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Resolution and Diffraction
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.
Wide open at f/4, contrast is very good, but slightly low; this is expected of virtually every lens.
Contrast peaks quickly at f/5.6 (below).
Aperture f/8 (below) is a very strong choice, but whites are a little less white and blacks are a little less black than aperture f/5.6 above.
Observe that even f/8 (above) has gone slightly gray, but that f/11 (below) shows near black areas going gray, bright speckles disappearing, etc. This is diffraction at work, and no lens can avoid it at f/11.
Aperture f/16 is still acceptable, but contrast is low; there are no blacks or whites anymore. The effects of diffraction are at work here.
Aperture f/22 has held up well, but compare to the f/5.6 result, shown again below it, to see just how much as been lost in terms of contrast and biting detail. There are those that maintain that such detail can be recovered, but that is difficult without also losing subtle tonal values and in this case, looks impossible.
Compare f/22 above to f/5.6 below.
The Hartblei 120/4 maintains high image quality under the duress of diffraction, but by f/22 it succumbs as all lenses do to the contrast and sharpness-robbing effects of diffraction.
There is no magic to be had, but you’re unlikely to find a lens that behaves better than the Hartblei 120/4 does when stopped down.