High Dynamic Range


 What is High Dynamic Range (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 20EV, Digital cameras (and Monitors) can cope with a lot less; a raw file can record a maximum of about 6-10EV (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 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.

 How?

Several photos 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 to 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 subsequently 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 photos?

Ideally, the images you take (probably 3 or more) need to be identical in all other respects apart from the 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 2EV 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 then 3 photos to capture the full range

I’ve got the Images, What’s next?

The images are merged and tone mapped using software, either through an image editing software (e.g. Photoshop) or stand alone software. An example of a popular stand alone software is Photomatix (www.hdrsoft.com), which is the software I use and I’ve found works well.

My Workflow

Using a Canon 5D mark II  I begin with settings of ISO 100, Aperture Priority,  auto exposure bracketing of 2 steps  above and below, varying the shutter speed not aperture, and 2 seconds remote release, shooting raw files. I register the settings so I can retrieve them by switching the mode dial. This is a start point and for a lot of purposes works well. If the conditions are particularly challenging I will adjust the exposure compensation and take further images.

If I have taken a lot of images I batch process in Photomatix- which will happily get on with processing whilst I can do something else. I have developed my own presets for different purposes, and favour a fairly neutral natural setting with quite low contrast. I save the files as Tiff, and then open the file in Adobe Camera Raw. This is particularly useful if there is a need for highlight recovery (if I haven’t calculated the exposure accurately), possible noise reduction (CS5 is particularly effective) or White balance adjustments (colour casts are not uncommon).

I use HDR for doing panoramas, landscapes, interior photography and night scenes.

 

Further Information

www.adobe.com  has info (put hdr in the search option), and access to tutorials/ online video demonstrations. Cambidge In Colour website has information on using photoshop for processing HDR.

www.hdrsoft.com is the Photomatix website with loads of info.

I’ve found ‘High Dynamic Range Digital Photography’ by Ferrell Mc Collough a useful book

Bas Montgomery