The camera lens acts much like a projection lens it that it projects an image of the outside world onto the surface of a light sensitive surface such as photo film or digital sensor.
The power of the camera lens is expressed by its focal length. This is a measurement taken when the lens is imaging an object at an infinite (as far as the eye can see in Latin) distance, such as a star. This object is seen because it outputs light rays. The rays from an object at infinity arrive at the camera lens as a bundle of parallel rays. For objects closer than infinity, its light rays arrive as bundle of diverging rays.
As these rays traverse the lens their direction of trave is altered. This action is called refraction from the Latin to bend backwards. These light waves pass on from the lens as a bundle of converging rays. A ray trace reveals that their shape conical. Focus is achieved and an image is seen when an interpreting screen is located at the apex of this cone of light. In photography this will be the light sensitive surface of film or digital sensor.
Now the lens has limited ability to refract light. If the object is closer than infinity, the distance from lens to apex will be elongated. In other words, to obtain sharp focus for object closer than infinity we must readjust the film or sensor distance from the lens. This is the act of focusing. Should we desire to image at life-size (unity), the object will be positioned 2X focal lengths forward of the lens and the light sensitive surface will be 2X focal lengths downstream of the lens. The distance object to focused image will measure 4X the published focal length of the lens.
Another way to put this factorial: The focal length is only valid if the object being imaged is at infinity. When we image objects closer than infinity, we must adjust lens to image distance to accommodate the revised distance lens to apex of the image forming rays. This adjustment is called focus travel. We can obtain life-size or even greater magnification provided the lens and camera body provide a way to accommodate the needed focus travel.
We accomplish this ordinary lenses by dismounting the lens and inserting a spacer called “rings” or “tubes” or bellows. A macro lens is specially designed to provide the needed extra focus travel needed when imaging super close. Additionally, this act results in a loss of image brilliance that will result in under exposure unless compensation is applied. The macro lens is designed to auto offset to some degree this loss of image brilliance called “bellows factor”.
While virtual and real image phenomenon exists in the optical world, camera imaging only deals with real image, those that can be focused on film or sensor.
If you place the object super close to the camera, same distance as lens to apex, the light rays arrive at the image plane as parallel rays (afocal). These will not form an image. If you look backwards through the open camera, you will see the lens acting as a magnifier showing you a magnified view of the object.