Brighter images have brighter pixels, therefore the bars to the right are higher than those at the left.
Darker images have darker pixels, therefore the bars to the left are higher than those to the right.
In addition, if we take a look at the two images below:
Image 1 | Image 2 |
We can see that in Image 1, there is a higher contrast between the brighter and darker part of the image. Bright and dark pixels occupy a larger portion of the total image. Therefore, in the histogram there are higher bars in the left (darker side) and right (brighter side), and in the centre the bars are relatively much lower, which means that there are only a few midtone pixels in the image.
In image 2, pixels in objects possess greater levels of brightness and darkness. The distribution is more even than that in Image 1 as well. Therefore, the brighter, midtone and darker pixels share evenly the total pixels in the image. This will result in the histogram for Image 2 appearing smoother.