Frame grabbers perform an essential role in image acquisition from CCD-cameras and other imaging devices or intermediate storage systems such as VCRs. They directly affect the radiometric and geometric quality of the digital imagery. This paper gives an overview of the most important functional elements of frame grabbers. Their influence on the image quality and ways for their assessment are discussed. Results from two frame grabbers are used to explain the methodology and to give indications on typical performance measures. Errors found in the investigations include DC-restoration, effects of low-pass-filters, and synchronization.