At View from the Porch, Tam explains why digital camera terminology sometimes seems to be deliberately deceptive:
The disconnect comes when you run into the “luxury” or “enthusiast” end of the compact camera market, where the physical size of the 1″ sensor in cameras like Sony’s RX100 line or the Canon PowerShot G7/G9 is touted as a selling point.
Because the sensor itself is not physically an inch in any dimension. For that matter, a tiny 1/2.5″ sensor isn’t two fifths of an inch in any dimension either.
Small CCD/CMOS video sensors are labeled based on the size of video tube they replace. These tubes had a rectilinear imaging surface inside the cylindrical glass vacuum tube. Inside a 1″ tube would be an imaging surface measuring 16mm diagonally, or a little less. When solid state sensors started replacing tubes forty years ago, they were labeled according to the tube they’d replace.
So to this day, a sensor 16mm (or a bit less) diagonally is still called a 1″ sensor.
For that matter, “35mm” film is only 35mm if you measure from edge-to-edge, sprocket holes and all. “Full Frame/35mm” sensors are only about 29mm diagonally; there aren’t any digital sprocket holes.
Just like we still “dial” and “hang up” our cell phones, even though phones with dials and handsets that you hang on the wall are a vanishing memory, digital imaging technology is still named after the analog technologies it supplanted.
