Mindbender. I v'e never heard of blend if. Where is it and under what tools?
It's in the same place as your layer effects. Double click on the layer in question, at the top of the dialogue pop up you will see "advanced blending options" label. In that area, near the bottom you'll see two sliders with black/white gradients and arrows under them. They're labeled "Blend-If". What they do is allow you to specify certain levels of certain channels in that particular layer to be blended (made invisible) or not blended with the underlying or from the underlying layers. Confused yet? hehe.
Here it is in simple terms. On the "this layer" slider. If you choose the black slider and slide up, it makes dark stuff more and more transparent. If you choose the white slider and slide down, it makes lighter stuff more and more transparent. It does this with a very harsh edge, so what you do is hold option (alt) and "split" the sliders into two sliders. This allows you to make more subtle and eye pleasing transparency. You can do this for the gray, red, green, or blue channels for
that layer.
If you choose to work on the "underlying layer" it blends based on what pixels are
under the layer you're currently working on, but it works the same way.
This is an amazingly powerful tool and can allow you to fine tune and create effects that are very hard in other ways.
It sounds overwhelming at first and can be a little confusing when you start to use it, but experiment with it as I've described and pretty soon you'll wonder how you ever lived without it!
One of these days I'm going to get around to writing up a tutorial on the Blend-If sliders. (I know there have beena couple others, but I'll write one just so I can remember the URL haha

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