Why is a Four Thirds sensor called 4/3 inch when its actual size is smaller?
Asked 8/25/2025
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A Four Thirds sensor is typically about 18 × 13.5 mm, with a diagonal around 22.5 mm, which is clearly less than 4/3 inch. What does the “4/3” actually refer to, and why doesn’t it match the sensor’s physical dimensions?
Originally by 93Iq2Gg2cZtLMO. Source · Licensed CC BY-SA 4.0
93Iq2Gg2cZtLMO
9mo ago
2 Answers
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This is covered in the very Wikipedia page you link to, in the Sensor size and aspect ratio section:
The name of the system stems from the size of the image sensor used in the cameras, which is commonly referred to as a 4/3" type or 4/3 type sensor. The common inch-based sizing system is derived from vacuum image-sensing video camera tubes, which are now obsolete. The imaging area of a Four Thirds sensor is equal to that of a video camera tube of 4/3 inch diameter.
See also our canonical question on the "inch" sensor sizes.
Originally by Philip Kendall. Source · Licensed CC BY-SA 4.0
Philip Kendall
9mo ago
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Generated from our catalog & community — verify before relying on it.
The “4/3” in Four Thirds does not describe the sensor’s actual width, height, or diagonal. It comes from the old inch-based naming system used for video camera tubes.
In that system, a sensor described as “4/3 inch type” was named after the approximate outer diameter of the image pickup tube it replaced, not the active imaging area itself. So a Four Thirds sensor has an imaging area roughly equivalent to what a 4/3-inch video tube would have delivered, even though the real sensor dimensions are much smaller than 4/3 inch.
So in short:
- “Four Thirds” refers to the legacy sensor format designation
- It also happens to have a 4:3 aspect ratio
- It does not mean any sensor dimension literally measures 4/3 inch
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UniqueBot
AI9mo ago
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