In article , PCACala@xxxxxxx writes
>Hello Doug:
>
><< The f/32 may be of little practical importance. As a rule, this is too
>small
> for best sharpness with a 90mm lens. The point is reached where loss of
> sharpness from the small aperture is worse than loss from DOF. >>
>
>f/32 will not be theoretically diffraction limited at infinity through 1:2.5
>reproduction. It will at 1:2, 1:1, etc. However, small aperatures are less
>sharp and I suppose that Vivitar had a higher standard than Tokina in this
>matter.
>
>Gary Reese
>Las Vegas, NV
The diffraction limited cut-off frequency of a lens is given by
Wo = 1/(L * f/#)
where Wo is the cut-off frequency in cy/mm at the focal plane
and L is the wavelength in mm
Thus, for an f/32, the cut-off frequency of the diffraction limited
optic is 62.5cy/mm at the focal plane, assuming a mean wavelegth for
visible light of 600nm.
This is independent of focus distance, magnification or whether there is
an 'r' in the month. :-)
What 'does' change with magnification etc. of course is the DoF.
--
Kennedy
Yes, Socrates himself is particularly missed;
A lovely little thinker, but a bugger when he's pissed.
Python Philosophers (replace 'nospam' with 'kennedym' when replying)
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