Reading the Histogram panel in Paint by Zaweba

A histogram is a graph of a picture's tones. It's one of the most useful things to keep on screen while editing, because it shows facts your eyes can miss, like whether highlights are clipping or shadows are crushed. The Histogram panel in Paint by Zaweba lives at View > Panels > Histogram.

What it shows

The horizontal axis runs from black on the left to white on the right. The height at each point shows how many pixels have that brightness. A photo with lots of shadow has a pile on the left; a bright, high key photo has a pile on the right.

You can choose what to measure: the active layer or the whole image. If there's a selection, the histogram covers only what's inside it, which is handy for checking a face or a sky on its own.

You can view it as colours, showing red, green and blue together, as luminosity, showing overall brightness, or as a single channel.

The numbers

Below the graph are statistics. The mean is the average brightness. The standard deviation shows how spread out the tones are; a low value means a flat picture, a high one means strong contrast. The median is the middle value. The pixel count is how many pixels are being measured.

Move the mouse over the graph and the panel shows the level under the pointer, how many pixels have that level, and the percentile, meaning what share of pixels are at or below it.

Using it while you edit

Exposure checking is the main use. A tall spike pressed against the right edge means clipped highlights, areas of pure white with no detail. A spike against the left edge means crushed shadows. Some clipping is fine in small areas like the sun or a deep shadow; broad clipping usually isn't.

Contrast checking comes next. A histogram bunched in the middle with empty space at both ends is a flat photo. Levels or Curves can spread it out.

Colour checking uses the colour or channel view. If one channel sits far to the right of the others in a picture that should be neutral, there's a cast.

Selections make it precise. Select a patch of white wall and look at the channel views; if they don't line up, the wall isn't neutral.

Reading it well

There's no correct shape. A night scene should be weighted to the left. A snow scene should be weighted to the right. Judge the histogram against what the picture is meant to be.

Gaps and spikes in a comb pattern can show that a picture has been stretched by heavy edits, especially at 8 bits. If you see that, consider working at 16 bits.

Misreading the graph

Trying to make every histogram fill the whole width can ruin moody or high key photos. Let the picture decide.

Forgetting which layer is measured is the other trap. If the numbers look wrong, check whether the panel is set to the active layer or the whole image, and whether a selection is limiting it.

Keep the panel open as you work in Paint by Zaweba, and glance at it after each adjustment. Before long, you'll read it as easily as the picture itself.