I have a (fortunately funny, not smoking hole in the ground) story about that.
We had a fly-in at our airfield when I lived in Houston, which included a spot landing contest and a flour bombing contest. The flour bombing contest was pretty straightforward, drop a 1/2 lb bag of flour from the plane at 100 feet and whoever gets closest to the target wins. The aircraft should contain a pilot and bombadier, so the pilot can concentrate purely on piloting.
On this day, a friend of mine is doing it in a Champ, with the bombadier in the back seat. He sticks his head out the window to get a better view of the target as he flew towards it…and his glasses were instantly whipped off his face. He didn’t have a spare in the plane. I asked him afterwards how well he could see without his glasses, and he told me (I was standing about 5 feet away), “I can see that you’re there, but I wouldn’t be able to recognise you”. I asked him how he landed the tailwheel aircraft without glasses and he said he just looked for the blurred shape he presumed were the runway edges. I guess at least he had a lookout in the back seat who could yell if he was trying to touch down on a runway 15 feet above where it actually was…
I have contact lenses. I have a spare pair of glasses (“spectacles”) in my flight bag as per the requirement on my medical, but I don’t take my flight bag with me when I fly aerobatics. In fact, I don’t want any loose objects in the cockpit whatsoever for safety reasons. So I put a small case with two spare lenses in my pants pockets.
If ever ramp checked, I’m going to argue that my solution is much safer than the “legal” solution, even though it’s technically not legal.
Yes several times. Glasses and backup glasses. Got reprimanded in Germany for having the latter in my flight bag and not on my person in immediate reach. All my medical said is VNL but I still need backups.
Btw same thing for driving if it’s in your license. Guy I know got fined and denied onward travel in Germany by the Autobahnpolizei for not carrying a spare set.
I was asked to show spare glasses in addition to ones I wear. I always keep them in my flight-bag although I practically don’t need them because I have the correction only on one eye. However, my medical says I have to have them.
I have a lazy left eye, so I have glasses where the left eye is corrected (seemingly required somewhere by law) but the binocular vision is thereby not improved. And my right eye has always overcompensated the laziness of my left eye. Depth view further than 1,5 meter has nothing to do with the working of both eyes together. Anyways, I have 2 glasses, which I always keep in my flight bag and never ever wear, even though I should. So, technically I get involved in an accident and they will notice that I wear no glasses, but on the other hand, when wearing them I get this headache and my eyes get tired from the corrected lens. So, I have decided to leave them in the bag. Never been ramp checked on them.
It is one thing whether you carry them on board. The other is whether you actually wear them. 
Fortunately, they have no way of checking that…
Sometimes the medical will say two pairs must be carried, and the report was that this was being checked.
Peter wrote:
specifically checking that the glasses specified in the pilot’s medical were on board
Nothing “optometric” is specified on medicals except the need to a carry of a second pair
(pointless for those on corrective lenses but very useful for glasses if you break yours)
Actually if you open the glove box of any rented aircraft you will find a bunch of them to run the Paris fashion week 
„Glasses? Yes sir, on board sir! One set for Champagne, a couple for Wine and a variety for Spirits, all located at the Midwing Bar, 3rd door to the right. Please refer to the Hostess for demonstration…and, uhm Sir, please knock before going in…“
Yes, I’ve been asked to show them as part of a SAFA check before. It’s on their checklist…
