How to use the image brightness and contrast adjuster
- Choose an image or drop it on the box. JPG, PNG, WebP, GIF, BMP and AVIF files up to 25 MB work. SVG drawings and other file types are refused.
- Move the Brightness slider to make the picture lighter or darker, and the Contrast slider to make the difference between light and dark areas smaller or larger. Each goes from -100 to +100, and the preview updates as you move.
- Choose PNG to keep transparency or JPG for a smaller photo file. Press Adjust and download. The saved file has the same pixel size as the original, not the smaller preview.
- Use Reset to return both sliders to 0.
Formula
new value = (old value + brightness x 2.55 - 128) x (100 + contrast) / 100 + 128
rounded half up and limited to 0 through 255, for each of red, green and blue
Brightness adds that percent of the full 0 to 255 range to every color channel. Contrast multiplies the distance from mid-gray (128) by a factor from 0 at -100 to 2 at +100. Transparency is left alone.
Brightness plus 20
Brightness +20 adds 20% of 255, which is 51, to every channel. A pixel of red 100, green 150 and blue 200 becomes 151, 201 and 251. A pixel of 0 becomes 51 and any value above 204 reaches the limit of 255, so very bright areas lose detail.
Contrast +50 multiplies the distance from 128 by 1.5. A value of 100 becomes 86, a value of 200 becomes 236 and a value of 128 stays 128. Contrast -100 flattens every value to 128, a plain mid-gray, and contrast -50 compresses 0 to 64 and 255 to 192. These tables were checked value by value against exact fraction arithmetic and against the libvips image library.
Brightness and contrast, in plain terms
Brightness shifts every part of the picture lighter or darker by the same amount. Contrast changes how far apart the dark and light parts are. Raising contrast makes darks darker and lights lighter around the middle gray, which can make a flat picture look sharper. Lowering it brings everything toward gray.
Both adjustments can lose information. When a value is pushed past 0 or 255 it is cut off, which turns a range of dark or bright shades into one flat shade. Small changes are safer than large ones, and a modest increase in both often helps a dim photo more than a large change in one.
What is saved
The preview is a smaller copy so the sliders stay quick. When you save, the full image is processed in strips, so even a large photo uses little memory, and the output has the original width and height. PNG keeps transparency and is not recompressed with loss. JPG is smaller for photos and fills any transparent area with white.
Information stored inside the file, such as the camera model, date and location, is not copied to the new file. Photos taken on a phone and stored sideways are turned upright, as your browser shows them. To change the size or the file type, see the image resizer, the image compressor and the image converter. To cut part of a picture away, use the image cropper.
Privacy and limits
The picture is read and changed in your browser. Nothing is uploaded. Pictures are limited to 16,384 pixels on a side and 120 megapixels, and the file must be 25 MB or less. Pixels that are only partly transparent may change slightly in color, because the browser stores them premultiplied.
Assumptions and limits
- The same change is applied to red, green and blue. Color balance, saturation and curves are not adjusted.
- Values are rounded half up to whole numbers from 0 to 255.
- The saved image has the original pixel size. The preview is a reduced copy.
- Only the first frame of an animated GIF is used.
Frequently asked questions
What does brightness plus 20 do to pixel values?
It adds 20% of the full range to each color channel, which is 51 out of 255. A channel at 100 becomes 151, and anything above 204 is cut off at 255.
Why does my picture look washed out after I raise brightness?
Bright areas hit the maximum and lose their detail, and the darks lift toward gray. Add some contrast, or use a smaller brightness change, to bring depth back.
Is the original file changed?
No. The tool reads your file, makes a new image in memory and saves it as a separate download. The original stays as it was.
Why is an SVG not accepted?
An SVG is a drawing made of shapes, not pixels, so brightness and contrast on pixels do not apply to it. Convert it to a PNG first if you need to adjust it.