The human visual system is often equated to a photographic camera. This is a poor analogy because the differences are far greater than the similarities. The processing of the human visual system is complex and non-linear so that even optical transfer function concepts must be applied with caution. Holographic optics offers some extra degrees of freedom with respect to refractive optics. Unlike refractive optics, diffractive effects are not, in the first order, dependent on material and geometric shape and require no significant volume. On the other hand they may suffer from fractional efficiencies and strong wavelength dependencies. The Pilkington 'Diffrax' lens invented by the author is an example of a product which steers between the disadvantages and maximises the advantages to provide the world's first diffractive bifocal contact lens. Indications for other visual applications are not very propitious although time and development may show this to be incorrect. This paper will review the interaction between the preferences and antipathies of the human visual system and the optical effects of diffractive systems.