Photography is all about capturing light. Every image is a collection of pixels that have recorded the amount and colour of light falling on the camera sensor (or film).
In most cases though, the camera will have a problem with scenes that are lit with extremes.
Take a typical landscape scene. The foreground is usually dark, and the sky is usually bright (unless you live in England, in which case it’s overcast and featureless!). When you set the exposure settings on the camera, you’re telling the camera how much light to allow onto the sensor. If you’re on auto, then the camera makes the decision. In cases of extreme contrast (lots of bright lights and lots of dark shadows) the camera gets confused, and doesn’t know what to expose for. In most cases, a single shot of the scene will expose for one or the other, leaving you with half an image as you either can’t see into the inky black shadows, or the bright areas just show as white.
HDR is a means of capturing both the light and dark areas, revealing more closely the scene as you remember it.
The process is twofold. Firstly, you need to take multiple exposures of the scene. Then, you feed those exposures into some HDR software which blends the exposures together.
Take a look at the example image below of Castlerigg stone circle in the Lake District.
As you can see, there are bright areas in the sky, and yet the shadows behind the rocks still reveal detail and colour. This is an HDR image.
Some photographers argue that using a neutral density graduated filter can achieve the same result. Quite simply, it can’t. A graduate filter blocks light from a certain part of the image, and is generally used to “darken down” a sky to bring the exposure times in-line with the foreground.
The problem with ND Grads is that they can leave a “band” across your image. Light is measured (in photographic terms) in “Stops”, where each stop doubles or halves (depending on whether you increase or decrease) the amount of light. So if there are 3 stops of difference between the foreground and sky, quite a severe filter is required to “reign in” the exposure difference.
With HDR, multiple exposures are taken, in “stop” increments, to capture all the detail from the darkest to the lightest areas. When processed in the appropriate software, this ensures a complete picture of the scene.
HDR has been lambasted in the past, and every photographer I know who has tried it has initially gone too far. However, as a tool to recreate the scene before you, and when used appropriately, HDR is an excellent weapon in a photographers arsenal.