I have an image of smoke over a black background. If I set the blending mode of this layer to screen, the image's background is perfectly edited out, however if there isn't another layer below this, the screen mode does nothing. How can I make the background truly transparent, so it is just smoke on a transparent background? Colour Range selection is useless in this scenario, because smoke needs different levels of transparency throughout, and not just a clear cut selection. Thanks
You don't see the transparency effect if there's nothing below the layer because there's nothing there for Photoshop to calculate (all these blend modes involve calculations based on the values of the corresponding pixels on each layer), so it just shows you the image.
In this particular case (as Farray pointed out while I was writing this!), you can pull an easy trick on Photoshop and make yourself a smoke-filled image with all the transparency intact. It's easy because you already have gray smoke on black, just like a channel.
Here's what you start with: I've taken a "smoke on black" image (Layer 1) and applied it in Screen mode over an arbitrary background (Layer 0).
Step 1. Hide all layers but the smoke.
Step 2. Switch to the Channels Palette and Ctrl-Click (Cmd-Click on Mac) on any of the channel thumbnails ("any" because in this case they're all the same).
What you've just done is load a selection based on the grayscale values in the channel you chose. Just like a mask, white is fully selected (opaque) and black is fully unselected (transparent), with the gray shades between.
Step 3. Create a new, blank layer, set your foreground color to white, and fill the selection. Hide all the other layers.
Step 4. Deselect, turn on your original background, admire the result, and move on to greater things!
Farray's suggestion is very similar. Do steps 1 and 2 then use your selection to create a mask. The advantage of this approach is that you can then and "tweak" the mask in the Masks panel.
I had a similar question a while back trying to extract clouds this worked extremely well for me you might find this useful.
Alan Gilbertson's answer worked well for a single-color image, but I needed to go through this process for a full-color image on a black background.
The PS action reviewed at http://www.howtogeek.com/59634/remove-backgrounds-automatically-with-a-free-photoshop-action/ (Author's download page here) ended up working beautifully.
I found this to be a quick simple process: