All the names for different formats and sensor sizes get confusing.
Large Format: 4" x 5" and 8" x 10" sheet film cameras
Medium Format: 120 roll film in 6x4.5, 6x6, 6x7, 6x8, 6x9 (those measurements are in cm. and the actual frames are a bit smaller)
Small Format: 2.4cm x 3.6cm (This is commonly known as 35mm)
These format sizes were all popularized during the era of film photography. Then when digital cameras emerged they had much smaller sensors. APS-C became the standard sensor size for DSLR cameras and it is approximately 18mm x 24mm. These are often called cropped sensors. When manufacturers finally started to make DSLR cameras with sensors that were more like the original 35mm film cameras they started to call them full frame, which makes it sound like they are huge. But in the bigger picture these are just the old "small format" otherwise known as the "Leica format".which was historically used for compact cameras such as the Leica.
Then to further confuse things manufacturers began making "medium format" digital cameras. But like their predecessors in the smaller format, almost all of the so called "medium format" digital cameras also have cropped sensors. True 645 cameras have a frame size of 56mm x 42mm whereas the digital Pentax 645D uses a sensor that is only 44mm x 33mm.. Of course this is still bigger than small format (35mm) but it is significantly smaller than true medium format. Eventually Pentax and other manufactures will probably release medium format cameras with normal sized sensors, and will likely call them "Full frame medium format".
Of course it should be remembered that 645 is the smallest of all medium format sizes, and in Japan it is referred to as "Semi-format", implying that it is "almost" medium format...
Last edited by revdocjim; 12-18-2012 at 11:34 PM.