While many of the early professional digital cameras were developed from 35mm film cameras, conversion meant that rather than a digital back, the body was actually added onto a large, bulky digital unit. This was often larger than the part that was actually the camera! These were not, however, after market conversions; they were actually built this way in a factory.
One development was the EFS-1. This was inserted into the camera in place of the film, giving the camera storage for 24 images and 1.3-megapixel resolution. These units were developed from about 1998 until 2001, when the company began developing a true digital back.
Included in the category of professional modular digital camera systems are some of the highest-end equipment costing up to $40,000.00. These cameras can be assembled from components and are seldom found in the hands of the normal consumer.
Developed for medium- to large-format film, they capture greater detail and therefore the prints can be enlarged more that your standard 35mm film. These cameras are normally found in studios and are used for commercial production; they are very large and seldom used for action or nature type photography.
The terms "digital back" or "film back" came from the ability to change the backs of these professional cameras to either digital or film use.