I have a Zenni pair, I think they are 1.74 and around -6 correction in each eye. They are really cheap compared to other places, but they have very high chromatic aberration on the sides and I don't thing they are perfectly centered.

I think it's something that's more noticeable when you need relatively high power lenses.

It's good to have a high quality pair from a local shop so you have something to compare Zenni or other online shops to. YMMV of course, but I've had good experiences with Zenni, and only had one pair out of dozens for my family that had issues (seemed to be uncentered or had the PD incorrect), and that was over 5 years ago.

It's nice being able to grab a cheap pair of glasses for activities where there is a high probability of them getting damaged or lost, and having a backup pair handy. The reduced mental load of not having to worry is worth it!

>so you have something to compare Zenni or other online shops to

That's for sure. I bought a pair from 38dollarglasses.com and skipped the antireflective coating option. As it turns out, if you've worn glasses with AR coatings all your life, plain lenses are really, really surprisingly annoying.

Yep. Once your lenses get above -6 diopter or so it starts to become a serious concern. By -10 diopter it's the driving choice in lens material and design.

At -10.5, -11 I always get relatively low refractive index (~1.5) CR-39 based lenses because it's Abbe number is 60. This means I can still read, say, street signs or flanking computer monitors out of the corner of my eye despite the light going through literally >1 cm of material.

This is what I suspected would be the case. You absolutely do get what you pay for when it comes to optics, but equally, you can over-pay for utter rubbish. If you've got the vocabulary (which you do) to describe and understand what to look for, you can't go wrong.