geonix said:Many (most?) lenses get sharper stopping down some from max aperature because lens aberrations reduce sharpness at or near wide open. The aberrations are competing with diffraction for which is worse at a given aperature. If you need depth of field, then giving up some sharpness to get it may be a reasonable trade.BeenThere said:Yes it could be so good as to be diffraction limited at f/4.sulla said:he sharpness at f/4 is phenomenal. The sharpness falls off with increasing f number, and rapidly above f/11.
Hm. I am thinking about a reason why the lens is sharpest at f/4 and falls off right after this.
Hmmmmmmmmm. Still no clue. Might it be that diffraction of the apertures smaller than 4 and the diffraction of the DO-element somehow amplify each other?? I don't think this lens is diffraction-limited already at 5.6, or is it??
The posted chart is an approximation because at f/4 the chart says corners and center are the same. That can't be strictly correct because even the theoretical MTF for the lens shows some degradation at edges and corners of the frame. Maybe within measurement error the author gets equal results across the frame.
The MTF Chart in this review indicates that the lens ist still "excellent" at f8 and f11 but I still find that odd. Often people stop down to f8 or f11 to increase depth of field so if sharpness decreases after f5.6 ...
Also I don't know what the MTF@140mm beneath the chart is about. ???
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