Where Does the 0.61 in the Diffraction Limit Formula Come From?

[ppy]

An infinity corrected microscope objective has a magnification of 100× for a tube lens
with focal length 180 mm. The numerical aperture of the objective is 0.90. Calculate the
the diffraction limited spatial resolution if the objective is
used with red light (660 nm). (Ans.: f=1.8 mm; d=447 nm)

Note I have already calculated the focal length of the objective lens as 1.8mm.

The answer reads d=(0.61*wavelength)/NA
A=aperture Can someone please tell me where the number 0.61 is coming from?

Thanks

 

[orsanyuksek2013]

You can read:

http://en.wikipedia.org/wiki/Diffraction-limited_system#Implications_for_digital_photography

---------------------
Örsan Yüksek