  We study theoretically the accuracy of the method based on the Fourier
property of lenses that is commonly used for the far field measurement. We
consider a simple optical setup in which the far-field intensity pattern of a
light beam passing through a Kerr medium is recorded by a CCD camera located in
the back focal plane of a thin lens. Using Fresnel diffraction formula and
numerical computations, we investigate the influence of a slight longitudinal
mispositioning of the CCD camera. Considering a coherent gaussian beam, we show
that a tiny error in the position of the CCD camera can produce a narrowing of
the transverse pattern instead of the anticipated and well-understood
broadening. This phenomenon is robust enough to persist for incoherent beams
strongly modified by the presence of noise. The existence of this phenomenon
has important consequences for the design and the realization of experiments in
the field of optical wave turbulence in which equilibrium spectra reached by
incoherent waves can only be determined from a careful far-field analysis. In
particular, the unexpected narrowing of the far field may be mistaken for the
remarkable phenomenon of classical condensation of waves. Finally, we show that
the finite-size of optical components used in experiments produces diffraction
patterns having wings decaying in a way comparable to the Rayleigh-Jeans
distribution reached by incoherent wave systems at thermodynamical equilibrium.
