Viewing a Z-Channel
Although you can't see the alpha in a Viewer, you can place a Key - DepthKey on an image and adjust the depth parameters as you view the alpha channel. If you keep the image in float (4 bytes) with the Bytes operator and do a Reorder rgbz, you will be able to see the numeric values when you scrub with the cursor on the alpha channel. This is an ideal candidate for a Viewer Script...
If the Z channel isn't directly imported with the FileIn, you can copy it over with the Copy command, using z as your channel.
If you are importing from Maya, they have a wacky Z setting which is -1/distance. This basically means that the numbers in DepthKey are inpractical. Therefore, jump onto www.highend2d.com and grab the MayaDepthKey macro which operates exactly like the normal DepthKey, but does the conversion math for you.
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