[This post is the "TECH TALK" extension of the article "Punchy sunsets with HDR" from the blog]
You might have seen this: When taking pictures pointing at the sun, or even when it is a bright day, only parts of the photo are exposed correctly. Either the sky is white and the foreground is correctly exposed, or the foreground is black and the sky is correctly exposed. The image sensor of your camera can only display a limited brightness range, which is much lower of that of the human eye.
For example: While the sun is setting, the sky is still decently lit but the foreground is much darker. To correctly capture the foreground you need to use a longer exposure time, which leads to an overexposed sky. You can fixed this issue by shooting images with varying exposures and combining them with a computer software.
The images of the article “Punchy Sunsets with HDR” where shot with as a 3 picture exposure sequence called “bracketing” or “exposure bracketing”. One image over-, one under- and one is correctly exposed.