Photography techniques – HDR images

One photography technique that I have really started to dig recently is HDR – high dynamic range images.

Basically, it’s a photo that has been processed to portray a large range of brightness (really dynamic range, but I’ll use the term brightness for simplicity).   If you look at the environment around you, there are parts that are light/bright, for example, where direct sunlight is hitting objects.  And there may be parts of the environment that are quite dark, for example, objects in the shadows.  I like to think of all the brightness available as a line, going from dark to bright.

Now I know the diagram below is not mathematically correct, but it’s a simple way to think about dynamic range.

Brightness line

Brightness line

Film and digital camera sensors (of today, 2008!) currently are unable to capture the detail in extreme bright parts of the environment, and the detail in the extreme dark parts of the environment simultaneously in one photo.  If you want to be able to “see” things in the dark areas or shadows, you will probably end up with the bright parts being extremely bright or white and washed out.   Alternately, if you want to be able to “see” things in the bright areas in a scene, the objects in the shadows will be all dark.

Using the simplified Brightness Line diagram above, I like to think of the camera as being able to capture only a portion of that brightness line.   And the “exposure” settings of the camera determining where that region is – whether it’s more on the dark portion, or more on the bright portion of that line.

A typical example of an image with this problem is if you take an indoor photo looking through a window.

Subject exposed, background washed out

Subject exposed, background washed out

Here you can see the features and detail of my daughter, but nothing through the window.  It was too bright.   If I had exposed the outside, my daughter would have turned into a silhouette.

The are two main sources of the problem.  The first is that the digital camera sensor cannot capture such large differences in lighting – that is, it has a small dynamic range (typically 12 bits worth).   The second, is that the screens we view pictures on, and the paper we print photos on, also have a small dynamic range (typically 8 bits worth).

So why bother with high dynamic ranges?  Really, it’s a photographic style that tries to recreate all the details as “seen” by the human eye.   Our eyes don’t see a washed out window background in real life.  A combination of the eyes ability to adjust “exposure” dynamically through the use of the pupil, the actual sensitivity of our retina and the cleverness of our brain allows us to resolve a much higher dynamic range than currently possible on a camera sensor or display.

HDR images are really images with a large dynamic range.  This is all good, but our computer screens cannot display a large dynamic range.  So what people do now is to process the high dynamic range and convert it into a low dynamic range, but in a way that doesn’t lose detail from the extremely bright and extremely dark areas of the scene.   This is known as tone mapping.

And as for how HDR images are captured in the first place if there is a limitation due to digital camera sensors?  By taking multiple photos of the same scene with multiple exposures.  This way, we are moving that fixed region along the brightness line, and taking snapshots along the way.   Afterwards, the photos can be “mixed” together and then tone mapped to produce a single photo with detail in both the shadows and highlights.

Multiple exposures to increase dynamic range

Multiple exposures to increase dynamic range

There are heaps of amazing photos on the internet and lots of links to learn more about HDR images.  Here is an example of a beautiful capture of a boat by Peter Van Allen, found on flickr.   If only a single shot was taken, the boat would most probably be very dark or silhouetted.

The Chesil Beach at Portland by Peter Van Allen

The Chesil Beach at Portland by Peter Van Allen on Flickr

In the end, HDR is another photographic technique that has really popped up due to digital photo editing software making it easier to combine images.   Not everyone likes HDR images and I’ve seen some that are more artistic and stylistic than realistic, but there are some other images that are just awesome.   And sometimes, we may really want to blow out the background so that it emphasises the subject in the foreground.   In any case, it’s another technique to play around with!  See the links below for some other great examples.

Photos

35 Fantastic HDR Photos – http://www.smashingmagazine.com/2008/03/10/35-fantastic-hdr-pictures/

Flickr’s HDR Photo Pool – http://www.flickr.com/groups/hdr/pool/

Flickr’s Photomatix Photo Pool – http://www.flickr.com/groups/photomatix/pool/

More detailed explanations

Detailed FAQ on HDR -http://www.hdrsoft.com/resources/dri.html#dr

Detailed explanation on bit depth and dynamic range – http://www.normankoren.com/digital_tonality.html

Software

Dynamic Photo HDR – http://www.popphoto.com/popularphotographyfeatures/4472/hdr-how-to-dynamic-photo-hdr.html

Tutorial using Photoshop CS2 – http://backingwinds.blogspot.com/2006/10/how-to-create-professional-hdr-images.html

Photomatix – http://www.hdrsoft.com/resources/tut_mac/index.html

How to

A collection of HDR tutorials – http://tutorialblog.org/hdr-tutorials-roundup/

A very clear step by step walk through using Photomatix and Photoshop – http://www.flickr.com/photos/sandmania/2734520399/in/photostream