When a lens is imaging an object that is that is an infinite distance away, we measure the distance, lens to film/sensor and pronounce this measurement distance to be the focal length. Thus if we mount a 50mm lens and focus on a star, the back focus distance is 50mm. When we close focus using this same lens, the back focus distance increases. This is because all lenses have limited ability to refract (bend inward) the image forming rays. As an example, to obtain a life-size (1:1) image of an object with a 50mm lens, the back focus distance increases to 100mm. The now increased back focus is technically not the focal length, but nevertheless the f-numbers associated with this lens are no longer valid. This induces a substantial loss in image brilliance when we close focus.
The formula to figure out how much compensation to apply is (M+1) X (M+1). M = magnification. Thus for the life-size set-up M = 1. To solve with M = 1. (1+1) X (1+1) = 2 X 2 =4. In other words the light loss is 4X. Since each f-stop equals a 2X change, a life-size setup requires that we open up 2 f-stops.
Macro lens to the rescue: The macro lens has two key characteristics. 1. The macro is optimized for close focusing. Ordinary camera lenses are optimized to image a world wherein the subjects are spread out over different distances. The macro is optimized to image flat objects like stamps and/or objects that display slight contour. The macro lens then projects an image onto the flat surface of film or digital sensor. 2. The macro is designed to maintain constant image brilliance as you close focus.
How is the constant image brilliance accomplished? The front lens group of the macro magnifies the size of the entrance pupil (aperture). This action is a variable, as you focus closer and closer, the outside world sees a larger and larger entrance pupil. It is this action that maintains a constant exposure as you close focus.
Bottom line: Most all macro lenses are optimized for close focus work and slightly compromised when tasked to image distant objects however image brightness is upheld as you close focus.