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I have a scene which is taller than 16:9 to allow for a vertical camera move, but the output, of course, wants to be 16:9. When I open Keyframer, the frame position guide is the same size as the scene. That is, not 16:9, but taller. How can I make the Keyframer are 16:9 within the taller scene?

Its maybe a bit counter intuitive, but you need to create a new "camera project" in the desired output resolution (16:9)
and do the keyframe camera move in there, choosing the vertically bigger project as your source in the KF rendertab.

You should remember that the output is defined by the project, the keyframer cannot change the project output.
So if you need a specific output ratio you need to create a project for that and do your (KeyFrame)camera moves in that project.

Ah okay. You are right about it being counter intuitive, but I guess once you know. So when I use the Keyframer for a move, it allows to choose the other open project as the source, but to do a multiplane effect, it does not, or am I missing something? Does this mean i need to do the multiplane and render it within the project, but then do the camera move in a new project?

Hold on, I experimented and it opened, but it is odd because the multiplane wizard gives me the choice to open from two projects, both called "untitled", so I chose one of them and it opened the project I wanted. I don't understand that, but it worked.

Wait, not it did not work. Aargh! It opened, but as one single layer rather than all the layers, which need so I can move them. I obviously have a big disconnect about how this whole thing works.

To be clear, I have a scene which is not 16:9. I want to zoom in and I want to slide foreground elements out of the way as I do the move. Obviously I need the output to be 16:9. I have done this before, but I guess the project itself was the the right aspect ratio, so I did not need to open a second project. In the second project, I am unable to get the layers to show up in the wizard.