Definition of half plate in English:

half plate

noun

British
  • A photographic plate measuring 43/4 × 61/2 inches (c. 10.8 × 16.5 cm).

    • ‘These were all made using a 5 * 4 (half plate) rather than a 10 * 8 (full plate) camera, so that the entire area of the painting is not always covered in the photograph.’
    • ‘I somehow thought portraits would have been in half-plate or even full-plate.’