What is HDR?

Dynamic range refers to the variation in luminance from the brightest to the darkest areas of a scene. High Dynamic Range simply means a wide range of brightness values. HDR Photography is the process of taking several pictures of a scene at various exposure levels, then merging the images into one file to maximize the dynamic range of the captured scene.

Why Bother?

Whilst the human eye can see exposure value (EV) up to 20 EV, Digital cameras (and monitors) can cope with a lot less; a raw file can record a maximum of about 6 - 10 EV (less than negative film). So, if we take our camera into a church with stained glass windows and try to photograph the scene, the chances are that either the windows will have blown out the highlights (not recording any pixels) with the shadows recorded, or if the exposure settings on the camera are adjusted for the highlights, then the shadow areas will be too dark and afflicted with noise. HDR photography enables you to capture all the exposure values in a scene which has a high dynamic range.

Phew! How?

Several photo's are taken of the same scene at different exposure settings, so that the highlights are recorded as well as the shadow areas: for example, one image would be exposed for the highlights without any concern for the dark areas; another exposure calculated to expose the darkest areas without concern for the highlights, and a third in between to capture the midtones. These files are then merged into one 32 bit HDR file. After merging the images, tone mapping must be done to the HDR file.

What's Tone Mapping?

Tone mapping is the process of scaling back a 32 bit image into a lower one, or, converting the tonal values of an image from a high range to a lower one, so that the image can be seen on the monitor and subseuently printed, with the highlights not blown and the shadow or dark areas well exposed and noise free.

So what's the best way to take the photo's?

Ideally, the images you take (probably 3 or more)need to be identical in all other aspects apart from exposure, which has to vary to record the high dynamic range. Therefore, using a tripod is essential, and any other techniques to minimise camera shake should be employed - mirror-up, remote release etc. If your camera has AEB (Auto Exposure Bracketing) and can be adjusted to 2 EV spacings, then this would be a good start point; but you would need to evaluate the range of exposures you are attempting to capture in an individual scene (either pre shooting or post shooting by checking your histogram on the camera). If you don't use AEB, you need to adjust the exposure manually - but because you want to avoid variation in your depth of field, you vary the shutter speed rather than your aperture, or use Aperture Priority. It may be necessary to take more than 3 photo's to capture the full range.

Is it possible without a tripod?

Aparently so, and it would be best then to use AEB with AP, set to continuous shooting, support the camera as well as you can, shoot and hope for the best.

I've got the images, What's next?

The images are merged and tonemapped using software, either through an imaging software (eg Photoshop) or a stand alone software. However, CS3 is pretty useless at HDR, and the stand alone software that has become the most popular is Photomatix. ( Link ) I haven't tried CS4 and according to Martin Evening, (Adobe Photoshop for Photographers, 2009, p368-382)CS4 is supposed to be better than CS3.

Unfortunately, Photomatix isn't free, although you can download a free trial version. I've used Photomatix a couple of times and it works very well at merging the images, thereafter the software has a number of different controls for tone mapping, and it is possible then to create almost surreal effects, there are a lot of examples on the website including a quick tutorial. You can use Photomatix tone mapping with one image, and use the software controls to produce different effects.

Isn't there a simpler way than all this HDR stuff?

It is possible to process the same image differently in Camera Raw and open the image in Photoshop and blend them together, eg a landscape image could be processed in Camera Raw so that the highlights look correct (but the shadows too dark), and a second Camera Raw conversion done so that the shadows have detail but the highlights are blown out and lose detail. Depending on the image (eg an uninterrupted horizon) it would be fairly easy to blend the 2 images in Photoshop and by using layer masks, reveal the correct highlights of one image with the correct shadow areas of the other image.

Further information

Adobe has info (put HDR inn the search option) and access to tutorials/online video demonstrations.

HDRsoft is the Photomatix website with loads of info.

High Dynamic Range Digital Photography by Ferrell McCullough is a useful book available through Amazon.

Bas Montgomery Nov 2009