In Brightness mode the photo is first turned gray, and Gray from chooses the recipe. There are five, and the same photo gives five different grays, which means five different notans. Choosing one is like choosing a filter for black-and-white film.
What it does
- Luminance (the default) weighs the three channels as the eye roughly does: 0.2126 of red, 0.7152 of green and 0.0722 of blue. These are the weights of the sRGB and HDTV standards: green counts most, blue least. Strictly, because they are applied to the photo's stored values rather than to linear light, the result is what video engineers call luma. It is close to how light each color looks.
- Red, Green and Blue each use one channel alone. Anything rich in that channel turns light. With Red, reds and yellows go pale and greens and blues go dark; with Blue, a sky goes pale and yellows go dark.
- Saturation measures how vivid each pixel is: the difference between its strongest and weakest channel, as a share of the strongest. Vivid color turns light; every neutral, white, black or gray, turns black.
Remember that dark is front. A recipe that makes your subject light will trace the background instead, unless you tick Invert. With Saturation this is almost always so: white paper, black ink and gray shadow all become front, and the colorful thing becomes back. Invert to trace the colorful thing.
Brightness and Contrast then work on whichever gray you chose, and the Adjusted preview shows the result.
Why it works this way
Luminance agrees with your eye most of the time, which is why it is the default. But the eye's idea of lightness blends the channels, and a blend can hide a difference. A red subject on green grass can have the same luminance as the grass; the red channel pulls them apart, because the subject is rich in red and the grass is not. Each single channel is a way of asking a sharper question than the eye does.
Saturation asks a different kind of question altogether: not how light, but how colored. It is for a bright object on a neutral ground, a toy on a white table, a painted sign on a gray wall, where the subject's lightness may match its surroundings but its vividness never does.
The saturation used here is the one from the HSV color model, which ranks a pixel by its channels alone. That makes it quick and predictable, with one quirk worth knowing: very dark pixels have unreliable saturation, since a tiny difference between channels becomes a large share of a tiny maximum. Dark noise can come out speckled. Blur helps.
The metaphor
The black-and-white photographer's colored filter. A red filter over the lens lets red light through and holds blue back, so a blue sky prints dark and clouds leap out of it; a green filter lightens foliage; a yellow one darkens a sky a little. Choosing Red here is shooting through a red filter: things of that color print light, things of its opposite print dark.
The metaphor holds exactly, because a color channel is literally what a filter passes. It breaks in cost and in kind. A filter over the lens meant more exposure and a trip back to the scene if you chose wrong; here you switch in an instant. And no filter ever made Saturation: gray by vividness has no glass equivalent, and it is new to digital images.
A history
In art
For centuries, turning color into gray was a skilled trade. Reproductive engravers translated paintings into black-and-white prints, and each had to decide how dark a red cloak or a blue sky should be in lines on paper. Two engravers could render the same painting quite differently. Gray from is that decision, made explicit.
Painters played with the same translation. The outer wings of Jan and Hubert van Eyck's Ghent Altarpiece hold figures painted in grisaille, in grays that imitate stone statues: color deliberately withheld on the outside, with the full color of the painting waiting within.
In printing
Early photographic plates were sensitive mainly to blue light. Blue skies printed white and red things printed nearly black, so a blue-sensitive plate was a “Gray from Blue” whether the photographer liked it or not. In 1873 Hermann Wilhelm Vogel found that adding dyes to an emulsion extended its sensitivity into green, leading to orthochromatic plates. Wratten and Wainwright produced the first commercial panchromatic plates, sensitive across the spectrum, in 1906, and after Eastman Kodak bought the firm in 1912, its numbered Wratten filters became the standard vocabulary of filtering.
Sergei Prokudin-Gorsky photographed the Russian Empire in the early twentieth century with a camera that exposed one glass plate three times in quick succession, through blue, green and red filters. Each of his triple negatives, now in the Library of Congress, is three Gray from recipes side by side: the Blue, Green and Red of this menu, a century early.
In technology
American color television, standardized in 1953, had to send a picture that old black-and-white sets could still show. So the signal carried one brightness record built from about 30% red, 59% green and 11% blue, weights that survive as the Rec. 601 standard (0.299, 0.587, 0.114). The high-definition standard, Rec. 709, recalculated the weights for its own primaries as 0.2126, 0.7152 and 0.0722, and sRGB, drafted by Hewlett-Packard and Microsoft in 1996, adopted those primaries. Those are the numbers Luminance uses.
The HSV model, hue, saturation and value, was published by Alvy Ray Smith at SIGGRAPH in 1978, after it had been used in paint programs for some years. It gave painters on screen a way to talk about color the way they thought about it, rather than the way a monitor made it. Its idea of saturation is older than computers: Albert Munsell's A Color Notation (1905) described every color by hue, value and chroma, his word for vividness.
In professional software
Turning color into gray is called black-and-white conversion, and choosing how much each color counts is channel mixing. The lesson carries straight over: there is no single correct gray, only a recipe, and the right recipe is the one that pulls your subject away from its ground.
- Adobe Photoshop. ‘Image › Adjustments › Black & White’, ⌥⇧⌘B (Alt+Shift+Ctrl+B), gives a slider for each of Reds, Yellows, Greens, Cyans, Blues and Magentas, and presets named after lens filters. ‘Channel Mixer’, on the same menu, with ‘Monochrome’ ticked, sets the shares of red, green and blue directly, as Luminance’s weights do. In the Channels panel, ⌘3, ⌘4 and ⌘5 (Ctrl+3, Ctrl+4 and Ctrl+5) show the Red, Green and Blue channels alone, the same grays as those choices here.
- Adobe Lightroom Classic. Press
V, or set the Treatment to Black & White in the Basic panel, and the Color Mixer becomes a B&W mix of eight color sliders. Its targeted adjustment tool lets you drag up or down on the photo itself to lighten or darken whatever color lies under the pointer. - GIMP. ‘Colors › Desaturate › Desaturate’ offers several recipes, among them ‘Luminance’ and ‘Luma’, the same distinction this chapter draws. ‘Colors › Desaturate › Mono Mixer’ lets you set your own shares of red, green and blue.
What carries over. Try several recipes on the same photo and keep the one that separates, not the one that looks most natural. One pitfall: Photoshop’s ‘Image › Mode › Grayscale’ throws the color away with one fixed recipe. Convert with a Black & White adjustment layer instead, and you can change the recipe later, as you can here.
Try this
- Bring in a photo of a red flower among green leaves. Set Preview to Adjusted and step through Luminance, Red and Green.
- Photograph a brightly colored object on white paper. Choose Saturation, tick Invert, and look at Two-tone.
Going further
See Separate by, Brightness and contrast and HSV: hue, saturation, brightness. Ansel Adams, The Negative. Charles Poynton, Digital Video and HD: Algorithms and Interfaces. Albert H. Munsell, A Color Notation.