18.9 High dynamic range in traditional photography🔗⧉

Photographers were fighting dynamic range more than a century before anyone merged a bracket in software. The scene has always had more range than the print can show, and for most of that history it also had more range than the negative could record. What they did about it maps closely onto the capture methods of Part 17, High dynamic range capture and onto the tone-mapping chapters of this part: combining several exposures, measuring the scene and deciding where each tone should land, and compressing large-scale contrast while keeping local detail. Knowing the craft makes the algorithms easier to read, and several of their failure modes, the halo above all, were met in the darkroom first.

This chapter follows the photographer through the three places a traditional workflow can intervene: in front of the camera, at capture and development, and at printing. Chapter 5.5, Developing and editing a photograph describes the darkroom as a whole; here the focus is on range.

18.9.1 Three places to intervene🔗⧉

A black-and-white negative-positive process has two stages with very different ranges. A negative can record a wide range of scene luminance, commonly ten stops or more with care, because its response rolls off gradually at both ends. A print on paper reflects light, and its range runs from the deepest black the paper can hold to the white of the paper base: a density range of about 2 on glossy paper, a ratio of about 100:1 or seven stops, and less on matte paper. The print cannot show what the negative holds, so every print is already a tone mapping, whether the printer thinks of it that way or not. Color slide film is the harder case: the transparency is the final image, it records only five or six stops, and there is no printing stage in which to rescue anything.

That gives three moments for control, each with its own tools:

• Before the shutter: change the scene. Add light to the shadows, subdue the highlights, or wait for light whose range the process can hold.
• At capture and development: change what reaches the film and how the film responds. Filters, exposure placement and development time decide which scene luminances end up as usable densities.
• At printing: change how densities become print tones. Paper contrast, local exposure with dodging and burning, and masks decide where each part of the negative lands on the paper.

The digital pipeline has the same three moments, and the two HDR parts are organized around them: capturing more range (Chapter 17.1, Dynamic range and multiple-exposure capture, Chapter 17.3, Merging exposures) and rendering it (Chapter 18.2, Global tone mapping and the chapters after it).

All three moments can be tried in a simulated darkroom that starts from a high-dynamic-range scene (Figure 5.5.2⛶ fullscreen).

18.9.2 Combination printing: Le Gray and his successors🔗⧉

The first photographers had a harsher problem than a short print range. Wet collodion, the standard glass-plate process from the 1850s, was sensitive almost only to blue and ultraviolet light. A sky, rich in blue, recorded far more strongly than a landscape or a sea of the same apparent brightness, so an exposure long enough for the land left the sky as a featureless, overexposed white. Painted-in or blank skies were common in landscape photographs of the period.

Gustave Le Gray's seascapes of 1856 to 1858, taken on the Normandy coast and at Sète on the Mediterranean, answered this in two ways. Some were single exposures made in favorable light, with short exposures that held the sky and let the sea go dark. Others combined two negatives: one exposed for the sea, one for the sky, printed one after the other onto the same sheet with the horizon as the join. The horizon is a gift to this method, a nearly straight line where the two exposures can meet without an obvious seam, and Le Gray used the same sky negative with more than one foreground (Getty exhibition). The prints were admired in their day partly because viewers believed they showed sea and sky together as the eye saw them, which a single collodion plate could not (Figure 18.9.1⛶ fullscreen).

Others pushed the idea further. Oscar Rejlander's allegory The Two Ways of Life (1857) was assembled from more than thirty negatives (Figure 18.9.2⛶ fullscreen), and Henry Peach Robinson's Fading Away (1858) from five. Their purpose was mostly composition rather than range, and it shows the risk in the method: once several negatives go into one print, nothing guarantees that they show one moment or one scene. By the end of the century many landscape photographers kept a file of cloud negatives to print into blank skies, a practice that raised the same questions about what a photograph attests that composites raise today (Chapter 5.8, Photographic representation and its limits).

Combination printing is the direct ancestor of exposure bracketing and merging. Each negative measures part of the scene well; the print takes each region from the exposure that measured it best. The differences from a digital merge are instructive. Le Gray chose a single boundary by hand and switched from one negative to the other across it, which works only where the scene offers a clean edge such as a horizon. A digital merge instead weights every pixel by how reliable each exposure is there (Chapter 17.3, Merging exposures), and exposure fusion blends exposures smoothly across a multiscale decomposition (Chapter 18.4, Exposure fusion). And Le Gray's two negatives were not taken at the same moment, so any wave or cloud that moved between them had to be hidden at the seam, which is the ghosting problem of Chapter 17.4, Ghosts and motion met with scissors and judgment.

18.9.3 Reducing the range of the scene: lighting🔗⧉

The cheapest way to fit a scene into the medium is to make the scene itself less contrasty. Where the photographer controls the light, this is the first tool, and it is still the first tool of professional photography and cinema.

Fill light. A face lit by a single source has a bright side and a shadow side. Adding a second, weaker source on the shadow side lifts it without changing the highlights much. Studio photographers describe the result as a lighting ratio, the luminance of the lit side to the shadow side: 2:1 (one stop) looks soft, 4:1 (two stops) moderate, 8:1 (three stops) dramatic. A reflector does the same job with no second source by redirecting some of the key light into the shadows, and outdoors fill flash lifts a backlit subject toward the level of the bright background (Chapter 5.4, Working with light, Chapter 35.1, Flash photography).

Subduing the highlights. The other end can be brought down. A scrim or diffuser between the sun and a subject cuts the light on it by a stop or two, and a black flag keeps a bright surface from spilling light where it is not wanted. On film sets, windows are routinely covered with sheets of neutral-density gel so that the view outside comes down to within a few stops of the interior.

Choosing the moment. Where the light cannot be changed, it can be waited for. Overcast light reduces the range between sun and shade; architectural photographers often shoot interiors at dusk, when the fading daylight outside falls to roughly the level of the interior lights and both can be held in one exposure. Landscape photographers wait for the low sun that lights the foreground at the same time as the sky.

All of these act on the scene, so they avoid the costs of the later tools: nothing is merged, so nothing ghosts, and no tone is compressed, so no local contrast is lost. The costs are practical instead: equipment, access, time, and a changed scene. Fill light changes the look of a face as well as its range, and a photograph taken at dusk is a photograph of dusk.

18.9.4 Filters at capture: graduated neutral density and its relatives🔗⧉

When the bright part of a scene is in a predictable place, a filter can darken just that part. The graduated neutral-density filter, or grad, is clear on one half and gray on the other, with a transition between them. Held in front of the lens with the gray half over the sky, it darkens the sky by a fixed amount while leaving the foreground alone. Its strength is given as an optical density $D$, related to the fraction of light transmitted $T$ by

so a density of 0.3 transmits half the light (one stop), 0.6 a quarter (two stops) and 0.9 an eighth (three stops). A hard-edged grad has a short transition for a flat horizon such as the sea; a soft-edged grad spreads the change over a larger part of the frame for broken horizons. A reverse grad is darkest just above its transition and lighter toward the top, for a sunset whose brightest band sits on the horizon.

The transition is not sharp on the sensor, and the reason matters. The filter sits in front of the lens, far from the plane of focus, so its edge is thoroughly defocused: its transition width on the image depends on the focal length, the aperture and the filter's distance from the lens. A wide-angle lens at a small aperture keeps it relatively narrow; a long lens at a wide aperture spreads it out.

The grad's failure mode is also its assumption. It darkens everything above a straight line, including whatever crosses that line: a mountain, a tree or a building that rises into the sky comes out darkened at the top, often visibly. A digital gradient mask applied in editing has the same shape and the same failure (Chapter 8.3, Tone point operations); an HDR merge followed by local tone mapping does not assume a straight boundary, which is much of its appeal.

Two other filters change range or relative tones in black-and-white work. Colored filters change which colors record as light and dark: a yellow, orange or red filter darkens a blue sky relative to white clouds and foliage, restoring cloud detail that panchromatic film would record as a pale gray. This changes relative tones rather than compressing the range, but where the problem is a sky that prints too light, it solves it. A polarizer darkens the parts of a clear sky roughly at right angles to the sun and removes reflections from water and foliage, which can bring bright areas down by a stop or more.

18.9.5 Exposure and development: the negative's curve🔗⧉

How a negative responds to light was put on a quantitative footing by Ferdinand Hurter and Vero Driffield, who plotted the density of a developed negative against the logarithm of the exposure that produced it Hurter & Driffield 1890 (characteristic curve). Their characteristic curve, still called the H&D curve, has three parts: a toe at low exposure, where density rises slowly out of the unexposed fog level; a straight-line portion, where density increases linearly with log exposure; and a shoulder at high exposure, where it levels off. The slope of the straight-line portion is the film's contrast, called gamma:

where $H$ is the exposure, the light energy per unit area reaching the film. A negative with $\gamma=0.6$ turns ten stops of scene range, a factor of about 1,000 or 3.0 in $\log_{10}H$, into a density range of 1.8. This is a log encoding in the sense of Chapter 4.3, Encoding: linear, gamma or log: equal ratios of light become equal differences of density.

Two controls follow from the curve. Exposure slides the scene along the log-exposure axis; it decides where the shadows sit relative to the toe. Development time changes the slope: longer development gives a steeper straight line and more contrast, shorter development a shallower one. Because shadow densities form early in development and highlight densities keep growing, development mostly moves the highlights. That is the logic of the old rule "expose for the shadows, develop for the highlights": give enough exposure to lift the important shadows out of the toe, then choose the development time that brings the highlights to a density the paper can print. A contrasty scene gets shortened development, which compresses the range the negative has to deliver. A flat scene gets longer development.

Film chemistry offered further compression. Compensating developers exhaust quickly in the densest areas, so highlights develop less while shadows keep going; two-bath developers push the same idea further. A brief, uniform pre-exposure of the film to dim light, called pre-flashing, lifts the deepest shadows out of the toe and reduces the effective range. Each of these is a global tone curve applied to the scene before the negative is even printed.

18.9.6 The Zone System🔗⧉

Ansel Adams and Fred Archer turned these controls into a method around 1940 Adams, The Negative. The Zone System divides the scale of tones into eleven zones, numbered from 0 to X. At the scene end, adjacent zones are one stop apart in exposure. At the print end, Zone 0 is the paper's maximum black, Zone X the paper white, Zone V a middle gray, and the zones between are equal visual steps rather than a doubling of luminance. Zone III is textured shadow, the darkest tone that still shows detail; Zone VIII is the lightest tone that still shows texture.

The method has three steps.

1. Previsualize. Before exposing, decide how the finished print should look: which parts of the scene should be dark with detail, which should be white, which should be middle gray.
2. Place. Meter one important area with a spot meter and choose the zone it should occupy. A meter reading, by construction, places whatever it measures on Zone V (Chapter 8.10, Auto-exposure and auto white balance), so placing a shadow on Zone III means giving two stops less exposure than the meter suggests.
3. Let the rest fall, then adjust. Meter the important highlights and see which zone they fall on at that exposure. If a white wall meant for Zone VIII falls on Zone X, the scene has two stops too much range for normal processing, and the negative gets reduced development, noted N−2, to bring that highlight down to Zone VIII. A flat scene gets expanded development, N+1 or N+2.

In the terms of this book, the Zone System chooses a global tone curve per scene, from measurements, before the image exists. The placement sets where the curve is anchored, and the development sets its slope. The Reinhard operator's "key" is the same anchoring made automatic (Chapter 18.2, Global tone mapping). What the Zone System cannot do is make the curve depend on location in the image, which is the job of printing.

Adams was explicit that the system serves the print and not the other way around: the negative is prepared so the print can be made the way the photographer previsualized it, and the printing still has work to do. He compared the negative to a score and the print to its performance Adams, The Print.

18.9.7 Printing: paper grade, dodging and burning🔗⧉

Under the enlarger, the negative is projected onto paper, and the printer controls how its densities become print tones.

Paper grade. Photographic papers came in contrast grades, typically numbered 0 (softest) to 5 (hardest). A soft grade accepts a long density range and compresses it into the paper's tones; a hard grade expands a short one. Variable-contrast papers, introduced around 1940, carry emulsions of different contrast whose mix is selected with colored filters, so one box of paper covers every grade. Split-grade printing exposes the same sheet twice, through the softest filter to set the highlights and through the hardest to set the shadows, which gives separate control of the two ends of the curve.

Flashing the paper. A brief uniform exposure of the paper to unfocused light, below the threshold that would show as gray, brings the paper to the edge of its response so that faint highlight densities in the negative begin to print. It compresses the top of the range without affecting the shadows much.

Dodging and burning. Paper grade and flashing still apply one curve to the whole print. Local control comes from changing the exposure region by region. To dodge, the printer holds a card, a small disk on a wire or a hand between the enlarger and part of the paper for part of the exposure, so that region receives less light and prints lighter; shadows that would block up are held back. To burn, the printer gives extra exposure through a hole in a card or between cupped hands, so a region prints darker; skies and bright walls are brought down. A classic print can involve a dozen separate operations. Adams's Moonrise, Hernandez, New Mexico (1941) is the famous case: the sky in his later prints is burned far darker than in early ones, a decision made at the enlarger rather than at the camera. Marked-up work prints by darkroom printers such as Pablo Inirio at Magnum, with each region labeled with its extra seconds, show how systematic this became.

The dodging tool is kept moving throughout, so that the boundary between dodged and undodged areas is a soft gradient rather than a visible edge. This is the darkroom's version of a smooth mask, and it produces the darkroom's version of the halo. A region dodged with a soft boundary that does not follow the edge of the subject gets a glow around it: a dodged face against a dark background carries a light fringe onto the background, and a burned sky darkens the top of the trees that cross into it. Printers learned to keep boundaries along natural edges, or to use cut masks that follow them. It is the same failure that a blurred base layer produces in local tone mapping, and the same remedy, masks that respect edges, is what Chapter 10.1, Bilateral filtering and Chapter 10.4, Local Laplacian filters automate (Chapter 18.3, Two-scale and bilateral tone mapping).

Finally, bleaching works on the finished print. A dilute oxidizing bath such as potassium ferricyanide, applied with a brush or cotton swab, dissolves some of the silver in a small area and lightens it: a catchlight in an eye, a highlight that printed too dark. It is the one tool that removes density rather than adding it.

18.9.8 Masking: the darkroom's base and detail🔗⧉

For negatives whose range was too long for any paper, printers had a tool that anticipates local tone mapping almost exactly: the contrast mask. The printer contact-prints the negative onto a sheet of low-contrast film, with a thin spacer or a diffusing sheet between them so that the copy comes out slightly blurred. The result is a weak, blurred positive. Sandwiched in register with the original negative, it adds density where the negative is thin (the shadows) and little where it is dense (the highlights), so the combination has a much shorter density range. Because the mask is blurred, it does not cancel fine detail: the small-scale contrast of the negative passes through, while the large-scale contrast is reduced.

In the terms of Chapter 18.3, Two-scale and bilateral tone mapping, the blurred mask is a base layer, the negative minus that base is the detail, and the sandwich compresses the base while keeping the detail. It also shows the same trade-off. A mask blurred too little flattens detail along with range; a mask blurred too much leaves bright or dark fringes along strong edges, the halo again. Printers controlled it with the spacer's thickness, which is the blur radius by another name (Figure 18.9.7⛶ fullscreen).

Printed on a harder paper to restore the overall contrast, the sandwich also raises the contrast of fine detail relative to the large scale, which reads as sharper; that is where the name "unsharp mask" comes from (Chapter 8.8, Sharpening). Astronomers used it to bring out structure in nebulae and galaxies whose cores are many stops brighter than their outskirts; David Malin's color images from the Anglo-Australian Observatory in the 1970s and 1980s made the technique well known. Dye-transfer and color printers used related highlight and contrast masks for the same reasons.

18.9.9 What carried over to computation🔗⧉

Almost every tool in this chapter has a digital descendant, and the pairing is useful in both directions: the craft explains why the algorithms look the way they do, and the algorithms make precise what the craft did by eye.

Two things did not carry over unchanged. The first is where the judgment sits. A darkroom print encodes hundreds of decisions made by a person looking at the print; a tone-mapping operator encodes a few parameters and makes the rest of the decisions the same way everywhere. Automatic operators are faster and repeatable, and they are worse at knowing which part of the picture matters. The second is the measurement. A negative's densities are not a radiance map, and the Zone System's meter readings covered a handful of points. A merged bracket measures scene radiance at every pixel (Chapter 17.2, Camera response and radiance calibration), which is what allows the rendering decisions to be revisited, automated or made by a different photographer later.