1. Introduction A macro converter (also known as an auto-extension tube or close-up adapter) is an optical accessory placed between a camera body and a standard lens to enable macro photography —capturing small subjects at life-size or greater magnification. The “GD” (General Definition) designation refers to converters designed for general-purpose macro work without compromising infinity focus, often incorporating corrective optics to maintain image quality. 2. Optical Principle A macro converter increases the image distance (lens-to-sensor distance), thereby reducing the minimum focusing distance and increasing magnification. Magnification Formula: [ M = \frac\textExtension Length\textFocal Length ] Adding an extension (x) changes native magnification (M_0) to: [ M_\textnew = M_0 + \fracxf ]
