Capture One has four histogram panels, and their significance when evaluating the tonal characteristics of an image at different steps from the capture and adjustment prior to output should not be underestimated. Each of the histograms is similar in that they display the frequency of pixels in the vertical (y) axis, against their separate color (Red, Green, Blue) and luminance values along the horizontal (x) axis.
In general terms, an optimal histogram should display a full range of tones without being excessively bunched with spikes at either end. Any pixels captured at or adjusted beyond that range have no tonal information, and thus no detail and are said to be clipped. The histogram can also inform you of subtly light or dark images, under- or overexposure, or when bunched in the center with a long comb either side - when there’s a lack of contrast.
Although the histograms are alike in Capture One, each one reflects the distribution or tonal range of those pixels at certain stages in the processing pipeline:
Exposure Evaluation: The Exposure Evaluation tool shows the actual RAW data information as shot in camera together with the selected Film Curve and the White balance/Tint. The Exposure Evaluation histogram only changes if a crop is applied or a Film Curve is changed or if white balance/tint changes. No other adjustments will be reflected in the tool.
Levels histogram: The Levels tool takes more adjustments into account. Adding exposure, contrast, saturation, brightness, HDR, Vignetting and Clarity settings to the Levels histogram. However, it excludes itself from the displayed values. This enables the user to adjust the brightness from the darkest pixels to the brightest pixels and adjusting the midtones or inverting the image. The RGB Channel modes also enables to correct for color shifts between the channels.
Curve histogram: Similar to the histogram in the Levels tool, however, this one will also reflect any adjustments done to the black and white point (set in the Levels tool with Pick Shadow Level and Pick Highlight Level cursors).
Histogram tool: The Histogram tool reflects what you see in the preview including all adjustments such as Curves, Color Edits, ICC profiles etc. It shows the histogram based on ALL adjustments applied on top of the RAW data and shows all the changes you have made.