Why is Four Thirds format 17.3x13mm instead of 18x13.5mm?

Started Sep 25, 2013 | Discussions thread
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dark goob
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Why is Four Thirds format 17.3x13mm instead of 18x13.5mm?
Sep 25, 2013

My understanding is that it is called a 4/3"-sized sensor because the diagonal of the sensor size is two thirds of 4/3". That would mean the sensor size ought to be 22.58mm diagonally, and in the 4:3 aspect ratio, that would be an 18x13.5mm sensor.

Incidentally the Panasonic GH-1 and GH-2 both have 18x13.5mm sensors. But in 4:3 aspect ratio mode, they crop their sensor down to 17.3x13mm. Why?

And why does Olympus not utilize an 18x13.5mm sensor instead of 17.3x13mm? It seems to me the latter is only roughly 95% of what the sensor size should be, in order to qualify as a 4/3" sensor.

If I had to guess, I'd say Oly did it so they could have image stabilization on the sensor. What do you think?

EDIT: Several sources said various Olympus cameras use 18x13.5mm sensors in fact, including the E-1. But some of these sources also indicate the "usable area" of the sensor is just 17.3x13mm. That makes me even more confused; why only use part of the sensor?

It seems they are cropping the 4/3" sensor down to have exactly half the diagonal size of 135-format, so that they can say their focal lengths easily convert by 2x. However, I'd rather have a weird numbered conversion, and get to use the whole sensor.

So what's the right answer? Is there any way to access the whole sensor from the RAW files, instead of just the 17.3x13mm crop area?

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