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

Started Sep 25, 2013 | Discussions thread
OP dark goob Contributing Member • Posts: 980
Re: Incorrect...

Beach Bum wrote:

On MAS, the crop factor is the same with all aspect ratios, namely 2x, whereas with a normal sensor it's only a 2x crop at 4:3, which changes at 16:9 and 3:2. The reason, Panasonic used the MAS with the GH2 was so that you could maintain a 2x crop at 4:3, 3:2, and 16:9.

Actually GH2 at 16:9 only has a 1.9x focal length multiplier relative to 135-format. Because at 16:9, 135-format incurs a 5% crop factor. This is because 135-format is natively 3:2, and is not a multi-aspect-ratio format. Therefore at 16:9, 135-format is not full-frame, since it gets cropped down to 36x20.25mm (41.3mm diagonal).

However, with the MAS, the image circle increases in the horizontal dimension for both 3:2 (17.84x11.9mm) and 16:9 (18.7x10.5mm). The total usable area of a Panasonic MAS sensor is 18.7mmx13mm when you factor in all of the aspect ratios.

The image circle never changes! It's determined by the lens, not the sensor. The diagonal size (which corresponds with the diameter of the circle) does not change in an MAS sensor. Therefore the circle does not change, either.

My point is, can Panasonic guarantee that all future lenses (at all focal lengths) will cover 18.7mm in the horizontal dimension? The answer is no, which is why they dropped it.

I'm pretty sure you're wrong.

A 4/3"-diameter image circle has a usable imaging area 22.58mm in diameter*. However the diagonal size of the imaging area of a FourThirds camera's sensor is only 21.64mm, which is 95.84% of the safe usable imaging area of 4/3". The Panasonic GH-1/GH-2 sensor maintains this standard FourThirds diagonal sensor size at 3:2, 16:9. However it never exceeds the usable imaging area of a 4/3" image circle, nor even comes close to doing so.

If you read back in time, lens makers were told to make lenses for the 18x13.5mm size (at a 4:3 ratio), which is the sensor size listed for early FourThirds cameras from Olympus (for example). The imaging area of the sensor is smaller than this (17.3x13mm), and the question of this thread is: why? The best hypothesis I've heard so far is that this was done to allow for in-body image stabilization without the sensor leaving the safe imaging area.

However your suggestion that lenses are being made for less than a 4/3" image circle just seems wrong. DO you have any basis for this claim in fact?

*According to the usage of the "image circle" terminology as explained by DPReview here , an "image circle" size (e.g. 4/3", 1", 2/3", etc.) defines the area projected by a lens, within which there is a "imaging area" sub-circle whose diameter is appx. 2/3 of the image circle.

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