There are some really nice HDR applications for the Mac that can process photos taken with digital SLRs (Photomatix Pro and Hydra are two examples), but nothing really exciting for the iPhone. There have been some iPhone apps that claim to be HDR capable, but most are just enhancing the color and stretching the contrast.
Along comes TrueHDR [US$1.99, iTunes link], an app that actually gives you a taste of HDR photography using your iPhone. Your picture quality is never going to equal that of a high-end digital camera, but the results are really quite good.
Here's how TrueHDR works: launching the app opens the iPhone camera, at which time you are asked to frame your picture. Next, you tap your finger on the brightest spot in the frame, which might be the sky. This action adjusts the camera exposure for that level of brightness. After snapping the first picture, the camera comes up again, this time with instructions to tap on a darker area of the frame. You take the second picture, and then you can merge the two shots. The end result is one picture, usually much better than what you would get without using this app. Details are not lost in the shadows, skies are not overexposed, and the image quality goes well beyond what the iPhone camera could do by itself.
There are a couple things you need to know before considering TrueHDR. You don't need to use a tripod, but you do need to match the framing of each image as closely as you can, and you want to hold the phone as steady as possible. If there is any difference in the way a photo was framed, the software attempts to match the two images automatically. You can take your HDR photos in either landscape or portrait mode.
The processed image is not full resolution -- it's limited to 960x960 pixels. The developer says that higher resolution is coming in an update. You can save the merged photo to your library, or email it directly from the application. Here's a link to a lot of sample pictures, and I have a link to a gallery containing my own TrueHDR photos at the bottom of this post.
TrueHDR is an app that requires an iPhone 3GS because you need to be able to adjust the camera parameters by tapping on the preview. You can import images from other cameras to the iPhone, but it's really a kludge and, in my opinion, not worth the trouble.
I think TrueHDR is the best two bucks you can invest to improve your iPhone images. Once again, it doesn't work for sports photography, or any subject that is moving. You can only merge two pictures, while higher end HDR applications merge three or more images to get more dynamic range. Despite these limitations, TrueHDR really works, and you'll have better pictures to show for your small investment.
Check out the gallery below for a selection of photos showing how TrueHDR can improve your iPhone photography. The first photo in each group was taken normally with the 3GS camera, while the second was taken using TrueHDR.