Sunday June 03, 2007

You probably know the human eye can handle a much wider range of light levels than can be captured on film or CCD. Where we can see detail in a scene with both shadows and a bright sky, a camera will either render the shadows as a deep, featureless black or blow out the sky to a featureless white. It often can't capture both the shadow and sky detail simultaneously.

Landscape photographers have traditionally used graduated neutral density filters to deal with this problem. However, using these filters can be a finicky process. Luckily, there is an easier and more effective way to accomplish the same end digitally by taking several exposures of the same scene and blending them with image processing software.

Before explaining how to do this, here are two test photos I took recently and the resulting blended image. The first image captures interior details, but at the expense of over-exposing the scene outside the window. The second image correctly exposes the outside scene, but at the expense of all interior detail. The final image shows what can be done with blending.

Interior correctly exposed, exterior over-exposed

Interior under-exposed, exterior correctly exposed

Final blended image showing both interior and exterior detail

One of my favorite photo websites, The Luminous Landscape, describes several approaches to digital blending using Photoshop. I describe below how to use approach #2 with the GIMP (free image manipulation software that probably does more than you'll ever need and costs nothing unlike Photoshop, which is outrageously expensive).

We will start with two images, Light and Dark, and produce a final, blended image.

1. Open Light in the GIMP.
2. Use "Open as Layer" to open Dark.
3. Create a Layer Mask for the Dark layer.
4. Copy Light. This is easily done by clicking on Light's thumbnail in the Layers window and typing CTRL C.
5. Select the Layer Mask by clicking on it once.
6. Paste Light. When you do this it will appear as a layer called "Floating Selection (Pasted Layer)" in the Layers window.
7. Now anchor the layer by clicking on the Anchor icon on the Layers window or using the pulldown menus. If all goes well, the Light image will now appear as the Dark image's layer mask.
8. Click once on the Layer Mask to select it and then use Filter->Blur to apply a gaussian blur with radius 40 to the layer mask.

With both layers visible, you should now have a blended image that you can flatten and write out in the format of your choice. Once you learn the above sequence it is actually quite easy to use. However, if you would like an even simpler method, consider using JD Smith's script-fu GIMP plug-in, available here. His script, exposure-blend, takes three images as input (light, dark, normal) and produces a blended result.