There are many articles out in the web which explains Histogram in detail and technically as well. But this article is intended to demonstrate histogram in a simple way without going into in-depth technical details.
I have chosen a simple Black & White image to demonstrate it, because that is what the Histogram is fundamentally. Yes – “the graphical representation of luminescence level of each pixel in an any given image”.
Have a detailed look at the image, the top right portion of the image is covered by fog and it is bright. The bottom left of the image is covered by rocks and it is dark. Draw an imaginary diagonal line across the image from top left to bottom right. Now look again, it would be easy now to understand. The fog portion comes under whites and the rock portion comes under Blacks. Look at the respective Histogram below.
If you look at the photograph – almost the dark & light pixels are equal(quantity wise). Isn’t it.? Now look at the Histogram, isn’t the same data displayed here in the histogram clearly? The light pixels are on the right side of the histogram and the dark pixels are on the left side. That’s it. In between, the connecting pixels are mid tones, which you can again see it on the image as well. Across the diagonal line, the tones on both sides of the line will fall on mid tones except the top left of the image, where it will fall under blacks.
One more thing we have to understand is, any white area on the image will fall on the right side of the histogram, whereas blacks will always fall on the left side. For example, turn the image 180° horizontally. Now the fog will appear on your left side and rock on the right side but the histogram would remain the same.
Hope this article clear your initial blocks towards understanding the histogram. For a detailed understanding of Histogram and why should one push the histogram towards right, I would recommend reading this article in dPS. In another article will discuss about types of Histogram.
Cheers and Happy photographing.