Black and White Image Converter

Greyscale, monochrome and true black and white are three different things

People use the words interchangeably and then wonder why the result is not what they pictured. Greyscale keeps every tone between black and white — a photograph stays a photograph, it just loses its colour. Monochrome means a single hue, so sepia and cyanotype are monochrome but not greyscale. True black and white, properly called one-bit or bitonal, allows only two values: a pixel is either black or white, with nothing in between.

Choose by what the image is for. Greyscale for photographs. Sepia for a deliberately aged look. True black and white for scanned text, signatures, stencils, vinyl cutting and laser engraving, where anything grey has to be resolved one way or the other before the machine can use it.

Why the channel filters matter

Before digital, photographers screwed coloured glass onto the lens to change how black and white film rendered colours. A red filter darkened blue skies until the clouds leapt out; a green one lifted foliage. The same physics applies here, because converting to grey means deciding how much each colour channel contributes.

Getting a scanned document right

Scans are the most common reason people reach for true black and white, and the cut-off slider is the whole game. Set it too low and faint pencil or a light signature disappears; set it too high and the paper's own texture, shadows and show-through from the other side of the page turn into speckle.

Start at the default, then nudge until the text is solid and the background is clean. If one corner of the page is shadowed and refuses to clear, raise the brightness a little before adjusting the cut-off — that lifts the whole page relative to the threshold rather than sacrificing the text.

What black and white does to file size

• Greyscale JPG — typically 20 to 30% smaller than the colour original, because the chroma channels carry no information.
• Greyscale PNG — a similar saving on photographs, much larger on flat graphics.
• True black and white PNG — often a tenth the size of a colour scan, because vast areas of identical white compress almost to nothing.

That last one is why fax machines and document management systems standardised on bitonal images decades ago, and why it is still the right format for archiving a filing cabinet.

A note on printing

A greyscale file printed on a colour inkjet is often mixed from several inks, which is why prints can come out with a green or magenta cast. If the print matters, ask the printer driver for "black ink only" or "greyscale" rather than relying on the file alone. For anything going to a commercial press, talk to the printer before converting — they may want the original colour file so they can control the conversion themselves.