatives

It is interesting to look back on two distinct methods to create a silver-gelatin print from a digital file. In the case of the halftone negative, we have a very repeatable, robust method, which resists later manipulation and requires available imagesetter facilities. The process requires planning and careful execution to ensure the final has the required size and tonality.

Inkjet Negatives

The other is the inkjet negative. Following in the footsteps of Dan Burkholder and others, I evaluated the application of full-size inkjet negatives for contact printing, to create a fine art silver-gelatin print from a digital file. In common with other respected photographers, I was unable to consistently make convincing silver-gelatin prints from an inkjet negative. Unlike alternative print processes, on coated matt paper, silver-gelatin paper has a very high resolution and shows the smallest negative detail. During this research, several clear and white plastic substrates were tried and ultimately rejected as a suitable material for inkjet negatives:

1) Contact prints from clear film, and to some extent, white film, show evidence of their mechanistic origin, revealing regular inkjet dot and mild banding (fig. 1), which can be clearly seen in the final print.

2) White film’s diffusing properties disguise the inkjet dot pattern, but unless the face down negative is in perfect contact with the printing paper, the print resolution degrades to less than acceptable levels (fig. 2).

3) Plastic inkjet films work best with dye-based printers, which limit the maximum transmission density and require a high-contrast enlarger setting, further accentuating the inkjet-negative limitations.

4) It is difficult to make prints from inkjet negatives with smooth tonality in highlight regions. Digital imaging systems are optimised for positive images. They have about 5x more tonal resolution in highlight than in shadow areas, to match our eye’s ability to discriminate print tones. When one considers an inkjet negative, the opposite is required, to ensure fine tonal gradation in high-density areas.

(Figure 1) In this scan of a contact print, which was made by using a clear inkjet film, vertical banding and individual inkjet dots are clearly visible, especially in the midtones. The vertical banding was greatly reduced, but not completely removed, by using white inkjet film and applying some pressure through the weight of a thick glass cover. The random grain (in this case, Tri-X) of the copy print process masks printer issues such as ink dots or banding. The combination of a matt paper surface, ink dot grain and negative grain disguises the hardware issues.
(Figure 2) On the left, these magnified scans of contact-printed, silver-gelatin prints from inkjet negatives, have a resolution up to 8.0 lp/m. In the center image, clear film and white film clamped between the paper and picture glass. At the right, white film, relying on the weight of the glass alone, caused negative and paper to be close but not touching in all areas, resulting in a blurry image and lost resolution. Even with clamped glass, it is not uncommon to have uneven sharpness on fibre paper.

Copy Prints – The Inkjet Positive

Inkjet printers are designed to make prints, not negatives. It occurred to make an 11×14 inch inkjet print, copy it onto film and print the resulting negative. It works surprisingly well, and is a viable alternative to halftone and inkjet negatives, so long as one adjusts the tonality of the inkjet print. In practice, a large matt inkjet print is made of a digital master image and photographed onto monochrome film. The resulting copy negative is conventionally enlarged onto silver-gelatin paper. Since the photographic process compresses extreme print values, it is necessary to apply a transfer function to the digital image, prior to inkjet printing, to cancel out these tonal distortions and faithfully reproduce the original digital image.

The copy negative can be on 35mm, medium or large-format film, depending on the intended grain and final enlargement size. Figure 3 shows the imaging sequence of the copy-print process, from the digital master, through the inkjet proof and copy print, up to the copy negative, from which any number of analog prints can be produced.

With each trial, the copy-print process produced a silver print with superior gradation to that of an inkjet negative print, especially in the highlight regions, and all signs of the mechanistic properties of the inkjet printer vanished, masked by the inclusion of film grain and the slight ink bleed qualities of matt inkjet paper. By way of comparison, fig. 1 also compares the scan of a silver print enlargement from a Kodak Tri-X negative.

Copy Negative Process Step-by-Step

1. Prepare the Digital Master

Ensure your digital original is a 16-bit mono- chrome image and carefully manipulate in the photo-editing software, so that the highlight and shadow values are adjusted to recommended values. Check the file resolution which, assuming standard vision, requires about 300 ppi on an 8×10-inch print. The monochrome digital master file is saved for later use.

2. Make the Inkjet Proof Print

Print an inkjet proof print of the digital master with a high-quality printer, using a similar print surface as the final silver-gelatin print (normally gloss or lustre). Follow your standard technique for ensuring the proof print and the monitor image show good correlation. The proof print should be allowed to dry and should be assessed in similar lighting conditions to that of the final print destination. This is the reference to which you will match your final silver print.

3. Make the Inkjet Copy Print

Apply the transfer function (fig. 4), which was previously determined by the calibration process, to the digital master file. Print the adjusted image again, but this time, onto smooth matt paper no smaller than 8×10 and ideally about 11×14 to 13×19 inches. Apply the printer settings used for the copy print during the calibration process. For best results disable colour management and ensure a suitable media setting is selected for the matt paper surface.

Why matt? Glossy and lustre paper surfaces can achieve remarkable reflection densities in excess of 2.3 with dye-based inkjet printers. These surfaces, however, are difficult to copy without including unwanted reflections from surrounding objects and light sources. For- tunately, all inkjet printers work with matt paper surfaces, which not only allows the use of pigment-based printers, but more importantly,

it also removes the sensitivity to unwanted reflections. Quite unlike a standard copy setup, diffuse daylight is the most effective light source for copying matt prints.

4. Setting Up for Copying

It is feasible to copy a print with nothing more than a small mirror, a tripod, a few pieces of sticky tape, an empty wall and a large window on the opposite side. The mirror is used to ensure that the camera is square on to the inkjet print. With the mirror held flat to the wall and in the middle of the print, the camera is in the correct position when the lens appears centred in the mirror and the print fills the viewfinder. The camera is set to its optimum aperture.

5. Exposure and Developing the Copy Negative

Use an incident light meter, preferably with a flat diffuser, to determine the exposure and also to check for even illumination. It is a good idea to increase the exposure a little, in order to correct for the unavoidable and inherent exposure loss of close-up repro, and ensure that the darkest image values are not lost. A good starting point is +1 stop.

The negative development needs to be mod- ified too, to make up for the low contrast print. A matt print has a maximum reflection density (Dmax) of approximately 1.5, or 5-stops subject brightness range. Such a low-contrast subject requires an increase in negative development (N+1) or a boost in paper contrast to achieve a full-bodied print (about 35% more devel- opment time than normal).

Sufficient exposure will improve shadow separation, beyond which, print grain and resolution will deteriorate. Depending on the film emulsion, a harder print contrast and a normal negative contrast may produce a more pleasing result than the opposite combination.

6. Make the Final Print

During the calibration process, an optimum print exposure and contrast setting was used to obtain a final silver print. You should use a similar setting to print the copy negative, using the inkjet proof print as a reference. Unlike the halftone process, however, further creative expression can be introduced with global or local adjustments to silver-print exposure and contrast, just as with any conventional negative.

(Figure 4) This sample “Curves” adjustment dialog box in Adobe Photoshop shows a transfer function, correcting for my particular printer, film and darkroom setup. Although, your settings will most likely differ, the overall shape of the curve will be very similar, as it is determined by the general characteristics of the film and paper.
(Figure 5) The final print, made from the 4×5 Tri-X copy negative, achieved a resolution of 6.5 lp/ mm, which is only slightly less than that of a traditional contact print. This enlargement would produce a 30×40-inch print.

Resolution

The copy-print process introduces two additional steps into the imaging chain with potential for resolution loss, associated with the matt inkjet print and copying it to the negative. To confirm that the resolution capability of the copy-print process is sufficient to support the requirements for the final silver-gelatin print, a high-resolution image of the USAF/1951 test pattern was printed on matt paper and photographed onto 4×5-inch Kodak Tri-X. The negatives were then printed at the same scale as the original test chart. In each case, short telephoto lenses were used at their optimum aperture for maximum resolution and sharpness. The result is shown in fig. 5.

In practice, the silver-print’s resolution exceeds that required for standard observation at about 6.5 lp/mm. Similar results were achieved with fine grain film on roll-film. This represents the peak system performance, since in practice, lower printer hardware and image file resolution settings will potentially lower the system resolution.

1. Hardware Resolution Setting

Confusingly, inkjet printers spray complex patterns of ink, which defy any convenient theoretical resolution calculation. Several ink blobs of varying intensity and sizes are required to define a single coloured ‘dot’. Annoyingly, in a bid to outdo each other, print manufacturers often claim highly exaggerated resolutions, which bear little relation to the actual print resolution of their products. These spurious numbers together with their complex printing algorithms, ink bleed and paper surface effects, just to form an ink dot, utterly confuse the issue. To make matters worse, many inkjet printers have higher resolution in one direction than in the other. As a result, the user can change the file ppi setting prior to printing and in some cases change the printer ‘dpi’ setting in a printer control panel to produce an effective dpi which matches neither number! Suffice to say, in practice, all modern photo inkjet printers achieve sufficient mechanical resolution as long as their highest hardware resolution settings are used for this process.

2. Image File Resolution

As part of my book research into digital resolution, I established a theoretical relationship between the image file resolution and print res- olution and confirmed by experiment. Similar to the physics of digital capture, a printer, when laying down ink on paper, requires about 67 ppi per lp/mm to assure that image line pairs are printed with 50% contrast and 53 ppi per lp/ mm to print them with 10% print contrast. For example, an inkjet print can resolve 5 lp/mm in all directions using a 300 ppi file setting.

If an image file has sufficient resolution for a standard print, it can be used to make larger prints by deploying the image pixels over a larger area, with a corresponding reduction in print resolution, assuming a proportional increase in viewing distance. Higher resolutions will withstand closer scrutiny.

Making a Transfer Function

A halftone negative simulates minute tonality differences, as found in continuous-tone images, through a series of equally spaced dots of varying sizes, which makes it remarkably tolerant of variations in paper exposure and grade settings. The copy-print process, however, is not as tolerant and requires the user to make an individual calibration for each choice of material and process setting. Unlike the halftone process, the ability to create an accurate inkjet proof print invites a direct print-to-print comparison with the final silver-gelatin print, which suggests an obvious mechanism for process calibration and removes the potential for unreliable comparisons between the monitor and the print. Ideally, the proof print should have a similar surface finish to the final silver-gelatin print and should be prepared through a calibrated and profiled workflow.

Calibration Basics

The calibration process assumes a consistent darkroom process and repeatable negative and print processing, since it manipulates the inkjet copy print to account for the tonal distortion in negative and final print. The calibration compares the required greyscale values (K%) to produce equivalent print densities on the inkjet proof print and the final silver print. The re- sulting correction or transfer function consists of up to 16 pairs of input and output values, from white (0%) through to black (100%). The transfer function is a curve, which can be saved and applied to other photographic images, prior to printing the matt inkjet copy print. The creation of the transfer function is made easier when using a specially-made calibration tem- plate similiar the one shown in fig. 6. The calibration process is quite straightforward.

(Figure 6) This home-made calibration template has tonal spacings of 1% for the extreme highlight and shadow tones and 2% spacings in the midtone area. A black and white band is placed below the patches to accentuate small differences in highlight and shadow tones.
(Figure 7) The inkjet calibration print on the wall simulates a matt copy print to be captured onto roll-film. The camera is set up square to the wall, and the exposure is determined by an incident light meter. The same setup is used for copying the actual inkjet copy prints. Diffuse daylight is the most effective light source for copying matt surfaces. When copying inkjet prints, ensure the lens is set to its optimum aperture and use all available techniques to reduce camera vibration.

1. Inkjet Reference Print

Design a calibration template, similar to the one in fig. 6. Simulating the proof print, make an inkjet reference print from it, using a device profile that is customised for the combination of hardware and settings. The print dialog box in your software must be set to use this profile for colour management. It should be printed on glossy or semi-gloss media, allowed to dry and put aside for later reference.

2. Inkjet Calibration Print

Make another, unadjusted inkjet print, but this time, select the software options to ignore colour management. This print should be on matt paper and use a matt paper media setting to avoid over-inking. It is not an issue if the print has a colour hue. Also, the tonality of the copy print may be slightly different to the proof print, but its settings produce more consistent results for calibration purposes. The print is allowed to dry and is hung on an evenly lit wall.

3. Calibration Copy Negative

Set up your film-camera opposite and perpen- dicular to the inkjet calibration print and make two exposures, one using an incident light reading at the effective film speed and another giving 1 stop overexposure as in fig. 7. Develop the film with N+1 development. The resulting increase in negative contrast helps to avoid needing extreme contrast grades at the printing stage, and gives some freedom for further manipulation in the darkroom. In general, any film can be used, but it is of benefit to select the same film type as used for other direct silver prints in your collection. In this manner, a homogeneous exhibition of prints can be achieved. After calibration, keep the negative, as it can be used for subsequent calibrations with other silver-gelatin paper.

4. Silver-Gelatin Calibration Print

It may take several attempts to make a silver- gelatin print from the calibration copy negative. The exposure and contrast setting should just produce a full range of image tones from white to black. Note the contrast setting and metering method for later use. For films with an extended tone region, the overexposed negative may be more suitable.

5. The Transfer Function

The transfer function must compensate for the tonal distortion that the inkjet copy print, the copy negative and the silver-gelatin print process introduce into the copy-print process. The relationship between the inkjet reference print and the silver-gelatin calibration print provides a direct measure of the compensation required and forms the basis of the transfer function. One can generate a transfer function from taking density readings with a densitometer and plot a graph, as in fig. 8. In this case, the maximum density of the inkjet proof print has been scaled to match the silver-gelatin print’s for easy comparison. From this graph, pairs of K% values, which yield the same print density, can be determined and read off, to be typed into a Photoshop “curves” palette.

A more direct method to generate a transfer function, and one that does not require a densitometer, is to directly compare the inkjet proof and the silver-gelatin calibration print by laying them on top of one another, as in fig. 9, and noting the K% values that match density. To create a useful transfer function, one requires about 10-16 pairs from white through to black, with more points near the tonal extremes. For instance, if a 20% value is required on the silver- gelatin print to reproduce the tone of the 10% inkjet proof print, one of these input/output pairs will be 10/20. The values are entered into the imaging software, appropriately labelled and saved for later application. In either case, the transfer function will most likely resemble an inverse S-curve (fig.4), reducing midtone contrast and increasing highlight and shadow contrast.

A close look at this S-curve reveals a dramatic change in contrast between the tonal extremes and the midtones. As a result, the silver print tonality is quite responsive to the digital highlight image value and print exposure. To identify potential problems and provide data required to tune the transfer function, it is useful to print a small step wedge on the margins of the matt inkjet copy print (fig. 3), which is used for a direct comparison to the proof print.

(FIgure 8) One option to generate a transfer function is to take density readings with a densitometer and plot a graph. In this case, the density of the inkjet reference print has been scaled to match the silver- gelatin calibration print’s Dmax, for easy comparison. From this graph, pairs of K% values, which yield the same print density, can be determined and read off.
(Figure 9) The inkjet reference print and the silver-gelatin calibration print can be directly compared to generate a set of equivalent K% values, which are used to generate the transfer function. If a 20% grey on the proof print matches a 40% patch on the silver print, one input/output pair is 20/40 in the transfer dialog. This “matching” of tones is repeated over the entire tonal range.

Final Thoughts

The copy-print process is a tolerant and practical method to make fine silver prints from digital files with a variety of inkjet printers, inks and media. The extra steps involved introduce a mild resolution loss, but it is insufficient to impact the perceived resolution of a print at a normal viewing distance. Additional sharpening of the original digital image can compensate the mild softening. In cases where a portfolio is made from a mixture of classical and digital images, the lack of telltale inkjet dots and the presence of film grain disguise the reproduction process to create a homogeneous body of work with other conventional silver prints.

Although this copy process requires additional materials, matt inkjet papers cost significantly less than plastic film, and the resulting conventional negative allows final images to be printed at any size and classically manipulated under the enlarger. Furthermore, these negatives can be archivally processed, stored conveniently in standard negative filing systems, and the matt copy prints discarded.

Castle y Bere
Old Tree

About the Author

Chris Woodhouse
CWoodhouse
Chris Woodhouse ARPS, is an English electronic engineer and photographer, who designs f/stop timers and enlarger meters. He is the co-author of Way Beyond Monochrome. This article is a revised and shortened version of a new chapter that will appear in the second edition, which is due for completion this year. www.beyondmonochrome.co.uk