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To understand , and therefore ,make good use of the Histogram you have to understand that it represents the distribution of tones present on the original scene , but placed over the Exposure Graph according to our exposure decisions.

And to understand what proper exposure decision means , we have to discuss the different Tonal Distributions found in Nature .



A less common distribution is when one group of tones dominate .

When the Highlights are more present and intense than the rest of tones we have what is called a High Key Scene; Snow and Beach Scenes are good examples and so are Clouds:




Some Scenes have just a short range of Mid Tones and no Deep Shadows or Extreme Highlights , These are Soft , Low Contrast Scenes, usually the product of Fog , Smoke or deep Forests with dense canopies that only let in Diffuse light.



Here comes the important part :

Proper exposure decisions are those taken during the capture to correctly place a reading taken of an 18% Reflectance Gray Card , of which a weathered piece of Wood is a good example ,exactly at the Mid Tone Point of the Histogram , at Binary Value of 128,( the midpoint on an 8 Bit scale that goes from 0 to 255). This point brightness ,represents the relative brightness in Nature of a Middle gray and you want to reproduce it as such.

A hand held Exposure Meter reading of such a card will do just that , because its calibration is based on this principles , but a camera built in meter is very different because it breaks the Scene in Zones and attempts to identify a value that will put the exposure as far up the Histogram , without Over saturating the Sensor . In a Typical six stop Dynamic Range Scene , this will work out pretty much the same and a Mid tone Gray will be placed somewhat close to where it belongs.



By looking at the Scene and recognizing what kind of Scene we are dealing with we will be enabled to make the proper decisions needed to correctly place the Mid tone within the Histogram so that the entire distribution will look like the proper one.

Low key scenes should have Low key Histograms and placing their tonal values higher up in the exposure curve will not reproduce the Scene that you saw, it will , in effect be overexposed in reference to Reality.






However That Histogram could also be the wrong capture of a Scene consisting ONLY OF SHADOWS , by gross overexposure, placing the shadow values too high, and it will require adjusting the exposure down , during conversion.


If you try (Wrongly!!) to increase its Global contrast by adjusting the Clipping points inward in Levels, , because there are insufficient tones to begin with , the stretching will create separation of tones called banding. The gaps show colors or tones that were lost and will not be present in the converted image.

In the Histogram that will show like this: