Dynamic Range is defined as the measurable difference between the brightest highlight and the darkest shadow value.

Both situations are due to the smaller dynamic range that a camera can reproduce compared to the human eye. If the camera decides the object is the most important thing in the scene, then it will capture accordingly. If it decides that the sky is more important (because the object is too small, or off-centre), then it will expose for the sky.

Real-world scenes contain light ranges that exceed a 15 stop range (our natural vision). Digital cameras however can only capture about 3.5 stops. Where we can see lots of information in both the shadows and highlights of an image, a digital camera will only record pure black or pure white (no details).

In high dynamic range (HRD) photography you would take a 'normally' exposed picture, then take any number of pictures overexposed (which will record lots of information in the dark areas of the photograph), plus any number of underexposed pictures (which will record lots of information in the brightest areas of the picture).

These separate pictures are then combined inside Photoshop to form a single picture with a much wider dynamic range, often much wider than your screen can display (or you could even see, much less print). The highlights in the left-hand picture are blown out, while the shadows in the right picture are gone (crushed is the proper term)... BUT the highlights in the most right hand picture are perfectly visible, and there is definitely information in the darkest part of the left picture (the door).

By taking varying amounts of information from the highlights, mid-tones, and shadows we are able to see information in all areas of the picture. Unfortunately, most of the information has to be thrown out as it could never be printed. The amount of information that you take from each of the 3 ranges will determine if the picture will look natural, or surrealistic (Surrealistic HDR seems very popular right now).

Keys To Shooting HDR Photos:

1) Proper subject. HDR really shines when you have a very large dynamic range to work with. Photos taken at night with an area illuminated by artificial light, or a huge church with light streaming in through stained glass windows and dark shadowy nooks look great! A sunny afternoon in a wheat field would not.

2) Moving subjects don't work. For example, people walking through the frame will look like ghosts in the final picture.

3) Use a Tripod. These pictures have to line up perfectly... even the camera shake from your hand will misalign them to some extent. Photoshop can take this into account to a degree, but better to be safe.

4) Use a remote cable release (or put your camera on timer delay).

5) Use your mirror lockup feature if you can. (Again, less vibration)

6) Only vary the amount of time to get different exposures NOT the aperture. Changing the aperture will change the depth of field, which will mess up the final composite. A good way vary the time on a set of photographs is to use the bracketing function. Bracketing will take a 'normally' exposed picture, then one overexposed by up to 2 stops, then one underexposed by up to 2 stops in quick succession. If you need more than 3 exposures, do it manually.

If you only had time to take one image, and parts are either underexposed or overexposed, try this tutorial:
How to Fake HDR photos with one RAW image


If you like these types of photos and would like to do them yourself, try:
How to merge multiple files into one HDR image, from capture to print.

Flicker Group devoted to HDR (great samples!)

http://www.flickr.com/groups/hdr/

Stuck in Customs (STUNNING images, plus a great tutorial on photomatix)

http://stuckincustoms.com/2006/06/06/548/

Great resources page on HDR

http://www.hdrsoft.com/resources/index.html