The purpose of this research was to explore South African Grade 11 learners' conceptual understanding of "image formation by lenses". The participants for this study were 70 Grade 11 learners from a selected senior secondary school in Mthatha, Eastern Cape Province, South Africa. The qualitative approach employed in the study made use of a two-tier open-ended questionnaire as the data collection instrument. The study explored several alternative conceptions the learners had held in terms of the roles that the lens and the screen play in the image formation and the characteristics of the image formed when a lens with a larger diameter is used and when a portion of the lens is covered. Most of the participants could not respond correctly in the situations presented in the questionnaire. However, almost all of them were found to have adequate conceptual understanding about the role of a lens in the image formation.