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

Started Sep 25, 2013 | Discussions thread
dark goob
Contributing MemberPosts: 946
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Re: 4/3 sensor: where did your idea come from?
In reply to 3DrJ, Sep 26, 2013

3DrJ wrote:

OK. The phrase "approximately two thirds of the designated size" is a clue, the key word in italics. The size is not exact, there is a range of sizes within which the choice of size is arbitrary. The size could be somewhat more or less, say a millimeter one way or the other.

But it's no longer the 1950's. The technologies of that era don't have much in common with current sensors. The general ideas of sizes of light sensitive surfaces is only very generally applicable to current-day sensors.

Yep, I thought the same thing.

I don't know all the considerations, and it's not worth the trouble to try to dig it out, but the 18x13.5mm size probably refers to the chunk of silicon containing the surface, but there has to be room for data conduits and so on, so the 17.3x13mm area takes that into account. Again, it's a general kind of specification, and engineers like to leave a bit of "cushion", a conservative approach.

It does seem that way.

Well, see http://en.wikipedia.org/wiki/Four-thirds which gives a general description. (You may already know about that.) Right, no one actually says how the FourThirds name was chosen. But remember 4/3 was developed by/with Kodak, so their influence would have been important, and had been around in the early days of electronic imaging.

Marketing is likely the major factor in choosing product names, but you'd be right that how names are chosen is a deep, dark secret even within the producing company itself. However, "FourThirds" actually does refer to an old technology, and "4/3" did at one time describe (at least roughly) dimensions of a real product. I didn't make that up.

It was my idea that "FourThirds" is in part a nostalgic reference, and it works partly because it vaguely matches descriptions of ancient devices, and the 4/3 ratio of the image sensor. So I think it's an entirely suitable name, and a clever, admirable marketing choice. I like it. I'm in no way criticizing it, but simply saying the name is not a technical description, and undoubtedly not chosen based solely on technical criteria. We will never know exactly how the name came to be what it is.

Lets not debate the merits of the name as a marketing thing. I don't think that it would be useful since it's not like we can change it. I'm more just interested in why these various sensor sizes that purport to be 4/3", or 1", or 2/3", etc., generally are smaller by 4-8% than the precise definition of 2/3 of the image circle diameter. I think it's safe to say "it's for engineering reasons" and note the "approximate" word from the article.

A lot about mFT and FT matters to us, I'm not dismissing anything. My point was "it is what it is", if it's 17.3x13.0 rather than 18x13.5, well, that's the size it comes in. Asking why is a waste of time; there's no answer forthcoming.

Well, I think I've gotten enough of an answer on that regard. I was more just curious, it relates to a project I'm working on but I'm not here to promote that. I think that would violate forum rules.

In the article I pointed to, it says of FT, "Proponents describe it as an open standard, but companies may only use it under a non-disclosure agreement." I take it to mean, they have their secrets and they're not telling us what they are. But I'm willing to live with it as it's offered up, rather than not have it at all.

I think the non-disclosure is there to cover the communications methodologies between the bodies and lenses through the contacts to control aperture and focus. They don't want people to be able to manufacture lenses that are compatible with FT/mFT without paying the licensing fees. It's not an "open standard" as some have called it.

Thanks for taking the time to chime in on this.

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