How to convert a photo to black and white
Just desaturating a colour photo often looks flat in black and white, because colours that looked different can turn into the same shade of grey. Here is why, and a free tool that fixes it.
Why plain desaturation can look flat
The simplest way to remove colour, averaging the red, green and blue values of each pixel, ignores that the human eye does not perceive those colours as equally bright: a saturated red and a saturated green can average out to nearly the same grey even though they looked completely different in colour, which flattens contrast that was doing real work in the original photo.
How a proper conversion handles this
A weighted grayscale conversion, the kind built into most photo editors, gives more weight to green and less to blue when computing brightness, matching how human vision actually works, which keeps more of the original contrast intact. Beyond that, black-and-white photography traditionally used coloured filters (a red filter to darken blue sky, for example) to control how colours map to tones, an effect some black-and-white converters replicate digitally.
In Photoshop or Lightroom
Photoshop's Black & White adjustment layer and Lightroom's black-and-white panel both let you push individual colour channels lighter or darker in the conversion, which gives the most control, but both need a Creative Cloud subscription and some skill to use the sliders well.
Converting in the browser
Stayput's black and white photo tool offers grayscale, a higher-contrast look, pure black-and-white for scans and signatures, and a sepia tone, with a live preview so you can compare options before saving, and batch processing for more than one photo at a time. It runs entirely in the browser, so nothing is uploaded.
Questions
Why does my black and white photo look flat or low-contrast?
A simple desaturation can average different colours into nearly the same shade of grey, losing the contrast that was visible in the original. A weighted conversion that accounts for how the eye perceives colour keeps more of that contrast.
What is the difference between grayscale and pure black and white?
Grayscale keeps a full range of grey tones between black and white; pure black and white (sometimes called "threshold" or "1-bit") maps every pixel to either fully black or fully white, which suits scanned documents and signatures better than photos.
Can I convert several photos to black and white at once?
Yes, Stayput's black and white photo tool supports batch processing for multiple photos in one pass.
Does converting to black and white reduce the file size?
Slightly, since a grayscale image needs less colour data than a full-colour one, though the effect on file size is usually modest compared to other compression.