What is Fresnel distance ?

The distance of the screen from the slit, so that spreading of light due to diffraction from the centre of screen is just equal to size of the slit is called Fresnel distance (Z_F)
Fresnel distance is given by ${\mathrm{Z}}_{\mathrm{F}}=\frac{{\mathrm{a}}^2}{\mathrm{\lambda }}$
Where a = size of the aperture and $\mathrm{\lambda }$ = wave length of parallel beam of light used.