Judas window

Glossary Terms

The name Judas window originates in the British English term for the spy-hole in the door of a prison cell (or originally, a church door).

In reference to SLR cameras, it generally means an arrangement of relay prisms which allow a direct optical view of the current aperture-ring position, visible above or below the main viewfinder image. A small peepsight on the front of the pentaprism is required. Depending on ambient illumination levels and the clarity of the engraved numbers, the usefulness of this feature varies.

Front peepsight on Pentax KX image by Dirk HR Spennemann (Image rights)

Engravings for "f/8–f/11" marginally visible at top of Pentax KX viewfinder image by Pablo Coronel (Image rights)