All lenses have unresolved issues. There are seven major aberrations that the lens maker strives to mitigate. This is accomplished by making the camera lens using several lenses inserted in the lens barrel. The finished product is generally a compromise, optimized for distance and slightly compromised when tasked to work in close. Most top drawer lenses will sport 7 or more lens elements.
When we focus on nearby objects we are racking the lens forward, increasing the spacing between lens and image sensor / film. Lens makers generally halt close focusing when the resulting image is degraded below photo standard. The user now must resort to mount close-up supplemental lens (close-up filters) or add spacers that move the forward of its normal position.
As the lens is racked forward, the focal length and the engraved f-numbers become invalidated. Likely under-exposure results unless some compensation is applied. Light meters that read through the lens do this automatically. This is called “bellows factor”.
Generally this forward spacing induces worsening aberrations. Often the countermeasure is to reverse the lens so the rear element faces the work. This often helps because the forward facing lens is optimized to image a world with different object distances, whereas the rear facing elements are optimized to work a flat surface. In clos-up work the subject is likely shallow. Thus reversing the lens often improves close-up acuity.
Best is a macro lens. This design is optimized to make close-up pictures. The macro is immune from “bellows factor”. The macro is slightly compromised when tasked to image distant subjects.
