Desaturating is not converting
Drag saturation to zero and you get a grey picture. It is almost never a good one.
The difference is what happens to the colour information. Desaturating throws it away. A real conversion spends it, deciding how bright each colour should become in grey, which is the only creative decision black and white offers and the one every one-click button takes away from you.
A blue sky can be a pale wash or nearly black. Skin can glow or go grey and blotchy. Grass can separate from the path beside it or merge into it. Same photograph, same pixels, entirely different picture, and it is decided by the weighting.
The filters are the whole tool
Before digital, photographers screwed a coloured glass filter onto the lens when shooting black and white film. The filter passes its own colour and blocks the opposite one, so it lightens what matches it and darkens what does not.
Red is the dramatic one. Sky goes almost black, clouds jump out white, skin lifts. It is the look people associate with mountain and desert landscape photography and it is one click.
Orange is red with the volume down. It deepens sky and flatters skin at the same time, which is why it is the standard choice for people outdoors.
Yellow barely announces itself. A little separation in the sky, nothing that looks done. If you are unsure, this is the safe one.
Green lightens foliage and separates leaves from sky, which is genuinely useful in woodland. It is hard on skin, so keep it away from faces.
Blue lightens sky and haze and darkens everything else. Rarely what you want, and exactly what you want for fog.
Why grass goes muddy on a straight conversion
The default weighting is not arbitrary. It is built on how the eye actually perceives brightness, and the eye is overwhelmingly most sensitive to green: roughly 71% green, 21% red, 7% blue.
That is correct for brightness and awkward for pictures. It means a green field, a red brick wall and a mid-grey road can all land on nearly the same tone, and a photograph where three different things become one tone has lost its structure.
The flat average, every channel equal, is technically wrong for the eye and sometimes right for the picture, because it keeps separation the weighted mix crushes. Both are here. Try both.
Contrast does more work in black and white than in colour
A colour photograph can rely on colour to separate one thing from another. Take that away and only tone is left, so the picture leans much harder on contrast than it did before.
Most photographs that look weak in black and white are not badly converted, they are under-contrasted. Pushing contrast further than you would ever push it in colour is normal here, and the classic film look is a good deal harder than most people expect.
Watch the clipping readout while you do it. Contrast pushes tones away from the middle, which means it pushes the extremes off the end of the scale, and pixels that reach pure black or pure white hold no detail at all afterwards.
Grain is not damage
Film grain is silver crystals, and it gave black and white photographs a texture that digital images do not have. A perfectly clean digital conversion can look oddly plastic, particularly in large flat areas like sky.
A little grain puts that texture back. It is monochrome here rather than coloured speckle, which is the difference between grain and noise, and it is seeded so the same settings give you the same result twice rather than shimmering every time the preview redraws.
Keep the original
The colour is gone from the converted file and there is no getting it back out.
Nothing here touches your original, and the download is a separate file. The risk is entirely human: the black and white version looks good, so the colour one gets deleted, and six months later somebody wants it in colour.
Common questions
Is black and white just turning the saturation down?
No. It is the single reason most conversions look flat. Pulling saturation to zero throws the colour information away. A proper conversion uses it: it decides how bright each colour becomes in grey, so you can make a blue sky dark and dramatic or light and airy from the same photo. Once the colour is gone you have no say left.
What do the coloured filters do?
They are the darkroom filters photographers screwed onto the lens for black and white film, and they work the same way here. A red filter lightens anything red and darkens anything blue, so skin lifts and sky goes nearly black. Orange is the gentler version and the usual choice for people. Green lightens foliage. Blue lightens sky and haze, which is rarely flattering and occasionally exactly right.
Why does a plain conversion make grass look muddy?
Because the standard weighting is built around how the eye sees brightness, and the eye is far more sensitive to green than to anything else. Green gets weighted around 71%, red 21% and blue 7%. That is correct for perceived brightness but it means a green field and a mid-grey wall can end up the same tone with nothing separating them. A green or yellow filter pulls them apart again.
Which photos work in black and white?
Ones that hold up on shape, texture, light and contrast rather than on colour. A picture whose whole point is a red door against a green wall becomes two identical greys and stops working. A picture with strong side light, texture or a clear subject usually gets better, because taking the colour away removes the distraction.
Can I get the colour back afterwards?
Not from the saved file. Converting to grey discards the colour permanently, so keep the original. Nothing here overwrites it, and the download is a new file, but it is worth saying because people delete the original once the black and white version looks good.
Does the picture leave my device?
No. It is decoded, converted and saved in your browser. Nothing is sent anywhere, there is no account and no size limit beyond what your device can handle.