Making your own sailor's hat out of newspaper? Easy. Crafting a 90mm f/2.8 lens from nothing except the raw parts? OK, that might register a bit higher on the difficulty scale. Evidently, Mats Wernersson decided it wasn't beyond his abilities and thankfully for us, he documented his build and uploaded the video.
As Michael Zhang details on PetaPixel, the lens is made from scratch (yes, even the glass), though the helicoid, blades and mount had to be ordered whole.
Here's what the final product looks like.
And the photos the lens produces.
Sure, you need your own fully-kitted workshop (or access to one), but it's mind-blowing what's possible with some ingenuity, talent and time.