The frame proportions of existing 4/3 sensors are 4:3 but that's purely
coincidental as the 4/3 actually refers to the sensor size in a
round-about almost incomprehensible way.
4/3 actually means 4/3 inches. By some bizarre means I don't
understand, the sizes of sensors are derived from the old TV Vidicon
tubes. In this case the 4/3" specifies the diameter of the tube. The
tube has a sensing area smaller than its diameter. A 4/3" Vidicon tube
has a diameter of (amazingly) 4/3 = 1.33", a sensing image circle of
22.5mm and a sensing area of 18 x 13.5mm which is the size of the
current sensors.
While current sensors are 4:3 ratio I'm sure that if you asked Olympus
about the 4/3 standard the only thing that really matters is the size of
the image circle. If someone wanted to make a 3:2 sensor with a 22.5mm
image circle I'm sure it would receive the 4/3 label. But it would have
even fewer pixels and you probably couldn't sell it. I don't expect to
see one anytime soon.
For more on this mystery and the source of most of my knowledge on the
subject see: <http://www.dpreview.com/news/0210/02100402sensorsizes.asp>
Other factoids about the four thirds system can be found here:
<http://www.four-thirds.org/en/about.html>
Chuck Norcutt
Alan Magayne-Roshak wrote:
> Can someone clarify the frame proportion in the 4/3 System? Does that
> designation mean it's 4:3? I like the 2:3 ratio of 35mm film rather than
> the supposed ideal format shape of 6x7, etc. so 4:3 would be too much a
> square for me.
>
> I loved the OM system for its size and functionality (no need to turn the
> camera on to take a picture). I did a motorcycle trip through Britain and
> couldn't have carried the equivalent gear in Nikon or Canon as easily - two
> bodies, 21mm, 50mm, 100mm, and 300mm. I do wish all the models had diopter
> adjustment. My OM-4 electronics died and I have to wear a special pair of
> glasses now to see clearly through the OM-1, 2 and 2S, with my aging eyes.
>
> Alan
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