Digital photography software has created an alternative to dealing with high contrast scenes. In the past, the only way to deal with a very bright sky and dark foreground was to use a Graduated ND Filter. Now, digital photography tools offers another solution: HDR
HDR stands for High Dynamic Range and it is a technique for compressing a high contrast scene digitally. The best implementation of HDR is to take multiple pictures of the same scene and to vary the exposure. HDR images can be created from only 2 or 3 pictures, but often work best with 3 to 5 pictures taken in one stop increments to capture the full range of light. The lowest exposure will show detail in the shadows, but will have many highlights blown out to full white. Each picture with increasing exposure will reveal more detail and color in the lighter portions of the picture at the expense of shadow detail. The end result is that all the pictures, when combined, will show the full range of detail.
Once the images are captured, they need to be combined with software. There are many specific packages and plug-ins designed just for HDR; however, HDR capabilities are built into Photoshop version CS2 and later.
There are several qualities that makes HDR desirable over using a ND Grad filter. The biggest advantage is when working with abnormal compositions and shapes with dark and light areas. For the landscape scene with a straight horizon, a ND Grad filter works very nice as the filter transition matches the contour of the scene. What if, on the other hand, you are taking a picture of a backlit mountain? Using a ND Grad filter will darken the top of the mountain if you place the dark-to-light transition too low - too high and the sky is blown out around the mountain.
Another example is when elements extend off the top of the image frame. The horizon may be straight to allow a ND Grad filter, but often you have trees at the edges extending up into the dark portion of the filter. The ND filter will make these elements silhouettes. If you use HDR, the sky around the trees will be captured and the shadows in tree trunks will also be preserved. It really is an amazing phenomenon to be able to show detail throughout the scene with complex shapes.
I definitely think there are many reasons to pick up the ND Grad filters over using the HDR technique. Here are a few disadvantages to HDR:
Implementation of HDR involves multiple exposures. This means that for the software to match the images up, both the subject and the camera need to be still. If the wind changes the position of grass or trees between exposures, the final combined image quality will suffer. For the camera to remain in the same position, it will usually need to be on a tripod. Some HDR software can cope and combine pictures that have been taken hand-held. I still find that the best results will be with the camera on a tripod.
It takes more time to post-process and create HDR pictures, and there are more image files to tag and deal with. With a ND Grad, the only extra time and effort is for you to dig the thing out of the bag and put it on your lens. Often it is a great advantage being able to create a picture that is the final product with little post-processing.
- HDR pictures tend to have a feel all there own. This can be a good thing as they tend to have a very ethereal, heavenly feel. While it is nice, the effect can be overused. Specifically, many people using HDR end up with uninteresting pictures that lack contrast. ND Grads, on the other hand, have a distinct feel that retains shadows and contrast that make for a great landscape photo.
Basically, the answer is both are great tools, but you should avoid overusing any one tool at the expense of capturing a scene in its best light. I don't like working with extensive post-processing techniques and prefer to get an image as good as I can the first time around. It's for this reason that I use the ND Grad filter much more often than HDR.
The disadvantage of using ND Grads is that they will often darken portions of your subject that rise above the transition line. I am OK with this and actually like the silhouette effect that I get. I like the increased contrast that you can get using a filter. Frankly, as more and more people use HDR, a proper use of filters can really make an image stand out.
In the end, your decision hinges on you taking control of the concept you create in your mind for what you want your picture to look like. You should then choose the best tool to fit that concept.