I’ve been chasing details of varifocal contact lenses and I’m sceptical that these work at all. According to this article, the lenses need to actually move on the eye’s surface, which is a dreadful idea.
Instead, today’s invention is contact lenses containing a fluid filled envelope.
As the eye looks ahead, the fluid does not get in the way of incoming rays. Look down, however, and the fluid has an extra refractive effect. This could be continuously attenuated by controlling the radial geometry of the envelope or even by having a fluid which is naturally stratified, providing variable optics.
I had a long chat to my optician about this. It seems these work by containing concentric rings with different refractive properties One set of rings focuses ‘near’ and the other set focuses ‘far’. There are more near-focusing rings towards the centre of the lens.
This works because we tend automatically to close down the pupil a bit when focusing on near objects, thus boosting the proportion of near-focusing rings at work under those circumstances.