Here we examine the progression in image sharpness and contrast from f/4 through f/22 on the Hartblei 120/4. Lens design has subtle interactions with diffraction, and some lenses do hold up slightly better than others; the Hartblei 120/4 holds up exceptionally well when stopped down.
This subject, the power-coated metal surface of a storage rack, was chosen because of its “fractal” nature and its contrast. The 24.4MP Nikon D3x was used for this test.
The subtle graying of detail is visible even by f/8, as is expected with any top-quality lens (lesser lenses start out crappy and improve by stopping down to f/8 or f/11).
By f/11 the graying of black and white is obvious, along with the loss of contrast in fine detail, most easily seen by noting the loss of the fine bright speckles.
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