deandra.homeip.net

There are numerous detailed articles on HDR (high-dynamic-range photography) out there on the internet, like the Wikipedia article and the HOWTO on instructables.com, so I won't bore you with too many details. What I'm going to do here is give a very brief rundown on what HDR is and how you can do it.

HDR

HDR commonly refers to a post-processing technique to boost the dynamic range of your digital photos. Digital cameras suffer from a rather narrow range of brightness values, compared to, say, the human eye. This is why the light areas of your pictures can appear "blown-out" (overexposed) and the dark areas can appear all black and featureless. The "dynamic" part comes from the fact that you can adjust the upper and lower bounds of the range with the exposure settings (shutter speed, aperture, and exposure compensation) settings on the camera. You cannot, however, increase or decrease the range; to do that, you can take multiple photos of the same scene using different exposure settings, and blend the images together to produce a final result with a much greater range of brightness than your camera is capable of producing in a single exposure.

Tools Used

• Canon SX10 IS - This camera has an exposure bracketing mode which is very useful for gathering the images needed for the composite.
• Tripod. This is essential to make this process easy. It eliminates the need to shift the layers around later to correct for jitter between the different images.
• GIMP - Among the myriad other features available in the GNU Image Manipulation Program are those necessary to compile the composite image, and make it easy to do as well.

Steps

Gather the images

On my camera, I turned on exposure bracketing (in the FUNC/SET menu) and set the width to +-2EV. This means that the camera will shoot three images in rapid succession: 1 with the correct exposure, 1 underexposed by -2EV, and one overexposed by +2EV. Note that this camera will not combine exposure bracketing and flash photography. Once I had the images I loaded them onto my computer. To make things easier I renamed the files "normal", "dark", and "light".

Create the composite

I started by starting the GIMP and selecting "Open as Layers" from the File menu. I selected my three images and loaded them into the program. I then moved the "normal" image to the bottom of the stack in the Layers window.

Next, I created masks of the dark and light layers. The masks allow certain parts of the background image to show through, while blocking others. In a mask, the dark parts show through while the light parts are blocked. I right-clicked on the dark image and selected "Add Mask". I selected to create a "grayscale copy of layer". I did the same thing for the light image, only this time I also selected "invert mask", so that this mask would block the light parts instead of the dark parts. At this point you can fine-tune your masks by selecting them and opening the Brightness/Contrast dialog to adjust how much they let through and how much they block.

Once this was done, I exported a new JPG and had a nice composite image with a very broad range of light levels.

You can see the images I used for this experiment in a special Album in my Gallery