Our eyes and brain are very good at allowing us to perceive a great deal of detail in the real world. We can see details in the highlights of a real scene and other details in deep shadows. It is very difficult to get the same level of detail in a photographic image either on film or with digital techniques.

The range of tones in an image is called the dynamic range. If the range of tones in the scene is less than the range that the recording medium can manage then there will be no problem and you will get a reasonable record of the original scene.

Often this is not the case, especially when there the scene contains a lot of contrast, in ‘difficult’ lighting conditions – exactly the conditions when great pictures are possible. In these cases the dynamic range is determined by the recording medium – the film or the sensor – because the scene contains a larger range of tones than the recording medium can store. Something has to be sacrificed. Either the highlights will be blown or the shadows will block up or, in extreme cases, both.

In the days of film there was a limited amount that could be done to manipulate the dynamic range. A little bit of dodging or burning in could adjust the local brightness of the image or the grade of paper could be adjusted for more or less contrast but that was about it.

With digital imaging it is possible to increase the apparent dynamic range of an image. This does not actually increase the range of tones in the image but makes the best use of them to fool our eyes and brain into perceiving an image with a high dynamic range.

The trick here is to take two images, one exposed for the highlights and one exposed for the shadows, and then combining them. The idea is that you will keep the highlights from the first image and the shadows from the second. Sometimes this works with two different conversions of the same RAW file but often the level of noise in the dark areas becomes unacceptable.

Image exposed for the highlights:

Image exposed for the shadows:

You have to be careful when making the exposure that the two images will line up. It is easiest to use a tripod and a cable release for this. You also need to minimise subject movement between shots which can be difficult on a windy day. Determining the correct exposure for the two images can be interesting. The easiest way is to use your usual metering method for the ‘highlight’ exposure, possibly exposing to the right. On this exposure the shadows may well block up but they will be taken care of in the next shot. Next take a spot meter reading from a shadow area for the ‘shadow’ exposure. On this shot the highlights will probably burn out but they are safe in the first exposure so it does not matter.

There are several methods of combining the two images that you have made. Adobe Photoshop CS2 has a built in HDR script. I also have an HDR action that was recorded using Photoshop CS. You can download it here complete with instructions. Use it at your own risk! My action uses the method outlined in this article from the Luminous Landscape, which also shows another two methods of combining the images.

Combined image:

Copyright © Greg Wright 2006 - All rights reserved