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

Started Sep 25, 2013 | Discussions thread
Re: 4/3 sensor: where did your idea come from?

3DrJ wrote:

dark goob wrote:

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. ...

Where the heck did that idea come from?

http://www.dpreview.com/glossary/camera-system/sensor-sizes

which states:

Sensors are often referred to with a "type" designation using imperial fractions such as 1/1.8" or 2/3" which are larger than the actual sensor diameters. The type designation harks back to a set of standard sizes given to TV camera tubes in the 50's. These sizes were typically 1/2", 2/3" etc. The size designation does not define the diagonal of the sensor area but rather the outer diameter of the long glass envelope of the tube. Engineers soon discovered that for various reasons the usable area of this imaging plane was approximately two thirds of the designated size.

If you look it up, "The FourThirds refers to both the size of the imager and the aspect ratio of the sensor". In particular, "The imaging area of a Four Thirds sensor is equal to that of a video camera tube of 4/3" diameter." And "The usual size of the sensor is 18×13.5 mm (22.5 mm diagonal), with an imaging area of 17.3×13.0 mm (21.63 mm diagonal)".

So the sensor size is 18x13.5mm? They just don't use the entire thing...? Why is that exactly?

The "FourThirds" name is a reference to an old technology unrelated to the digital camera sensor. In other words, the FourThirds name is a "branding" phenomenon, size of the sensor happens to approximate the old Vidicon tubes, so the name sounds nostalgic to those who know the history. The exact sensor size chosen was more or less arbitrary; the size is not defined mathematically but by specification.

What is your evidence for this claim? I mean are you simply hypothesizing, or do you actually have some sources to back this up? It sounds plausible enough, but I would question whether you are correct. It does not seem to be to be purely coincidental nor does "nostalgia" seem a strong enough motivator to stick this name on every product you make.

BTW, it happens that the mFT sensor size very closely approximates the format of 110 film. I shot a lot of 110 Kodachrome in the early 80's, and the film performed magnificently well. On the whole, however, 110 was not successful with color and B&W negative films, the resolution was just too low.

Think about it. The E-M5 (and now E-M1) is the living descendant of that crummy 110 format, and though mFT shines, it could just as well have been called new110 or something like that. But it wasn't--FourThirds was a much sexier name indeed.

Anyway, for the sensor, does a millimeter (or less) matter? Why should we care?

Nothing really matters, at least, not that gets talked about on the DPReview.com forums. But this isn't a philosophy or religion forum so lets not go there

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