I have imported a PDF image, and I’ve created a face floating on top of that image. It’s hovering a few inches above it actually. I want to paint the floating object a transparent color so I can trace some of the elements in the imported image, but when no matter how I adjust the opacity there are really only two outcomes: Either 100% or 0. I’ve done this before and am not sure what’s going wrong here.
Why are you trying to do this as if you are working on a light box? Why not just draw right on the imported image?
A different approach would be to layer the model object directly below the imported image (as image), explode the image, right-click on the face and choose Projected from the Texture menu. Use the paint bucket and Command-click on the image, then paint bucket the shape. You will then have exactly the shape part of the PDF as a texture on the shape, to then trace.
The imported image is quite busy and I’d really like it to be much lighter so my drawing contrasts sharply with it.
You could edit the image to make it lighter. I also find when tracing images that it helps to set the edge color to something that shows up well against the image and to set Profiles to 2 or 3 so the edges are thicker.
What material did you apply to the face and are you trying to edit the In Model material or the one if the library?
If you really need to draw a rectangle over the image to draw on, you could put it right on the image as I did below and set the face style to X-ray which will naturally fade the image out for you.