Contrast changes the separation between lighter and darker values. It can make a flat image feel clearer, but too much can remove gentle tones that describe a face, fabric or shaded product.
Image Contrast presents a contrast increase control while using PHP GD's contrast filter underneath. Positive values in this interface increase contrast; negative values reduce it.
Judge several regions together
Try a modest increase such as 20 on a flat photograph. Compare a shadow, a midtone and a highlight, rather than judging only the darkest area.
The source dimensions remain unchanged. Contrast is a colour value operation, not sharpening, resizing or a way to recover missing resolution.
At an extreme setting, shadows can become nearly solid and highlights can become clipped. The result may look dramatic while showing less useful detail than the original.
Text scans and portraits need different treatment
A weak document scan may benefit from stronger tonal separation. A portrait can become harsh under the same setting. Batch use applies one adjustment to every source, so a mixed batch deserves individual review.
The filter's numeric scale is not a universal camera or editor setting. A value of 20 here should not be assumed equivalent to 20 in an unrelated application.
Keep the original and compare at normal viewing size. If your main goal is to clarify edges, the sharpening tool addresses a different part of the image and should be assessed separately.
Use the smallest contrast change that helps the intended task. A form reviewer needs readable detail, while a social photograph may need a more restrained appearance. A higher number is not automatically a better correction.