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The reason wide apertures usually result in soft focus is because the edges of the lens front glass elements are thinner than the middle of the front glass elements. The difference in lens thickness makes the point of focus from the edges fall at a slightly different distance than the point of focus from the middle of the lens.
Lens designers add additional lens elements to correct that focus point imbalance, but it cannot be completely eliminated without causing other optical aberration issues.
Stopping down eliminates using that thinner part of the lens.
Lens designers add additional lens elements to correct that focus point imbalance, but it cannot be completely eliminated without causing other optical aberration issues.
Stopping down eliminates using that thinner part of the lens.