Toronto. The most important part of any single lens reflex (SLR) camera design is its mirror box. This amazing idea allows the rays from the subject, through the lens, redirected, and focussed on a ground glass at full aperture. With the lens stopped down as necessary, the mirror could be flipped out of the way so the rays would hit the shutter and then the glass plate, or film.
The upside was a sharp view through the lens for telephoto lenses, crisp focussing of the narrow depth of field, and accurate subject framing. The down side was that the film to lens mount distance exceeded the focal length of traditionally designed normal to wide angle lenses; the lenses had to be ‘wide open’ to maximize brightness and minimize depth of field, then stopped down and (along with shutter speed) adjust the light intensity to match the needs of the plate/film making camera operation slow compared to a rangefinder (e.g. a Leica or Contax).
The lens design for many normal lenses and most wide-angle lenses was ‘retrofocus’ (lens mount to film distance greater than the lens’s focal length) resulting in various degrees of geometric distortion and less than the best resolution. Some normal lenses used a slightly longer focal length to allow traditional lens designs to be used and lessen geometric distortion.
When sensor technology allowed sensors to increase in size, eventually becoming ‘full frame’, digital took over the professional market and cameras became digital SLRs (DSLRs) with same mirror box and lens mount to sensor distance so film lenses intended for SLRs could be used on DSLRs and focussed to infinity.
The introduction of interchangeable lens mirrorless designs like the Micro 4/3 cameras and Sony’s NEX line made the camera body thin enough that an adaptor could allow many rangefinder film lenses to be mounted and focused to infinity or closer. Recently the mirrorless designs have even had full frame sensors!