Effect of spherical aberration on the imaging properties of scanning optical microscopes. 
The effect of primary spherical aberration and defocus on the imaging properties of scanning optical microscopes with weak objects is considered.  Optically there are two types of scanning microscope.  Type 1 scanning microscopes behave identically to conventional microscopes, but in type 2 scanning microscopes animaginary part is introduced into the transfer function.  In general, therefore, it is important that the lenses be adequately corrected, but for objects with very weak amplitude contrast the effect may be a useful way of obtaining phase imaging.  Experimental demonstration of the effect of spherical aberration is reported.