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The frustum is visible as a yellow frame when a camera is selected
Camera focus ring
When viewing your scene through a camera you can get an idea if the center of your render will be in focus or not (Press I on your keyboard if the focus ring is not visible when looking through the camera). When the focus ring is red it means the camera target is placed in front of the area/objects within the focus ring circle. When it is blue it means the camera target is placed further away from the area/objects within the focus ring. You can adjust the target distance parameter from the Attributes Panel: Camera interactively to see the focus ring color change. When it has no color it means the area within the focus ring is in perfect focus.
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Left: Camera target is placed at the third sphere which is also the area where the focus ring is pointing: perfect focus. Middle: Camera target placed too close, Left: Camera target placed too far back.
Moving a Camera
Please see the Viewport panel and navigation page for details.
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These and the rest of the cameras features are explained in the Scene tree page and the Attributes Panel: Camera page.
Hide/Unhide and Lock/Unlock a camera on its Camera Parameters panel
Projection
You can choose whether the selected camera should be a perspective camera (3D projection) or any of the 2D orthographic projections (Top, Left, Right etc.)