Why are lenses soft when wide-open?

Started May 28, 2008 | Discussions thread
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pacolucia
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Why are lenses soft when wide-open?
May 28, 2008

Hi,

I gathered from looking at multiple lens reviews (here and elsewhere) that most lenses are softer when wide open, the hit a "peak" in sharpness around 1 to 2 stops from wide-open and then degrade again in sharpness.

I also understand that, for small apertures, the softness is due to the effects of diffraction. What I don't know is what causes the softness when the lens is wide open? Why is this typically the weak point in sharpness?

Thanks,

Paul

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