Certified Professional Photographer (CPP) Practice Exam

A circular polarizer is recommended for autofocus DSLRs primarily because it allows the camera's autofocus system and metering systems to function properly. Unlike linear polarizers, which can interfere with these systems by affecting the light that reaches the autofocus sensors, circular polarizers are designed to minimize this interference.

Circular polarizers consist of two layers: a polarizing filter and a quarter-wave plate. The quarter-wave plate converts the linearly polarized light from a linear polarizer into circularly polarized light. This conversion ensures that the autofocus mechanism receives the correct information, enabling precise focusing and exposure metering. This is particularly important when shooting in varying lighting conditions or with reflective surfaces, as a circular polarizer also enhances contrast and reduces glare without compromising the camera's performance.

While square polarizers can be used in certain situations, they do not provide the same advantages when it comes to compatibility with autofocus systems. Hexagonal polarizers are not a standard type and do not serve any specific function relevant to this context. Therefore, the circular polarizer stands out as the best choice for maximizing both image quality and camera functionality in autofocus DSLRs.