How to Make a Photo Black and White

Three things people mean by black and white

Choosing the wrong one is the commonest reason a conversion disappoints. A scanned contract wants bitonal; a portrait wants greyscale; a period-styled family photo might want sepia.

• Greyscale — every tone between black and white survives, the picture just loses its colour. This is what most people want.
• Monochrome — a single hue. Sepia and cyanotype are monochrome but not greyscale.
• True black and white, properly called one-bit or bitonal — every pixel is forced to pure black or pure white with nothing in between. This is for scans, stencils, laser cutting and vinyl.

Why the conversion method changes the picture

Turning colour into grey means deciding how much each colour channel contributes to the brightness of each pixel. The standard formula weights green at about 72%, red at 21% and blue at 7%, because that is roughly how the eye perceives brightness. It is a good default and a terrible one in specific cases.

Consider a red apple on green grass. To the eye they are strikingly different. Under standard greyscale they can land on almost the same grey, and the picture falls flat. Switching to a red-channel conversion makes the apple bright and the grass dark, and the photograph works again.

The channel filters, and what they came from

Before digital, photographers screwed coloured glass onto the lens. A red filter blocked blue light, so a blue sky exposed less and printed darker, which is where those dramatic black skies with white clouds come from. A green filter lifted foliage. A yellow filter was the mild everyday choice.

What makes a photo work in black and white

• Contrast between the subject and its background — in brightness, not just hue.
• Texture. Stone, fabric, bark, skin and weathered paint all gain when colour stops competing for attention.
• Shape and line. Silhouettes and strong geometry read more clearly without colour.
• Directional light. Side light and backlight create the tonal separation the picture now depends on entirely.
• A reason. A photograph whose whole point is a red door is not improved by removing the red.

Adjusting after the conversion

Almost every straight conversion benefits from a contrast increase. Colour was doing separation work, and once it is gone the tonal range has to do that job alone — which usually means stretching it. Somewhere between 110% and 150% is normal.

Brightness is the other lever, and it is worth adjusting before contrast rather than after: brightness shifts the whole range, contrast stretches it around the middle, so doing it in that order gives you more predictable control.

Scanned documents are a different job

If what you are converting is a page of text rather than a photograph, use threshold mode and forget everything above. The only control that matters is the cut-off: too low and faint pencil vanishes, too high and paper texture and show-through from the reverse turn into speckle.

The payoff is size. A colour scan of an A4 page can be several megabytes; the same page as a one-bit black and white PNG is often a tenth of that, because vast areas of identical white compress almost to nothing. That is why document management systems have used bitonal images for decades.

Printing black and white

A greyscale file sent to a colour inkjet is usually mixed from several inks, which is why prints come back with a green or magenta cast. If the print matters, find the printer driver's black-ink-only or greyscale setting rather than relying on the file. For a commercial press, talk to the printer before converting — they may prefer the colour original so they can control the conversion themselves.