ASSESS LENS QUALITY

Understand and use MTF charts

MTF (modular transfer function) charts plot the resolution and contrast of a lens from the centre to the edges of the frame. In this example the blue lines represent performance when stopped down to f/8, while the black lines show results when shooting at maximum aperture. The thick lines show contrast, while resolution is demonstrated by the thin lines. Each of these lines has a solid and dotted version, showing sagittal (parallel to the image diagonal) and meridional (perpendicular to the image diagonal) performance respectively. A perfect lens would have all of the lines travelling horizontally across the top of the chart.

THE CLOSER THE SOLID AND DOTTED LINES ARE TO EACH OTHER, THE MORE NATURAL OUT-OF-FOCUS AREAS APPEAR. THE STRAIGHTER THE LINES ARE, THE BETTER THE CENTRE-TO-EDGE PERFORMANCE. THE HIGHER THE LINES APPEAR IN THE GRAPH, THE BETTER THE CONTRAST OR SHARPNESS IS.