Histogram panel (Photoshop)
The histogram panel in Photoshop shows the distribution of tones in your image, from black on the left to white on the right. It reveals whether your shadows are crushed or your highlights are clipping. It is your objective measure alongside what you see on screen.
Open Window, Histogram and choose the expanded view to also measure per channel. If the graph piles up against the left edge you lose shadow detail, against the right edge means clipped highlights. While working with curves or levels you see the histogram move along, so you know how far you can push. Tip: a spike right against an edge is a warning that you are clipping there, always check that before you export.
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The presets on this site set these adjustments up for you as a starting point, which you then fine-tune to taste.