Speaking of a feeling of control, something to be aware of for people who wear strong glasses is that the fact you're not allowed to wear them during the procedure might affect your psychological experience. (When it comes to contact lenses, I've had a quick look online and while it seems to vary depending on the place and the procedure, I think people often seem to be allowed to keep them in.)
It does mention "no glasses" (among other things) in the leaflet, but it can't tell you how you'll actually feel in that situation, if you wear strong glasses that you'll need to take off. Of course, some people are going to be fine with it, but for me at least, being without my glasses is disconcerting in itself, let alone in a situation like an endoscopy.
I always feel profoundly vulnerable without them anyway — unable to see what's happening, or anticipate what people are about to do. I don't even understand speech as well, especially if the person has an accent I'm not used to, because I can't see people's faces, let alone mouth movements. With my glasses off, anything more than 8cm from my eyes is a chaotic blur. I can't tell what anyone around me is doing — all I see is giant blaring lights, a few human-sized blobs floating in and out of existence, and if my head is turned to the side, probably the weave of the fabric that my head is resting on.
My vision with glasses is fine, and I only take them off to sleep, so I'm no better equipped for this situation than any other sighted person with no experience of navigating the world with limited vision (though I admit that don't know how much having that experience actually helps). Both times I've had an endoscopy, I've tried to explain that I can hardly see anything, and asked them to please talk through what they're doing, but it doesn't help much with the feelings of loss of control and vulnerability.
TBH I'm not sure the endoscopy teams (or anyone else really) always fully realise just how poor it's possible for someone's uncorrected vision to be, when they see perfectly normally with glasses on. Or how scary and disempowering being without your glasses can feel, if people suddenly touch you or do things to you and you couldn't see it coming. (I mean, even just knowing that my glasses aren't within arm's reach and safe from damage is anxiety-inducing for me…)
And even if they do realise and understand how little I can see, and try to talk through as much as they can, they can't fix the fundamental problem that having unpleasant things done to you when you can't see anything that's going on can be pretty unsettling.