13.2.4. The histogram

Below the frame buffer viewer, the histogram pane plots the distribution of pixel values in the current frame – or, when a region is selected in the frame buffer, in just that region. It updates live with the preview, so you can read what the camera sees numerically: what colour an object is, how much it varies, and how it separates from the background.

The histogram in RGB: one plot per channel with its statistics underneath. The readout line on top ends with the focus number.

The selector in the pane’s title bar chooses the colour space the statistics are computed in: RGB, Grayscale, LAB, or YUV. Each channel gets its own plot, with the channel’s statistics underneath: mean, median, mode, standard deviation, minimum, maximum, and the lower and upper quartiles. Select a region in the frame buffer to read the values of one object instead of the whole scene.

13.2.4.1. The focus number

The readout line above the histogram ends with a focus metric – a sharpness score computed from the image. Its absolute value means nothing; its direction means everything. While adjusting a lens, watch the number: it rises as the image gets sharper and peaks at best focus. Point the camera at a detailed target at the working distance, turn the lens slowly, and stop at the maximum.