What each slider actually changes
Brightness shifts every tone in the image up or down together. It is the right control when the whole photo is too dark or too pale, and the wrong one when only the shadows need help, since it washes out the highlights at the same time.
Contrast stretches the gap between dark and light. Increasing it makes blacks blacker and whites whiter, which reads as sharper even though no detail has been added. Too much of it crushes shadow detail and blows out highlights permanently.
Saturation controls colour intensity alone, leaving lightness untouched. Dropping it towards zero approaches greyscale, while pushing it up can quickly make skin tones look unnatural.
Fixing a document scan
Scans photographed with a phone almost always need the same treatment. The paper comes out grey rather than white and the text looks soft. Raise contrast until the paper reads as white, then nudge brightness if the page still looks dull, and lower saturation to remove any colour cast from the room lighting.
Stop as soon as the text is clearly readable. Pushing further starts to eat the thin strokes in letters, and a document that is technically legible but visibly manipulated can raise questions at verification.
Fixing a photo for a form
Form photographs need an evenly lit face against a plain background. If the photo was taken indoors and the face sits in shadow, a modest brightness lift usually solves it. Avoid heavy contrast here, since it deepens shadows under the eyes and can make the face harder to recognise.
Frequently asked questions
What is the difference between brightness and contrast?
Brightness lifts or lowers every tone by the same amount, so the whole image becomes lighter or darker. Contrast pushes dark tones darker and light tones lighter, which makes the image look sharper without changing its overall lightness.
My document scan looks grey. What should I change?
Raise the contrast first. A grey scan usually has a washed out range where the paper is not white and the ink is not black. Contrast pulls those apart. Add a little brightness afterwards if the paper still looks dull.
Can I rescue a very dark photo?
Partly. Brightness reveals detail that was recorded but hidden in shadow. Where the sensor recorded nothing at all, raising brightness only produces grey noise, so a severely underexposed photo has limited scope for recovery.
What does saturation do?
It controls how intense the colours are. Lowering it moves the image towards grey, raising it makes colours more vivid. For document scans, reducing saturation usually gives a cleaner result.
Should I adjust before or after compressing?
Adjust first, then compress. Raising contrast on an already compressed file amplifies the compression artefacts along with everything else.
Is the preview accurate?
Yes. The preview uses the same values the output is rendered with, so what you see is what the downloaded file will look like.