Creating HDR Photos With Open Source Tools

Mike Bedford investigates how to generate dramatic photos, containing much more of the tonal variation we actually see in a scene.

- Mike Bedford

According to a large majority of digital camera users, the quality of photos depends on one thing and one thing only: resolution, in megapixels. If only things were that simple. In reality there are loads of facts and figures that differentiate a good camera from a mediocre one, but we’d like to think specifically about just one here: the dynamic range. In plain English, this is a measure of the difference between the dimmest and the brightest element in an image that can be recorded.

You could find a range of up to 100,000:1 in some daylight scenes, while the human eye can cope with many thousands to one. The depressing fact is that cameras don’t come close to the eye, even though a good DSLR will perform better, in this respect, than most phones or point-and-click cameras. The upshot is that it’s often impossible to correctly record the subtleties of tone in both the brightest and darkest areas of many scenes. The limited dynamic range of a camera can result in a serious lack of detail compared to what you saw with your eyes.

In particular, depending on the exposure, either lighter areas such as the sky could be an almost uniform white, or the darker areas, for example shadows, a uniform black. HDR – that’s High Dynamic Range photography – overcomes this problem, as we’re about to see.

Say cheese!

The basic principle of HDR photography is to take several photos at different exposures, so that between them, you’ve captured much more of the tonal variation that was present in the scene than can be recorded in a single shot. Later on we’ll investigate ways in which those multiple shots can be combined to create a single HDR photo, but to start we need to give some guidance on taking a set of photos at different exposures, or a set of bracketed shots as photographers would say.