Roy Mathers wrote in post #11801137
Does it matter how accurate the focal length is, or is your question purely academic and out of interest. It seems to me that, for instance, saying 30 frames per second for NTSC video is easier that saying 29.9700299700299 frames per second. Similary, it's easier to call a lens 100mm than, say, 102.36.
It's kind of academic interest. I want to know, when I do calculations, what level of accuracy is the norm.
Yes, it is easier to say 30 fps. But when I generate a synthesized slate clock by computer, I need to know that it is 30000/1001. So 30 fps was the "rate class" as opposed to the specific rate. Now ATSC, the digital standard here, supports the exact rates of 24, 30, and 60, as well as the "NTSC compatible" rates of 23.976[...], 29.97[...], and 59.94[...]. So just saying "30" isn't as certain as it once was (but is still far more often really 29.97 or so). The video engineers have to know these numbers.
So I presume lens engineers know if their design is 100.0000 on the nose, or is 102.3333 just because that was the easiest or best way to tweak the design into having less CA and/or less BD. It's still a "100mm class" lens that I'm sure most of us could not see any difference with (though it might show in critical 3D or photometry shots).