What does HDR actually do on a phone camera?

A single exposure cannot record a bright sky and a shaded face at once — the sensor's range is not wide enough. HDR is the workaround, and on a modern phone it is running on almost every shot whether or not there is a button for it.

Step by step

1. Understand the mechanism.Several frames are captured within a fraction of a second at different exposures, aligned, and merged. Bright areas are taken from the short exposures, dark areas from the long ones, and the result is compressed back into a normal image file.
2. Recognise where it earns its place.Anything with a large brightness range — a person against a window, a room with a bright doorway, a landscape with sky, a backlit subject. Without it these are either a silhouette or a white sky.
3. Watch for ghosting.If something moved between frames — leaves, water, a hand, a passing car — the merge can leave a faint double image or a smeared patch. It shows up most at edges of moving objects.
4. Notice the flattening.Aggressive merging lifts every shadow and pulls down every highlight, so the picture ends up with detail everywhere and contrast nowhere. Real photographs have dark areas. A shot where nothing is dark reads as artificial even to people who cannot say why.
5. Take control with exposure lock.You cannot usually switch the merging off on a modern phone, but you can bias it. Lock on the important part of the scene and slide the exposure down a little; the shadows go darker, the highlights stay clean, and the result looks more like a photograph.
6. Check what your phone saves.Some phones store an HDR file that displays brighter on their own screens and flat or odd elsewhere. If your photos look great on the phone and washed out on a laptop or when shared, that is what is happening, and there is usually a setting to save a standard file instead.
7. Turn it off for deliberate silhouettes.A sunset with a figure in front of it is meant to be a silhouette. HDR will helpfully rescue the figure and destroy the picture. Lock the exposure on the sky instead.
8. Do not confuse it with the HDR on your television.Same three letters, different problem. On a TV it means the display can show a wider brightness range. On a camera it means several exposures were merged into a normal one.

Tips

• For anything moving, HDR is a liability. Lock exposure for the subject and accept a brighter or darker background.
• If a phone photo looks strangely flat, reducing the shadow lift in editing usually restores it in one slider.
• Bracketing on a proper camera is the same idea done manually, and gives you control over the merge.

Common mistakes

• Blaming the lens for a soft, doubled edge — Ghosting from a merged burst looks like motion blur or poor optics. It appears only at the boundaries of things that moved, which is how you tell.
• Expecting HDR to fix a backlit face at night — The merge needs the shadow detail to exist in one of the frames. If the face is genuinely unlit, no amount of merging invents light — you need to add some or move the subject.

Questions people ask