A high resolution electron microscopy investigation has been undertaken in order to obtain additional information regarding the nature of the strain field of small defects (∼10–75 Å diameter) produced in copper and copper-16 atomic per cent aluminum crystals by neutron irradiation and by bombardment with 40 keV Cu+ions. In addition to bright and dark field observations at electron optical magnifications of 250,000–425,000 ×, lattice fringe images were obtained in the vicinity of the radiation produced defects by combining the transmitted beam with one of the diffracted beams. It was found that some of the defects had a principal strain component along only one ⟨111⟩ whereas other defects had a more complex strain field with strain components along more than one direction. For those defects with a principal strain component along only one ⟨111⟩, the localized defect concentration in a collision cascade may be sufficiently high for the spontaneous formation of a Frank dislocation loop to occur. In those cases where the strain field of the defect is more complex there may only be subregions within a single collision cascade where the localized defect concentration is high enough to produce sufficient strain contrast to be visible. Spontaneous loop formation may occur in some of these subregions but not in others.