The Four Thirds system

In response to a misconception voiced in another article,, it may surprise some readers to learn that the image sensors in Four Thirds digital cameras do not have a diagonal size of four thirds of an inch.

[Article by Martin Ward]

The naming of the Four Thirds camera system is a good example of a practice that is often used when a manufacturer wants to hide the true size of a product from the consumer. Namely, an obscure or deceptive imperial measuring convention is used instead of the original metric design size. The image sensors in Four Thirds cameras are actually 18 mm x 13.5 mm (22.5 mm diagonal), with an imaging area of 17.3 mm x 13.0 mm (21.6 mm diagonal) – quite a lot smaller than a diagonal size of four-thirds of an inch (33.9 mm), and significantly smaller than the 22.5 x 15.0 mm and 23.6 mm x 15.8 mm sensors used in the equivalent cameras of competitors Canon and Nikon.

The practice of describing image sensors in this deceptive manner continues from the days when all image sensors consisted of vacuum tubes and were described by the physical diameter of the glass tube, which was always larger than the “diameter” of the image sensor inside. Thus the imaging area of a Four Thirds camera sensor (17.3 mm x 13 mm) is the same as the imaging area of a hypothetical vacuum image-sensing tube of 4/3 inch diameter.

As with LCD and plasma TVs, technology has changed beyond the point where it makes any sense to continue using the old measuring conventions. It would be far more useful to modern consumers to describe the sizes of TV screens and camera image sensors in terms of width x height in standard metric units, than in terms of the diameters of hypothetical vacuum tubes, especially when these products now come in different aspect ratios. But, as is often the case, the needs of consumers do not always coincide with those of the marketing departments of manufacturers.

Further information on the Four Thirds system can be found at

One thought on “The Four Thirds system”

  1. There are various instances where product manufactures use obscure values to “hide the truth”, or at least make it less obvious. One example is tyre sizing. Apart from being based on a bizarre mixture of imperial and metric, there is a letter which represents the maximum speed at which the tyre should be driven. It would make more sense for this figure to be directly stated, but no, a code must be used to make it less obvious, goodness knows why. You can at least check this out at a tyre fitting depot, where they usually have posters on display explaining this tyre marking system.


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