Brightness and Contrast adjust the gray image before it is split. Brightness slides every tone lighter or darker by the same amount; Contrast pulls tones apart from middle gray or squeezes them toward it. They are the familiar pair from every photo editor, working here the plain, old-fashioned way, on purpose.
What it does
Both appear only in Brightness mode, under Gray from. Both run from −100 to 100 and start at 0.
- Brightness adds the same amount to every tone. At 100 every pixel is 128 levels lighter (out of 255); at −100, 128 darker. A tone pushed past white or black stops there.
- Contrast stretches tones away from middle gray, level 128, along a straight line. Above 0, lights get lighter and darks darker; below 0, everything drifts toward gray. At −100 every pixel becomes the same middle gray; at 100 the stretch is so steep, about 130 times, that nearly every tone is flung to black or white.
They come first in a fixed order: gray, then Brightness and Contrast, then levels, then blur, then the split. The faint layer of the histogram shows the tones after Brightness and Contrast but before levels, so you can watch the whole mountain of tones slide and stretch as you drag. Reset returns both to 0.
Why it works this way
Here the gray image has one job: to be split in two. What matters is the order of the tones and where the gap between the two groups falls, not how pretty the gray looks. A plain slide and a plain stretch are predictable: every tone moves the same way, so you can read exactly what happened on the histogram.
A modern photo editor's Brightness bends its curve instead, so that highlights and shadows are not lost off the ends. For a photo you mean to print, that is kinder. For a split, losing the ends can be exactly the point: a strong Contrast is a quick way to decide the doubtful middle tones one way or the other. For curved, gentler control there is Mid, among the levels.
Contrast pivots on middle gray, the center of the scale. When the split falls near the middle, as it often does, stretching about the middle pulls the two groups of tones apart around the very line that will divide them.
The metaphor
The darkroom. A printer controlled two things above all: how long to expose the paper, which made the whole print lighter or darker, and which grade of paper to use, from soft to hard, which set how steeply the tones ran from white to black. Brightness is the exposure; Contrast is the paper grade.
It holds in spirit: one control for level, one for spread. It breaks in the shape of the curve. Photographic paper has a toe and a shoulder, a gentle roll into black and into white, so detail fades gradually rather than stopping dead. Here the line is straight and the ends are hard. And darkroom exposure multiplies light rather than adding to it, so its effect is not an even slide at all. The app's version is simpler than the chemistry, and for splitting, simpler is clearer.
A history
In art
Painters work in a narrow range. The whitest paint is nowhere near as bright as a sunlit wall, and the blackest nowhere near as dark as a shadowed doorway, so every painting compresses the world's tones into what pigment can hold, and the painter chooses where. Caravaggio pushed the contrast so hard that middle tones vanish into darkness and figures appear as if caught by a single lamp; the Italian word for this dramatic light is chiaroscuro, and its most extreme form is called tenebrism. Notan is the same move taken to its end: two values and nothing between.
In printing
In 1890 Ferdinand Hurter and Vero Charles Driffield published their study of photographic plates, and with it the characteristic curve: the density of a developed plate plotted against the logarithm of the exposure it received. The straight middle stretch of that curve has a slope, which came to be called gamma. Contrast became a number.
Ansel Adams and Fred Archer turned that science into a working method, the Zone System, while teaching at the Art Center School in Los Angeles around 1939 to 1940. It divided the tones from black to white into zones, so a photographer could decide in advance where a scene's tones would fall in the print. Graded papers, and later variable-contrast papers with colored filters, gave the printer the Contrast control by hand.
In technology
On the old cathode-ray television, the knob labeled brightness actually set the black level, and the one labeled contrast set the gain, and with it the white level. The names were loose, and they stuck.
Photoshop's Brightness/Contrast was for many years much the plain kind used here, sliding and stretching every tone evenly and clipping the ends, and it had a poor reputation among photographers for that reason. Photoshop CS3 (2007) changed it to bend the curve instead, and kept the old behavior behind a “Use Legacy” box. The legacy version lives on where clipping doesn't matter, in masks and in splits like this one.
In professional software
Brightness and contrast belong to a family called tonal adjustments, alongside Levels and Curves. Professionals usually apply them non-destructively, as settings kept beside the photo rather than changes to its pixels, which is how Studio Notan treats them too.
- Adobe Photoshop. ‘Image › Adjustments › Brightness/Contrast’ bends the curve to protect highlights and shadows; tick ‘Use Legacy’ for the straight slide and stretch used here. Levels, ⌘L (Ctrl+L), and Curves, ⌘M (Ctrl+M), give finer control. Add any of them from ‘Layer › New Adjustment Layer’ and they can be changed or removed at any time.
- Adobe Lightroom. Tone lives in the Basic panel of Lightroom Classic and the Light panel of Lightroom: Exposure, Contrast, Highlights, Shadows, Whites and Blacks. Every edit is stored as an instruction, so the original file is never changed.
- Autodesk AutoCAD.
IMAGEADJUSTsets Brightness and Contrast for an attached photo, each from 0 to 100 and starting at 50. It changes only how the image shows in the drawing and on the plot, which is often enough to make faint lines easier to trace.
What carries over. Adjust with the histogram in view, and prefer edits you can change later. The difference: a photo editor’s tone controls aim for a pleasing picture and try to keep detail at both ends, while here they serve a split, where driving the doubtful middle tones to one side is often the point.
Try this
- Set a Threshold by dragging it, so Auto is off. Then drag Contrast up with Two-tone showing, and watch the doubtful middle tones take sides.
- Turn Auto back on and drag Brightness. Notice how little the notan changes until the histogram crowds against one end.
- Set Contrast to −100 and look at the Adjusted view: one flat gray, and nothing left to split.
Going further
See Histogram, Levels: black, mid, white and Threshold, Auto and Invert. Ansel Adams, The Negative and The Print.