Doug Kerr
Well-known member
We often hear, including from Olympus*, the "godfather" of the Four Thirds System, that the name "Four Thirds" is intended to reflect:
A. That the format size for the system is very nearly twice that of the infamous "2/3 inch" format size (which name comes from the nominal diameter of the glass bottle of an important early Vidicon imaging tube with a target ["sensor"] of nominal dimensions 8.8 mm × 6.6 mm, now thus considered to be the "2/3 inch" size in digital cameras).
B. That the prescribed frame size for the system has an aspect ratio of 4:3 ("4/3" when expressed as a fraction, which is what ratios are).
And since the nominal diagonal dimension of the "2/3 inch" sensor size is 11 mm, then the size of the Four Thirds System image can in fact be thought of (under this repugnant convention) as "4/3 inch".
But B is just not so. Sadly, I don't have the actual Four Thirds System Standard - it is only available to camera manufacturers under a strict non-disclosure agreement. But one of the following seems certainty true:
1. The standard does not prescribe an image aspect ratio.
2. The standard allows several defined aspect ratios.
In support of my opinion is the following passage from the Micro Four Thirds System section of the Four Thirds White Paper promulgated by the Four Thirds Consortium (published of course by Olympus, the real guardian of the Four Thirds Standard):
Further discussions in this document indicate that the standard is intended to allow the use of various (or maybe any) aspect ratios with a consistent frame diagonal dimension. That could mean that any Four Thirds System lens could be used in a camera offering any aspect ratio (for the standard frame diagonal dimension).
But I have seen it said that some Four Thirds System lenses in fact have internal glare baffles or a field stop that are only compatible with operation at an aspect ratio of 4:3.
Incidentally, for a 4:3 aspect ratio Four Thirds System camera, the image size is (to four significant figures) 17.30 mm × 12.98 mm (generally stated to one decimal place as 17.3 mm × 13.0 mm).
The size 18 mm × 13.5 mm, which we often see quoted, is the typical overall size of the sensor.
Best regards,
Doug
A. That the format size for the system is very nearly twice that of the infamous "2/3 inch" format size (which name comes from the nominal diameter of the glass bottle of an important early Vidicon imaging tube with a target ["sensor"] of nominal dimensions 8.8 mm × 6.6 mm, now thus considered to be the "2/3 inch" size in digital cameras).
B. That the prescribed frame size for the system has an aspect ratio of 4:3 ("4/3" when expressed as a fraction, which is what ratios are).
* For example (from the Wikipedia article on the Four Thirds System):
Well, A is quite true. Precisely (and this was a design choice for the system), the prescribed image diagonal dimension, 21.63 mm, is "exactly" half the diagonal dimension of the 36.00 mm × 24.00 mm frame we speak of (if we are being careful) as the full-frame 35-mm frame size (43.27 mm)."In an interview John Knaur, a Senior Product Manager at Olympus, stated that 'The FourThirds refers to both the size of the imager and the aspect ratio of the sensor'."
And since the nominal diagonal dimension of the "2/3 inch" sensor size is 11 mm, then the size of the Four Thirds System image can in fact be thought of (under this repugnant convention) as "4/3 inch".
But B is just not so. Sadly, I don't have the actual Four Thirds System Standard - it is only available to camera manufacturers under a strict non-disclosure agreement. But one of the following seems certainty true:
1. The standard does not prescribe an image aspect ratio.
2. The standard allows several defined aspect ratios.
In support of my opinion is the following passage from the Micro Four Thirds System section of the Four Thirds White Paper promulgated by the Four Thirds Consortium (published of course by Olympus, the real guardian of the Four Thirds Standard):
The Micro Four Thirds System is designed to be compatible with the aspect ratios specified for the Four Thirds System, including 4:3, 3:2 and 16:9.
Further discussions in this document indicate that the standard is intended to allow the use of various (or maybe any) aspect ratios with a consistent frame diagonal dimension. That could mean that any Four Thirds System lens could be used in a camera offering any aspect ratio (for the standard frame diagonal dimension).
But I have seen it said that some Four Thirds System lenses in fact have internal glare baffles or a field stop that are only compatible with operation at an aspect ratio of 4:3.
Incidentally, for a 4:3 aspect ratio Four Thirds System camera, the image size is (to four significant figures) 17.30 mm × 12.98 mm (generally stated to one decimal place as 17.3 mm × 13.0 mm).
The size 18 mm × 13.5 mm, which we often see quoted, is the typical overall size of the sensor.
Best regards,
Doug