We can get a more in-depth understanding of the Histogram by using synthetic test images. An image containing entirely 50% grey pixels will be represented as a single vertical line on the Histogram at the half-way point on the horizontal axis. This situation reveals one of the problems of Histograms: there is probably not enough vertical space on the graph to indicate 100% of the pixels of the image being the same luminance. Even if we decide to increase the height of the Histogram column by one pixel to represent every hundred pixels in the image which are at a given luminance, which sacrifices a lot of precision, an HD image would require a Histogram 20,736 pixels high to remove all possibility of the graph overflowing if all nearly-two-million pixels were at the same luminance (since 1920x1080÷100 = 20736) . Of course, this is not a realistic scenario for real-world photography, but it is possible to find photographic subjects that have a very limited luminance range. This can cause the top of many practical Histogram displays to be cut off, giving the impression that the luminance range is less limited than it is because we simply can't see the sharp peak on the graph.
The other problem with Histograms is that they provide no information about which part of the image is provoking a feature on the graph. Neither axis of the Histogram is related to the two axes of the photographic image. Histograms are a good, quick and easy way to get an idea of overall exposure, and an experienced eye will be able to estimate what's going on in most circumstances, but there is a better alternative.
This Histogram of a completely 50% grey image consists of a single line at 50%, indicating that all of the image's pixels are at 50% luminance