Digital sensor units

In another forum, there is discussion of focal lengths and crop factors, including for smartphones.

 

I started looking up some phones, and it seems usual to give the sensor size as an inverse of some number.

 

That is, it might be (1/2.9) inches. I add the (), as otherwise it looks like 1/(2.9"), but normally it is written 1/2.9".

 

Now, traditionally fractions were a ratio of integers, and that always made some sense,

but why this form with 1 for the numerator, and a non-integer denominator?

 

It seems to be done consistently.

Interesting. Via Google, I came across this article on phone sensors. The article also contains a 'deep dive link' to the history of the 1/x" notation. It seems that it started as marketing ploy to express the sensor size in a way that customers wouldn't understand. Or at least in a way that made the sensor size size seem larger than they actually were:).

The 1/x designation refers to the early days of television, in which the sensor was a vacuum tube, cylindrical, of a certain diameter. I say "early days" when most of the TV shows when I was in college were in B&W. Most common (and unusual) sensor sizes are listed in this article.

 

Image sensor format - Wikipedia