Dashmellow
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Ahh, a lens mount rather than lens class, using a standard metric thread; thanks for explaining:
ISO metric screw thread - Wikipedia
en.wikipedia.org
OK, perhaps "class" was not the best word to use but I've been hearing the term used in the photography profession for decades. When photography people hear the term "M 39 lens" it is usually assumed one is talking about the classic Leica lens mount used for many decades on many different cameras rather than the metric ISO standard so whether you are talking about the size of the lens threaded barrel or the mount it is just really just the other side of the same coin. The M39 lens pre-dates the International Organization for Standardization (ISO) by a great many years as well as it wasn't established until 1947. It is similar with M12 type lenses where people think of the lens type rather than the metric thread standard. People who know what they are understand you are talking about "Metric 12mm lenses". Obviously, that M involves the mount was assumed as when I said, "Basically any lens that has a screw-in mount and is designated in millimeters".
Anyway, as an amusing aside, manufacturing thread standards can be very useful in unexpected ways. Many people don't realize that many of the food items and other liquid products they purchase come in jars and containers that come with lids and bottle caps that are manufactured to standardized industrial screw neck finishes and thread sizes called continuous thread, screw-top or screw-on depending upon their thread design. Years ago, I began to notice that a wide range of different products I buy use the same size screw-on closures and many are interchangeable between products. For this reason, I sometimes keep the bottle caps and jar lids from certain screw-on products rather than throwing them all away because I know they will probably have alternative uses eventually. This happened just the other day when the pump bottle of some hand lotion I use stopped working because the damned pump tube didn't reach all the way to the bottom of the bottle thus leaving a surprising amount of unused product behind. (FYI: Dry cracked hands can be a real hazard here during a cold Vermont winter, especially if you work outside at all, hence the hand lotion.) So, I went through the kitchen drawer where I've saved a small collection of jar lids and container caps and found the perfect match; a screw-on cap with a pop-open top that came off a bottle of dish washing liquid and happens to be the exact same thread design and is the same size as the pump bottle cap. So, the wasteful pump bottle with several ounces of product still left over is now a very usable squeeze bottle thanks to the thread standards!
Without standards all of consumer product manufacturing would be total chaos.
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