High Dynamic Range (HDR)
In the article on metering light, I introduced the dilemma we face when photographing a scene with a dynamic range that exceeds the dynamic range of the camera - see exposing to the right. A typical example would be photographing a landscape on a bright day when we might wish to keep detail in the clouds as well as in the shadows. The dynamic range is the difference in Exposure Value between the brightest and darkest parts in the scene. This could be of the order of 15 to 16 stops but a typical DSLR might only be able to record about 10 stops. We are therefore faced with the undesirable options of blowing out the highlights in the clouds or loosing detail in the shadows.
The solution to this problem is to take a set of photographs at differing exposure settings such that the set covers the entire dynamic range of the scene. Traditionally this meant setting the camera on a tripod and taking at least 3 images, ensuring that one captured the shadow detail, one captured the mid-range and one captured the highlight detail. The images were then stacked in different layers in Photoshop or similar software and carefully masked by hand to produce a final image.
Happily, modern technology makes the technique much less painful. Some modern cameras have an HDR mode which can be selected to do this automatically, typically taking a set of 3 photos with exposure compensation set to -3, 0, +3 stops (for example). A single press of the shutter release then takes the photos in quick succession and the camera merges the individual photos together into an HDR image.
While in-camera processing is certainly convenient, for more critical work the set of images are merged in post processing using software such as Affinity Photo or Photoshop, which now have specific facilities for achieving the merge semi-automatically.
The picture below shows an HDR image of a table-top still life illuminated by a desk lamp.
As the camera was pointing directly at the bright filament of the lamp, it was not possible to set an exposure that would capture details of the items on the table without the lamp becoming completely blown out to white - see left-hand image below. I therefore took a set of images with exposures ranging from that needed to capture the details on the table all the way down to an exposure to capture the lamp filament, which needed an exposure 9 stops darker. The thumbnails below show the first image (exposed for the table top), an intermediate image (exposed for the relatively bright coasters) and the last image (exposed for the lamp filament).
In Affinity Photo, I opened a new HDR Merge file and imported the set of images. The software then conducts a technique known as "tone mapping" to compress the very wide range of tones into a smaller range that can be displayed on screen and printed. Some finishing adjustments were then made to the resulting tone-mapped image.