What is HDR?
In image processing, computer graphics and photography, high dynamic range (HDR) or high dynamic range imaging (HDRI) is a set of techniques that allows a greater dynamic range of exposures (the range of values between light and dark areas) than normal digital imaging techniques. The intention of HDR is to accurately represent the wide range of intensity levels found in real scenes ranging from direct sunlight to shadows. High Dynamic Range Imaging was originally developed in the 1930s and 1940s by Charles Wyckoff. Wyckoff's detailed pictures of nuclear explosions appeared on the cover of Life magazine in the early 1940s.
The process of tone mapping together with bracketed exposures of normal digital images, giving the end result a high, often exaggerated dynamic range, was first reported in 1993, and resulted in a mathematical theory of differently exposed pictures of the same subject matter that was published in 1995. In 1997 this technique of combining several differently exposed images to produce a single HDR image was presented to the computer graphics community by Paul Debevec. This method was developed to produce a high dynamic range image from a set of photographs taken with a range of exposures. With the rising popularity of digital cameras and easy-to-use desktop software, the term "HDR" is now popularly used to refer to this process. This composite technique is different from (and may be of lesser or greater quality than) the production of an image from a single exposure of a sensor that has a native high dynamic range. Tone mapping is also used to display HDR images on devices with a low native dynamic range, such as a computer screen.
The technique of taking HDR photos means taking bracketed shots at different exposures; one at the correct exposure and others which are underexposed and overexposed to create a number of images covering a range of exposures, e.g. -2EV, -1EV, 0EV, +1EV and +2EV. This technique is particularly useful when taking a high contrast scene - I’m sure we have all experienced those difficult shots with deep shadows and bright sky where you either have no detail in the shadows or blow out the sky completely.
Single RAW HDR
However rather than taking a number of bracketed shots when shooting in RAW format you have the opportunity to make a pseudo HDR image from just one image. As a RAW image has a higher dynamic range compared to a normal jpeg image you can produce a number of jpgs or tiffs from one RAW image at different exposures. You can produce 3 images at -2EV, 0EV, and +2EV or if you use 5 images, -3EV, -1.5EV, 0EV, +1.5EV and +3.0EV then use these images at different exposures to create a pseudo HDR image.
HDR Software Applications
There are a number of HDR processing applications but I use Photomatix which has a number of tools to assist you in this process. I also have PhotoMatix as a plugin for Aperture which provides easy access to the application directly from my main workflow application. Just recently I have been also using Nik Software's latest HDR application called HDR EFEX Pro which of course comes with Nik's wonderful U-Point technology allowing you to apply the HDR effect in precise locations in the image rather than the whole image.