ETR System film backs (except for Polaroid film backs) are comprised of three sub-components:
- Film Back Frame, which attaches to the back of the ETR camera body (for the 70mm film backs, the frame is two separate components)
- Film Holder (or “insert”), which holds the film and is inserted into the film back frame
- Dark Slide – which protects a loaded film from light exposure when the back is removed from the camera, allowing backs to be interchanged part way through exposure of the loaded films
All types and generations of film backs are compatible with all generations of ETR body. The exceptions to this are the ETR-C and ETRC bodies, where the film back frame is fixed to the body (although it is possible to switch between 120 and 220 inserts within the fixed back component of the camera). However, ETRSi generation film backs are the easiest to operate and most recently manufactured, making them the better choice where possible.
In some cases, it is possible to combine different generations of the three sub-components to form a complete back – for example, an ETRSi generation 120 Film Holder can be put into an ETRS Mk-II generation 120 Film Back Frame,
