Almost every phone has this feature, but most people don't know what it does

You might have noticed the three letters "HDR" on your smartphone's camera app when you're lining up a picture, and you might wonder whether to keep it on or off.

It's generally better to keep it on, and here's why:

HDR stands for "High Dynamic Range," and it helps make your pictures look better by making your smartphone take three pictures instead of just one. Each of the three pictures are taken with different levels of brightness (exposure).

Your smartphone's camera app then essentially mixes parts from the three pictures to create a single picture with the best balance of dark and light.

You won't see the three pictures with varying exposures in your phone's photo gallery, but one will be taken with high exposure, the other with normal exposure, and the last with low exposure. They'll likely look like the first three pictures below. The end-result is the 4th picture.

Here's a bigger version of the HDR photo, which take elements from all three differently exposed pictures:

It doesn't work with every situation

If your phone has an auto-HDR feature, you might not need to worry about the following situations. But it's good to know as the auto-HDR features in many smartphone aren't perfect.

HDR works best for dimly-lit environments without using flash (often indoors), close-up shots, most landscapes, and most portraits.

But if you want to keep stark contrasts like a silhouette against a bright window, or vivid colors, you're better off turning HDR off.

It's also not ideal if you or your subject is moving because your phone is taking three pictures instead of one, and you end up with some blurring.