Kodachrome remains a standard of quality in professional circles. Unlike other E-6 and C-41 color films, Kodachrome contains no built-in color couplers, only 3 color sensitized black and white emulsions. All the color is added in the Kodachrome processing process. Thus, Kodachrome layers are thinner and fewer, its grain finer, its sharpness unchallenged. Author explains Professional Kodachrome 150 25 and 150 64 are not new or different products; rather, critically selected batches of the standard production, tested and aged (stored) until they reach an ideal color balance (newly made film tends towards green coloration, aged film more purple). Comparing Professional 64 with Professional 25, author concludes K64 is brasher and more contrasty; K25 has a longer tonal scale, holds more highlight and shadow details, and has a slightly warmer tonal scale.