Object between F and the lens

If the object distance is less than or equal to the focal length the image is virtual.

When object is on F, the image is at infinity.

Object between F and 2F

If the object distance is greater than the focal length the image is real.

When object distance is equal to twice the focal length, the image distance is also equal to twice the focal length. In this case, the image is the same size as the object.

When object is at infinity, the image is in the focal plane.

Concave lenses always form virtual, diminished images of real objects.

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