Joseph, thank you for your explanination, I had read some where that
diffraction are much easier happen with small aperture size, for
example in short focal lenght lens, I didn't know it also happen so
easy with tele which has a physical large aperture hole size.
C.H.Ling
>=======
>Anyone care to comment about this? A long lens that
>would degrade after F4 by diffraction?
>=======
>
>it means that this lens is so sharp that it is diffraction
>limited at f/4. diffraction limits resolution and most
>lenses get better corrected as they are stopped down
>and diffraction gets worse as they are stopped down.
>this means that as you stop down a lens, it gets
>sharper until diffraction takes over. the sharper
>the lens, the wider the aperture where this
>crossover takes place. most lenses are not
>sharp enough at f/4 to hit the diffraction-limit
>at f/4, which corresponds to an aerial resolution
>of 400-425 lp/mm. At f/11, the diffraction-limit
>corresponds to about an aerial resolution of about
>150 lp/mm. so it takes a really well corrected lens to
>be diffraction-limited at f/4.
>
>joseph
>
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