Angelouscuitry,

load an object & make sure it's selected (e.g., its properties/parameters tabs are showing).

usually the preview window is from the Main Camera, but any one will work.

on menu bar, click Window>Python Scripts to open the Python Scripts window.

Click Render IO and then Calc DoF Focal Distance. A window will open that shows the distance to the object in Feet.decimalFeet. You can then use that number for your focus_Distance entry for the camera.

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