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Creating Images wth Extended Depth of Field with Adobe Photoshop CS4
This is a new feature in Photoshop CS4 which is essentially a variant of the Auto Blend feature from Photoshop CS3, except where as Auto Blend is used for stitching together seamless panoramas, we can use this feature to stack a bunch of images and try to get a result that is as sharp as possible everywhere.
The obvious use for this feature is in Macro work. When you are at 1:1 magnification with a 180mm macro lens, your depth of field even when shooting stopped down to f/22 can be insufficient to fully capture your subject. The problem only gets worse when you are shooting with a macro lens like the Canon 65mm MP-E with 5x magnification.
Lets use a flower as an example. I wanted to photograph this Dutch Iris with my Macro lens, but unfortunately I couldn't fit the entire flower in the depth of field at that angle and working distance. So I took a bunch of images at f/11. You definitely want to put everything on a tripod, figure out the exposure and set the camera to manual mode. You'll also want to use manual focus. Start at either the front or the back and take a shot, move the focus distance a little bit and repeat. This is where Live View can be very useful, since it lens you see when you have the right parts in focus.
I took 5 images and stacked them. You may wonder, why f/11 and not f/22? After all, if I had photographed at f/22, it would probably only take 2 images to stack. I chose f/11 because in addition to having the entire flower in focus, I wanted the background to remain quite blurry. At f/22 the background would also start to become less blurry. What this means is that you final stacked image could be one that cannot be reproduced by just having a lens that stops down to f/64 (or smaller), which is cool.
You may notice that the image shifts slightly as you move the focus point. This is an unfortunate property of all macro lenses I've tried and its something that in some circumstances can mess up the Extended Depth of Field algorithm.
The process is really easy, just load the images into a stack. There's a handy script built into Photoshop CS4 (and CS3) under File->Scripts->Load Files Into Stack. Check off the option to automatically align the images. Then select all the layers, go to Edit->Auto-Blend Layers and select the option for Stacking. Also make sure the checkbox to correct color is checked. Here is the resulting image:
For comparison, here is the same shot taken at f/22, I prefer the stacked version.
The algorithm isn't foolproof however. I have run into many situations where I have to manually adjust the layer masks to get a better result.
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