HDR

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.

Tone Mapping

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.

Bracketing

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.

There are a number of HDR processing applications but I use Photomatix which has a number of tools to assist you in this process.

Example

Now let's take this image of Ta Phrom temple in Siem Reap, Cambodia and see how we can do a pseudo HDR technique. Here is the photo taken at 0EV exposure and as you can see we have some very dark shadow areas and a very bright sky overhead resulting in an average exposure which does not bring out any detail in these two areas.

So taking the original RAW image in Aperture we can adjust the exposure setting to a number of settings such as here, -3EV, -1.5EV, +1.5EV and +3EV to produce four more jpg images.

-3EV

-1.5EV

+1.5EV

+3EV

We then import all the five jpg images into Photomatix where we can run through a tone mapping process which blends these 5 images into a single image. Here is the final HDR image which you can see clearly has well defined exposure of the sky and the dark shadow areas.