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.

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In coherent radiation sources (diffraction radiation, Smith-Purcell effect, etc.) based on relativistic electrons passing by a material "radiator'', the electron self-field is partly shadowed after each part of the radiator over a distance of the order of the formation length γ 2 λ. This effect has been investigated on coherent diffraction radiation (DR) by electron bunches. An absorbing half-plane screen was placed at various distances L before a standard DR source (inclined half-plane mirror). The DR intensity was reduced when the screen was at small L and on the same side as the mirror. No reduction was observed when the screen was on the opposite side. The shadowing effect can significantly reduce the total energy radiated in a long radiator.