Change camera focal length from degrees to mm

[SJS66]

How do you change your camera focal length in the measurement bar from degrees to mm?
I tried using Tools > Advanced Camera Tools > Select Camera Type & adding a metric camera type but my camera focal length (zoom tool) is still in degrees?

[Fletch]

Select SketchUp magnifying lens tool.
Hold down Shift while dragging down in your scene with the magnifying glass.
Focal length by default is shown in degrees... you are now disappointed.

"Fletch, this is NOT what I wanted! I know this already"

Hold your horses... now don't touch anything, simply type 50mm on your keyboard and hit Enter key.
Now the VCB will change to show focal length in "mm" units
Now type 20 and hit enter - no need for the "mm" SU will do that automatically now for rest of model edit session.

"awww... gotcha Fletch. That's what I'm talking about right there."

[JGA]

Excellent tip, thanks.