Right-click any viewport label. > Views > Choose a camera.
Keyboard > C
A camera viewport shows the view from the camera, looking at its target (even free cameras).
Camera viewport controls include:
Zoom Extents All, Zoom Extents All Selected
Activate camera or light views from the viewport right-click menu, under Views. If there are more than ten cameras or lights listed, the last entry in the list is "More Cameras" or "More Lights." Choose this to display the Choose a View dialog, which shows the complete list.
If you press C to activate a Camera view and you have more than one camera in the scene (and no camera is selected), choose a camera from the Select Camera dialog that appears.
Note: Switching to or from a camera view clears the Undo/Redo lists.
Procedure
To undo changes to a camera viewport, do one of the following:
Click Undo on the Main toolbar.
Press CTRL+Z.
Note: This is different from orthographic viewports, which require the use of Views menu > Undo, or SHIFT+Z.