MTF - 50mm F2.5G
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MTF - 50mm F2.5G

Hypothetical MTF plots. F2.5 (Left) and F8 (Right)

From a contrast and sharpness perspective, the 50mm F2.5G behaves very similarly to the 40mm F2.5G. Contrast is essentially perfect centrally wide open (left graph), and only fractionally less so at the corners, reaching perfect performance upon stopping down to F8 (right graph). Central sharpness is at 90% or better for the first 10mm of the imaging circle, only dipping below an average (or the radial and tangential traces) of 70% well out at the image peripheries. Stopping down to F8 yields outstanding performance, with ~90 - 98% contrast across the entire frame.

The 50mm lens requires little to no digital distortion correction, so the significant improvement in edge and corner sharpness upon stopping down isn't surprising.

A note on reading these graphs: sagittal or radial traces are solid while tangential traces are dotted. Orange traces are for 10 lp/mm detail, often indicative of lens' contrast performance. Green traces are for 30 lp/mm detail, often indicative of lens' sharpness performance. The horizontal axis ranging from 0 - 24 at the bottom of each graph represents radial distance (mm) from the optical center of the lens, while the vertical axis represents % contrast. The higher the traces, the better.