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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 Camera Parameters panel 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.
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 Viewports and Navigation page for details.
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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.).
Camera focus ring