Using Multiple Images and Masking to Make a Composite Image

April 11th, 2007

After my post about Photomatrix I wanted to make sure beginning real estate photographers know that there are other, better ways to deal with the problem of shooting in a situation where the brightness range is more than you camera can handle. Ideally you want to get the darkness of the sky and still get crisp whites. It’s almost impossible to do this with out making a composite image.

The image above is one I shot of my son’s 1880 Dutch Colonial farm house last week. The weather was broken clouds and the north facing front was brightly lit with ominous clouds in the sky. This is a classic case where it takes one exposure for the home and foreground and a second exposure to get the darkness and delicateness of the clouds. If I’d exposed for the nice greens and yellow of the home the sky would be burned out and boring. On the other hand, if I’d exposed for the sky the the House and foreground would be too dark and the yellow and white wouldn’t have been crisp. This is the very same situation that you run into when shooting interiors where the windows are bright and the interior is not as bright. In the interior situation a flash can be used to light the interior so it’s the same brightness as the outside but outside this won’t work.

When I was taking this shot I took several exposures in anticipation making a composite image example. To create this image I opened both shots in Photoshop and shift+dragged one image over the other to exactly register them. The crux of doing a composite of these two images is to select the complicated edge between the sky and the house/foreground. There are many ways to do this but I like the way described in Katrin Eismann’s book, Photoshop Masking & Compositing. This is a great book that has everything you need to know about using Photoshop to make composite images. On page 225 she describes a technique she got from John Warner to select the sky by duplicating the blue channel and then using curves to increase the contrast of the the blue channel copy so it is a solid mask for the sky. This is the technique I used on this image. This technique is a quick and relatively painless (took me about 15 minutes for this image) way to select the complicated edge between the sky and the trees. After selecting the sky, you make a layer mask that allows the sky from one layer/image show through to the layer/image of the house and foreground.

You can usually spot composite images of this sort because there is a temptation to take shortcuts in making the sky selection mask. Frequently, there will be a “halo” between the sky and the rest of the image because someone took a shortcut in making the sky mask. If you look very closely at a large file of this image there are still defects along the lacy edge where the trees meet the sky but in sizes used for the web it’s difficult to see the defects. The mask is delicate enough to let the dark sky show through the lacy trees on the far left and middle-left.

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