  The approach to obtain the image shape of an optical diffraction radiation
(ODR) source focused by lens on a detector with taking into account the
"pre-wave zone" effect has been developed. In full analogy with optical
transition radiation source the characteristic size of ODR slit image doesn't
depend on the Lorentz-factor and is defined by slit width, wavelength and lens
aperture. If a beam is passed through a narrow slit the shape of a slit image
depends on a beam size. ODR generated by extremely ultrarelativistic beam
passing through a narrow slit may be used to determine the beam size from ODR
slit image.
