Why is a "1-inch" camera sensor only 13.2×8.8mm?
Asked 6/2/2013
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My Sony RX100 is advertised as having a "1-inch" sensor, but the actual sensor size is about 13.2×8.8mm. A friend's Canon 5D has a 36×24mm sensor, which is easy to understand in millimeters. What does the "inch" designation on camera sensors actually refer to, and how does it relate to the real sensor dimensions?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
2 Answers
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Your mind is working fine, it's the inch-based measurement system that's dumb.
The 36x24 mm sensor measurement is straightforward; those are the length and width of the sensor in millimeters. The 36x24 mm size happens to be the same size that the image was on 35mm film, so that size is often called "full frame".
It's the inch-based system that's confusing and unintuitive. It goes back to the time that video sensors were housed in glass tubes. The inch measurement is the diameter of the glass tube that the sensor would be housed in, if it were housed in a glass tube, which it isn't anymore.
An approximate conversion is given by Wikipedia:
Sensor formats of digital cameras are mostly expressed in the non-standardized "inch" system as approximately 1.5 times the length of the diagonal of the sensor.
Wikipedia also has a nice table of nominal sensor sizes. It says a 1" sensor is about 12.8x9.6 mm. This is reasonably close to the specification you found for your camera (it's not exactly the same, but the inch-based scheme is approximate, to sort sensors into "classes" of size, not to specify precise dimensions).
Originally by user2138. Source · Licensed CC BY-SA 4.0
user2138
13y ago
0
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The millimeter measurement is the real sensor size: width × height. So a Canon 5D’s sensor is physically 36×24mm, often called “full frame” because it matches 35mm film.
The confusing part is the old “inch-type” naming used for many compact-camera sensors. A “1-inch” sensor is not 1 inch across in any direct way—not width, height, diagonal, or area. The term comes from an older video-camera tube standard, where the quoted size referred roughly to the outside diameter of a camera tube, not the active imaging area.
That old naming stuck, even though modern sensors are not tubes. As a result, the inch-type value is only a legacy format label, not a literal measurement.
A useful rule of thumb is that the quoted inch-type size is roughly 1.5× the sensor’s actual diagonal, but it’s approximate rather than exact.
So your RX100’s 13.2×8.8mm sensor really is the correct physical size; “1-inch” is just the historical format name for that class of sensor.
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