In general, lenses are designed for maximum performance at infinity. Accordingly,when the lense barrel is fully extended to the shortest focusing distance, resolutionis reduced. Although this is negligible for ordinary lenses, it becomes increasinglyimportant in lense specially designed for close distance photography. Lense designersadopted a system where mechanism moves certain lense components as a unit automaticallycorrecting for aberrations. This assures high lense performance throughout the focusingrange.mirglossary
All lenses have aberrations (defects) to a degree. Minimizing them is known as correction. High-quality lenses have greater correction. The effect of their aberrations is less evident.