Exposure blending

From Daniel's homepage

Increase the dynamic range of your camera with exposure blending. This tutorial describes the steps you'll have to take. We'll use Gimp as a graphics editor.

Creating the source pictures

Ordinary picture with blown out highlights and underexposed shadows	Balanced exposure using exposure blending, and black-/whitepoint adjustment
Ordinary picture with blown out highlights and underexposed shadows	almost finished image (needs saturation and black-/whitepoint adjustment)

Take three images of your scene with different exposure times. My camera can do exposure bracketing, taking images at -2,0, and 2 f-stops off the metered setting. For high contrast scenes you might want to increase this span by manually setting the exposure times.

Doubling exposure time means adding one f-stop, cutting it in half means subtracting one f-stop.

Assembling them in gimp

1. Load your middle exposure into Gimp. Then add first the darker then the lighter exposure as layers on top of your middle exposure.
2. Now select the second layer, which should be the darker exposure.
3. Right click the layer and select Add Layer Mask.... In the dialog which appears select Grayscale copy of layer and make sure Invert Mask is unchecked.
4. The layer mask should now be selected for editing (in the Layers dialog it should have a white outline). From the Filters menu select Blur -> Gaussian Blur.... In the upcoming dialog select 20x20 px as a radius and select ok. (you might have to fiddle around with blur radii depending on your images).
5. Now select the first layer, which should be the lighter exposure.
6. Right click the layer and select Add Layer Mask.... In the dialog which appears select Grayscale copy of layer and make sure Invert Mask is checked this time.
7. Again the layer mask you just created should now be selected for editing. Blur this one too like in step 4.
8. Now adjust the Opacity sliders of layer one and two to get a natural appearance
9. You might have to adjust the images saturation now, which is increased by this process. Also check the curves dialog and adjust white-point, black-point and gamma to improve the pictures appearance (but be careful not to create blown out highlights which we just tediously removed :-) )

Done!