The modern camera’s metering system measures exposing light thru-the-lens. We expect the results to be accurate, even when close focusing.

You should be aware that the f-numbers associated with a camera lens are based on imaging an object at infinity (symbol ∞). In other words, the math is performed when imaging a point source, like a star. When we image objects that are closer than ∞, the f-numbers become invalidated. The f-number error is super tiny when imaging objects 3 feet (1 meter) or further. When imaging object at about 1 meter, or closer, the error becomes significant.

This phenomenon is called “Bellows Factor” (BF). In the past, most camera makers stopped close focusing when the error approaches 1/3 f-stop. They did this because general purpose camera lenses are optimized for distance thus they are compromised when tasked to work close. Additionally the f-number error (BF) will likely result in an under-exposure unless a correction is applied. Modern cameras that read the exposure through the lens are generally immune as the metering system likely applies the needed correction.

On the other hand, a macro lens keeps the f-numbers valid when close focusing and they are optimized to image close-up., They are slightly challenged when tasked to image objects at a far distance.

Bellows Factor math: BF=(m+1)^2. M = magnification. If you close focus an achieve “unity” (life size or magnification 1), then m=1. At unity BF=(1+1)^2 (magnification + 1 squared). Solving for BF at unity: BF =( 1+1)^2 = 4 The BF is handled just like a filter factor; we multiply the exposure time by the BF to compute a revised shutter speed. If the factor is 4 the correction is 2 f-stops compensation (you open up the aperture since each f-stop is a 2X change). At unity the camera set to f/5.6 is actually functioning two f=stops more stopped down at f/11. Applying the BF to shutter speed: The shutter is set to 1/60 of a second, we multiply 1/60 by 4 = 1/15 (the revised shower shutter speed).

Why is your close-up images unexposed? Perhaps the camera’s logic does not fully compensate for BF. Perhaps the mundane background and it’s color / hue is fooling the metering logic. I suggest aperture priority thus allowing the camera logic to select the shutter speed. You can compensate in manual mode and open up the aperture or slow the shutter.