In more detail
On 8-bit images each channel runs from 0 to 255. Brightness adds or subtracts the same amount from every value; contrast scales values away from (or towards) the middle grey, roughly new = (old − 128) × factor + 128. Grayscale weights the channels by how the eye sees them, using 0.2126 R + 0.7152 G + 0.0722 B (Rec. 709) or the older 0.299 R + 0.587 G + 0.114 B (Rec. 601) that many tools still use, so green counts most. Blurring and Pixelation are a different kind of change: they remove detail.
Why it matters
Results are clipped to 0–255, so pushing brightness or contrast too far turns highlights pure white and shadows pure black, and that detail cannot be recovered from the saved file. Repeated adjustments on 8-bit data also cause banding in smooth skies. Work on a copy and make one combined adjustment rather than several in a row.
Example
Take the pixel (200, 100, 50):
- brightness +60 gives (260, 160, 110), clipped to (255, 160, 110)
- contrast × 1.5 gives (236, 86, 11)
- grayscale is 118 with the Rec. 709 weights, or 124 with Rec. 601