How to use Image Brightness
- Choose one or more photos.
- Drag Brightness (−100% to +100%, starts at +15%).
- Drag Contrast (−100% to +100%, starts at +10%).
- Pick Save as and Quality, then press Apply.
- Download the files ending in -adjusted. If highlights look blown out, lower Brightness and run it again.
What it does and when to use it
This is the quick fix for pictures taken in poor light: a room lit by one lamp, a cloudy day or a screenshot of a dim video. Raising brightness lifts every tone; raising contrast pushes darks and lights further apart around the middle grey, which adds punch. Negative contrast does the opposite and gives a soft, low-contrast look.
Brightness is a blunt tool, so check the result. For finer control use Image Gamma to lift only the midtones, or Image Contrast alone. The Image Histogram tells you before and after how many pixels are clipped to pure black or white.
How it works
The tool builds a 256-entry lookup table and applies it to the red, green and blue channels of every pixel inside a Web Worker. With b and c as the slider values:
out = ((v × (1 + b/100)) ÷ 255 − 0.5) × (1 + c/100) × 255 + 127.5
The result is rounded and kept between 0 and 255. This is the same maths as the CSS filter brightness() followed by contrast(). Transparency is not touched, and the file never leaves your device.
Worked examples
Values computed with the tool’s own lookup table.
Default settings (+15% brightness, +10% contrast).
| Input value | Output |
|---|---|
| 0 (black) | 0 |
| 50 | 50 |
| 128 (mid-grey) | 149 |
| 200 | 240 |
| 211 | 254 |
| 212 and above | 255 |
Shadows stay put because the contrast pulls them down about as much as the brightness pushes them up, while light tones rise strongly. That is why the default suits dull, grey-looking pictures.
Darker, no contrast change (−30%, 0%). 255 becomes 179, 200 becomes 140 and 128 becomes 90. Pure white turns light grey, which is handy for dimming a busy photo behind text.
Limits and tips
- Lost detail does not come back. Values clipped to 255 or 0 are flat in the saved file.
- JPEG photos show noise when strongly brightened; small steps work better than one large one.
- The adjusted copy carries no EXIF data such as the date or camera model.
- To shrink the file afterwards, use Compress Image to 200 KB.
Frequently asked questions
Why do light areas turn pure white?
What do the default values do?
How is this different from gamma?
Can I make a photo darker?
Is the original file changed?
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