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The "Critical focus zone" is related to the f ratio
Don Goldman gives delta f = 1.6 fr^2 x Lamda
Where Lamda is wavelength in nm.
Other references (based on 1/4wave tolerance) give slightly different values
Summary
f10 - 220 micron
f8 - 141micron
f5 - 55 micron
f4 - 35 micron
HTH
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