This script is by JD Smith and updated for GIMP 2.3+ by Alan Stewart. Main website: http://turtle.as.arizona.edu/jdsmith/exposure_blend.php
After installation, you can access the script from [Toolbox]/Xtns/Photography/Exposure Blend...
exposure-blend makes quick work of blending 3 bracketed exposures into a detailed, realistic, extended contrast image. It has many features to save you the drudgery of assembling contrast blended stacks yourself:
* Set masks from among any of the three images (normal, dark, bright).
* Several smoothing options, including edge protection for halo suppression.
* Stores smoothed versions of all masks used for quick automatic recovery, so you can feel free to experiment with different versions, quickly switching among the mask source and smoothing choices. These are saved along with the file, so you can pick up where you left off.
* Optionally pre-trim the masks' histograms to deliver the most contrast.
* Differencing alignment mode to enable easy manual alignment of the images. This also moves all the associated cached mask to keep things in sync.
* Trim the image to the area of overlap of all three layers.
* Save custom edited image masks for later recovery.
Compared to an automated contrast blending tool like PhotoMatix, exposure-blend has the following advantages:
* Similar "mostly-automatic" mode produces reasonable results (see example below).
* Infinitely tweakable final image, with good real-time feedback.
* Easily change the blend weights in certain regions of the image by directly editing the masks.
Unlike PhotoMatix, the differencing image alignment mode is not automatic, but is very quick, and gives good feedback on the quality of alignment (e.g. residual rotations, etc.).