Let the histogram help to determine proper image exposure. Tap 
A histogram is a simple graph showing the distribution of tones in the image. The left edge of the graph represents the shadows, with the furthest left point indicating pure black. The right edge represents the highlights in the image, with the furthest right point indicating pure white. That leaves the middle of the graph to represent the middle gray tones.
What should the histogram look like?
An ideal distribution reaches from end to end, ensuring there is a pure black point, a pure white point, and all tones in between. The distribution will change as adjustments like Brightness or Contrast are made to the image, so the actual shape of an ideal distribution can vary. A bell curve is a common distribution for a properly exposed image, for example, but not all properly-exposed images would result in a symmetrical curve.
Therefore, a better question may be, what should the histogram not look like? If the distribution does not reach one or both ends of the graph, and a normal exposure is desired, the image should be tuned so that the distribution stretches to touch both end points of the graph. Open the Tune Image Tool to make corrective adjustments.
Therefore, a better question may be, what should the histogram not look like? If the distribution does not reach one or both ends of the graph, and a normal exposure is desired, the image should be tuned so that the distribution stretches to touch both end points of the graph. Open the Tune Image Tool to make corrective adjustments.