If you’ve ever seen (or taken) a picture where the sky is blown-out white, or impenetrable shadows block out the detail, you have encountered a dynamic range problem.
Dynamic range is the amount of difference in brightness or color that a camera sensor can capture without losing detail. The camera can’t record anything brighter than white, or darker than black.
Why does this happen?
Unfortunately, a camera’s dynamic range is a lot smaller than that of the human eye. This can lead to disappointment when taking a quick snap of a sunny day, or a dark-furred pet – while you can see clearly, the picture is anything but.
HDR, or high dynamic range, is the way around this. Technically, it’s an editing technique using two or more images, similar to shooting a panorama, but many cameras have it as a setting and take care of it all as you take the shot.
So, how does it work?
HDR does its job by taking several images at different exposures so every part of the picture is perfectly exposed in at least one photograph. Later (or immediately, if the camera is taking care of it for you), those pictures are knitted together, replacing any part of the main image where the detail has been lost with pieces from a different exposure.
In the end, you are left with something very close to what your eye saw – and what you expected the camera to see in the first place.
An important thing to remember about HDR photography is that it works by taking more than one image, and those images need to match up. Holding steady is always important to taking high-quality, sharp images, but it’s especially important here, where the shutter speed is effectively slower.
When taking a photograph on your phone, or using a camera’s built in HDR setting, it is likely that two or three pictures are being taken instead of one, which means holding still for two or even three times as long.
Confusing the term
So far we’ve talking about capturing an image with high dynamic range, but there is another use for the term HDR with almost the opposite meaning.
For a while, applying a HDR effect was very popular in photography – it’s still done today, but it seems to have lost some of its popularity.
Instead of compressing the dynamic range of an image, making it display well in a limited color space, the HDR effect sets out to make a normal image appear to be brighter and darker than the camera can capture in one shot.
The end result? A photograph of a normal scene with impossibly dark shadows and extreme lighting. Often, the same idea is applied to the color, again with results you would never see in the natural world.
Certainly eye-catching, but not what we’re talking about here. It’s a shame that the same term is used for two different things.
When HDR shines
Bright sunlight, night scenes and any time when there are big differences in lighting are good places to try HDR.
A lot of people get tempted to leave the setting on all the time – which is a mistake. Not only does it introduce a time lag, but it does, by its nature, change the image you get. Maybe that won’t cause a problem, but sooner or later it will. Remember, HDR exists exclusively to make extreme lighting behave itself. It is a specialty tool.
Now you know how to tame those tricky lighting situations. No more blown-out, white skies, no more dogs discernable as only a floating pair of eyes in shadow. Just remember to turn it off afterwards.