HDR - HIGH DYNAMIC RANGE / HEINOUS DIGITAL RESULTS
For several years now, High Dynamic Range has been a prominent feature of landscape and architectural photography. It's not limited to these genres though as I frequently use it in a varied form, with portraiture, whereby multiple versions of the same photograph at different brightness are combined to increase the dynamic range of certain areas of an image.
True HDR photography though, involves multiple photographs at different exposures, often making use of the camera's inbuilt bracketing feature. The parameters can be defined so that 3 or more exposures are taken in quick succession. The first is the "correct" or middle exposure. The camera then takes another two images, one a stop below and one a stop above. So if the ISO is 100, aperture is f/8 and the shutter speed is 1/100 sec for the "correct" exposure, the other two will be at 1/50 sec and 1/200 sec respectively.
The affect of doing this is to make shadow details that are lost in the middle exposure visible and likewise with the highlights too. The images are then combined in post processing software. Newer versions of Photoshop have a HDR function. There are also plugins and standalone software like Photomatix too.
For best results the camera should be tripod mounted or at least unable to move and either activated with a release cable, remote or self timer. This way every static object in the frame will automatically line up in subsequent photographs. If there is a breeze then flowers, long grasses, tree branches, flags and things of this nature will obviously differ from image to image and depending on the shutter speed you may incur blur in these areas and make those parts of the image unusable. If this happens then you may need to increase your ISO to facilitate a faster shutter speed to avoid this across all photographs, so bear that in mind. It is also worth noting that a stop above and a stop below may not be sufficient to capture all the required detail in high contrast situations.
Once all the images are taken, they are combined in post processing software. The highlights from one are combined with the mid-tones and shadows of the other exposures to give the image a much broader dynamic range of tonal values than would be possible with just a single shot. It is at this junction though, where most people go "wrong" with HDR and create a very unnatural and overly processed look. I believe this is due to beginners falling into the trap of thinking that just because the sliders in the software go to 100%, that you have to push them that far!! In general, try to avoid this temptation.
Obviously if you are intentionally aiming for a cartoony/CGI type of look, then by all means do it, but for the most part the idea of HDR is to simply increase the range of tones and reveal detail that the eye would normally see, as it naturally has a higher dynamic range than a camera does. With some cameras, the dynamic range is already very high and so the use of bracketing may not be necessary. For example, the D800 has a dynamic range of 14.4 exposure values. Coming from a Nikon D90 which has 12.5 EV, it is quite amazing what can be recovered from a D800 RAW file. Still with high contrast lighting, like shooting a landscape or portrait outdoors on a sunny day, it's easy to get clipping at the extremes of the histogram. It's better to expose correctly for both the shadows and the highlights, as recovering details from a single RAW file can reveal noise in the shadows and make the end result look very gritty and colours look a bit weird.
Common traits of an over-processed HDR image include halos, softening and an almost ethereal glowing look, that resembles a graphic from a games console cut-away scene, than a photograph. I'm a big believer in subtlety and the "less is more" mindset. If a scene looks completely natural, it will tend to have a better visual impact upon the viewer than a scene that is blatantly over-processed. Like selectively coloured images, it's pretty much cliché or kitsch now to do heavily processed HDR images. They tend to look amateurish and ultimately detract from your portfolio, so I'd advise against doing either. Aim to give your images a high dynamic range, without incurring heinous digital results!