The standard camera lens/body combination stops the forward movement of the lens when 0.1 magnification is obtained. At this close up positon, a 10 cm height is imaged 1 cm on film/sensor. The f/numbers engraved on the lens barrel are only valid when the typical lens is imaging objects at infinity. As we move closer and closer by elongating the lens to film/sensor, the magnification obtained increases. Magnification means that the image forming light rays are being spread thinner. That translates to less light energy in any given unit area.

We can easily calculate the magnitude of the loss in image brightness due to magnification. We are talking about the Bellows Factor formula.
BF = (M +1) ^2 That’s magnification plus 1 squared. For the 0.1 magnification positon BF = (0.1+1) ^2 = 1.21 about 1/3 of an f/stop (we must compensate by aperture or shutter speed).

We handle the BF just like a filter factor, it is a multiplier. Suppose the exposure for this subject is is f/16 @ 1 second as determined by a hand-held light meter. We apply the BF = 1 X 1.21 = 1.21 seconds. In other words, at the 0.1 magnification position we are required to increase the exposure from 1 second to 1.21 seconds to compensate for light loss as the image forming rays transvers the lens. If we increase the lens to sensor distance an obtain magnification 0.5 the BF = 2.25. In other words, if we obtain magnification ½ life size we compensated by increasing the exposure from 1 second to 2.25 seconds. If we obtain life size, magnification 1, the BF is( 1 + 1) ^2 = 4. Now the exposure time must be increased from 1 second to 4 seconds. That’s two f/stops. If we obtain magnification 2 (twice life-size) the BF is (2 +1)^2 = 9. We increase the shutter speed from 1 second to 9 seconds.

All this is independent of the lens unless it is a macro. A macro is optimized to image at magnification 1 (unity). The design of the lens compensates for BF thus the f/numbers stay true.