The focal length of a lens is a calculation made when the lens is imaging an object at infinity. This is a distance as far “as the eye can see” symbol ∞. As we focus on objects nearer than infinity, we must lengthen the distance, lens to sensor (film). The now elongated distance is called “back focus”. The lens to sensor/film extension becomes large. As we focus to achieve “life-size”, often called “unity” or 1:1 magnification, the lens will be racked forward 1 complete focal length, and the distance object to sensor/film will be 4 times the focal length. What I am trying to tell you is, the amount of mechanical extension to reach magnification 1 (life-size), is one compete focal length.
So, to make a lens close focus and reach unity requires lots of room to rack the lens forward. This is actually not too difficult, but now for the rest of the story. The f/numbers we know and love, that are engraved on the lens, are calculated from the infinity focus position. As we close focus, the engraved position marks for the f/number settings become invalid. At magnification 1 (unity), the error is 2 f/stops. This is a problem because we tend to underexpose when we close focus.
This f/number error is called “bellows factor”. If the camera reads the exposure measuring thru-the-lens, bellows factor is not an issue. If the exposure is determined by an external light meter, it is a big problem. As rule of thumb -- most camera makers (lens makers) stop the forward travel of the lens when the bellows factor error approaches 1/3 of an f/stop. The macro lens design is clever in that the lens array portion ahead of the iris diaphragm is a strong magnifier. As we focus close-up the magnification makes the diameter of the aperture opening appear larger. This magnification of the aperture allows more light to transverse the lens. This is how the macro design nullifies the bellows factor error.
Naturally it costs more to incorporate this design; so many lens makers stop the forward movement as the bellows factor approaches 1/3 f/stop.