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| p.6 #1 · p.6 #1 · Sony FE 24mm f/1.4 GM Lens Big Bronco testing thread plus more | |
naturephoto1 wrote:
Hi Guy,
Isn't it usual that the lessening of sharpness and diffraction of faster lenses normally begin before they do on slower lenses (it is in part determined by how many stops closed down from being wide open)? That is one of the reasons that often slower lenses tend to be better than faster lenses for landscape work and apertures of about f8 and possibly f11.
Rich
No: diffraction has got nothing to do with how many stops down from wide open you are. It’s just the aperture.
AFAIK there are only two reasons a non-macro lens appears to diffracts ‘early’
One is that it is incredibly good. Diffraction is not on-off. It increases with every stop. If a lens is free of aberrations from wide apertures, it’s diffraction limited, and will only get worse with diffraction as you stop down. The aperture at which diffraction ‘appears’ is just the one where the effect stopping down has on reducing aberrations is outweighed by diffraction. If it has no aberrations to begin with, it will just get worse as you stop down. What this means is that the performance of a lens that seems to be getting wors e from diffraction may actually be better than one that is getting better!
The other, less usual, one is that there is enough residual vignetting that the aperture in the corners is effectively much smaller so diffraction sets in.
A complicating factor is that some cameras apply a special kind of sharpening to combat diffraction at some apertures in a built in lens profile. Canon does this, not sure about Sony. But it’s adds a complication - differences may be in the profile not the lens.
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