Raster histogram

1. Click the Add raster layer and select the file C:/OSGeo4W64/geodata/raster/s2/Subset_S2A_MSIL2A_20170619T.tif.
2. After clicking Open, the raster layer appears on the QGIS canvas.
3. Right click the layer name in the TOC and select Properties --> Histogramm

This dialog displays a histogram for all bands band. Left click and drag a box on the diagram to zoom in. The value range of the raster data is evenly divided into bins. The histogram shows how many pixels of the image fall into each of the bins.