Levels decide which tones count as black, which count as white, and how the tones between them are spread. They reshape the gray image before it is blurred and cut, so they move where the threshold falls, what “Auto” chooses and how soft the edges become. What they cannot do, on their own, is invent a split the photo did not already hold.
What it does
Three numbers sit under the histogram, and each has a handle on it.
- “Black” (0–254, default 0). Every level at or below it becomes pure black, 0.
- “White” (1–255, default 255). Every level at or above it becomes pure white, 255. It always stays at least one level above Black, and a dragged handle stops before the two meet.
- “Mid” (0.10–9.99, default 1.00). A gamma: it bends the tones between the two ends without moving the ends.
Between Black and White, each level is first stretched to fill the whole scale: its place between the two becomes a fraction from 0 to 1. Then Mid bends it: the fraction is raised to the power of one over Mid, and multiplied back up to 255. At 1.00 nothing bends. Above 1 the midtones lighten; below 1 they darken. The gray diamond shows the bend as a place on the scale: it sits at the input level that comes out as middle gray. At Mid 2.00 the diamond sits a quarter of the way from Black to White, so every tone above that quarter mark comes out lighter than middle gray.
Drag the handles, or type the values beside them. Typed, Mid moves in steps of 0.05; dragged, it is kept to two decimal places. The ink layer of the histogram and the “Adjusted” preview show the result.
Levels work on the score image in either mode: on the gray image, after “Brightness” and “Contrast”, when you separate by brightness, and on the color score when you separate by color. They come after those and before “Blur” and the threshold.
There is one thing Levels cannot do, and it is worth knowing. They never change the order of two tones: a pixel darker than another before is darker, or level with it, after. So when “Blur” is 0 and you set the threshold by hand, Levels cannot change which pixels become front. Any cut you make after Levels is a cut you could have made before them, at a different number. What Levels change is everything around that cut:
- Where “Auto” cuts. Otsu's method reads the histogram after levels. Clip a pale, blotchy background to pure white and it becomes a single spike at 255, which changes where the method finds its best division.
- What Blur averages. Blur comes after Levels, so it averages the adjusted values. Push a grainy background to white first and blurring it gives white, not a gray that wanders back and forth across the threshold.
- How much room the threshold has. If the tones that matter lie between 90 and 130, stretching them across the whole scale spreads a forty-level decision along the full length of the slider.
And one warning: what you clip, you merge. Every level below Black becomes the same black, and no threshold can separate those pixels again.
Why it works this way
Photos rarely use the scale the way a notan needs. A gray day gives a lump in the middle. A scan of a drawing gives a white that is really pale gray with a texture in it, and a black that is really dark gray. Levels are the oldest correction in digital imaging for exactly this, and the controls follow the convention of most photo editors: a black point, a white point and a midtone gamma, with 1.00 meaning no change. What you know from a photo editor works here, and what you learn here works there.
Mid is a gamma rather than a level because a gamma keeps its meaning when the ends move. 1.00 is always “no bend”, wherever Black and White are; a midtone given as a level would quietly change its effect every time you touched either end. The diamond still shows you the level it corresponds to, so you get both.
The metaphor
Levels borrow the darkroom print. Printing a negative, a photographer chose how long to expose the paper and which contrast grade to print on. Exposure decides where the darkest tones land; the grade decides how steeply the scale climbs from black to white. Adams spoke of placing a value, deciding that a shadow would print as, say, Zone III, and letting the other tones fall where they would. Black and White place the two ends; Mid tilts the middle.
The metaphor breaks at the ends. Photographic paper has a toe and a shoulder: tones fade gently into black and gently into white. Levels clip: at Black and at White the curve simply stops, and everything beyond is merged. Nor can one gamma make an S, lighter lights and darker darks at once; it bends only one way. For a notan that hardly matters, because the very next step throws away every tone but two.
A history
In art
Painters have always worked in a narrower range than the world. Paint cannot match the sun, or a street at night, so a painter chooses a key, high and pale or low and dark, and fits the scene's values inside it. Caravaggio pulled lit figures out of grounds that are almost black, a manner later called tenebrism: the whole picture is pressed toward the dark end of the scale, and a few lights carry everything. Printmakers made the same choice with fewer means. A woodcut has two values, paper and ink, and an engraver who wanted grays had to build them out of lines.
The notan exercise goes further than either. It asks the artist to decide, for every part of a subject, dark or light, and to live with the decision. Levels are the step where you decide how much of the photo's range deserves to be in play when that decision comes: which darks are all one dark, which lights are all one light, and where the middle of the argument lies.
In printing
The word gamma comes from Hurter and Driffield's study of 1890. Their characteristic curve, density against the logarithm of exposure, has a long straight middle, and they called the slope of that straight part gamma: steep for contrasty, shallow for soft. The idea passed into darkroom practice as the paper grade. Fixed-grade papers were made in a series numbered from 0, the softest, to 5, the hardest; variable-contrast papers later put the choice under the enlarger, where a yellow filter softens and a magenta one hardens.
Reproduction for the press added its own pair of limits. A press cannot hold the very smallest halftone dots, nor keep the very largest from filling in, so a photograph prepared for print had its lightest and darkest printable tones set before anything else. Those settings are the ancestors of the black and white points.
In technology
Gamma turned up again in television. A cathode-ray tube does not glow in proportion to the voltage it is given: its brightness follows a power of the voltage, with an exponent usually around 2.35 to 2.55. Signals were bent the other way to match, and “gamma correction” became part of video. When Hewlett-Packard and Microsoft proposed sRGB in 1996, standardized by the International Electrotechnical Commission in 1999, they fixed a curve close to a gamma of 2.2.
Most photos are encoded that way, which is why the 256 levels in this histogram are not evenly spaced in light. Level 128 sends out only about a fifth of the light of level 255, yet it looks near a middle gray, because the eye is more sensitive to differences among the darks. The Mid field applies the same kind of power curve, on purpose, to the picture's tones. Most image editors present Levels the same way: three handles under a histogram, the middle one shown as a gamma of 1.00.
In professional software
Levels is one of the oldest adjustments in digital imaging, and its handles mean the same thing almost everywhere: a black point, a white point and a midtone written as a gamma, where 1.00 means no change. Learn Black, Mid and White here and you already know the Levels dialog in Photoshop and GIMP.
- Adobe Photoshop. ‘Image › Adjustments › Levels’, or ⌘L (Ctrl+L), puts three ‘Input Levels’ handles under a histogram, the middle one a gamma that lightens the midtones above 1.00 and darkens them below it, as Mid does here. Two ‘Output Levels’ handles can stop black from reaching 0 or white from reaching 255, and holding ⌥ (Alt) while you drag the black or white input handle turns the image into a map of the pixels you are clipping. Made with ‘Layer › New Adjustment Layer › Levels’ instead, the same controls sit on a layer of their own and stay editable, leaving the pixels beneath untouched.
- Adobe Lightroom Classic. The nearest controls are the ‘Blacks’ and ‘Whites’ sliders in the ‘Basic’ panel of the Develop module, and dragging the two ends of the histogram moves them too. Hold ⌥ while dragging either slider and the photo turns into a map of what is being clipped.
- GIMP. ‘Colors › Levels’ has the same three input handles and two output handles, eyedroppers for picking the black, gray and white points from the image itself, and an ‘Auto Input Levels’ button. Its histogram switches between linear and logarithmic, like Studio Notan’s.
What carries over. Set the two ends first, then the middle, and watch for clipping as you go: every level pushed past Black or White merges with its neighbors for good. The pitfall is what Levels changes. In Photoshop, unless you use an adjustment layer, Levels rewrites the pixels, and every later step inherits the result. In Studio Notan, Levels reshape only the score image on its way to Blur and the threshold, and the photo itself is never altered.
Try this
- With “Preview” on “Adjusted”, drag “Mid” from 1.00 up to 3.00 and down to 0.33. Watch the diamond travel and the midtones lighten and darken while the two ends stay put.
- On a scanned drawing, bring “White” down to the left foot of the paper's hump in the faint layer. The paper texture turns pure white. Then press “Auto” and see where it cuts now.
- On a dark photo, push “Black” up to 60 and look at the two-tone preview. Everything that was darker than 60 is now one mass, and no threshold can divide it.
Going further
See Histogram, Brightness and contrast and Threshold, Auto and Invert.
- Ansel Adams, The Print
- Charles Poynton, Digital Video and HD: Algorithms and Interfaces
- Richard Benson, The Printed Picture