Monday, 5 March 2012
How is it (so much as) possible to see the world through vari-focal spectacles?
This is familiar. When I first began wearing glasses, I found the bending of straight lines in my surroundings (such as door frames) when I moved my head very distracting. After a while, not only did this not disturb but it became hard to see it at all. Or rather, whilst at first I had an experience as of the external world (impossibly) bending, later it never even seemed that way though I could, if I wanted, concentrate on the visual ‘impression’ and still see bendiness in it.
With a more recent pair spectacles which had a new lens coating designed to reduce glare, my first awareness of a corresponding distortion was finding myself thinking ‘Now I can see this scene as though it were in 3D’. That is the sort of experience one has of 3D cinema which sometimes takes a bit of work (to ‘assemble’ a single 3D experience rather than two non-overlapping 2D ones). But I was in fact looking around Preston station. It took a bit of reflection to realise that a fringing effect around the edges of pillars etc made them look like 2D pictures posted at different distances.
So the idea of getting used to a new correction isn’t a surprise. With vari-focal lenses, the top ‘row’ of the lens corrects short sightedness for infinite distance, the middle spot works at about computer screen distance and a lower central spot works for close up reading. All smoothly connect. But because of engineering limitations middle and lower areas to left and right are blurred.
Initial instruction is to improve focus of blurred objects by moving the head: chin down to focus on far distance; chin up and centring to focus at middle or, further up, close distance. Thus the first aspect of accommodation is to learn these sensory motor contingencies (as those nice enactivists would say) and then let that tacit knowledge become habitual second nature. But to do that, it is initially necessary to notice what is blurred and then correct it. It takes a bit of explicit finding. Only so, can the head movements be learned as more of less instinctual practical knowledge.
But there’s an obvious second element of accommodation. Unless the world is arranged such that the distance of elements is in the just the right function of polar coordinates (lower things nearer), some elements just will be blurred. Walking downstairs, for example, looking ahead to the (sharp/clear) bottom of the stairs, the step one is just about to use is blurred. Sootica, scurrying at my feet, is a perpetual blur. And given the limitations of the lenses, lower left and right is blurred at any distance (so, in fact, there’s no function that could yield a universally sharp picture). Moving the head to focus on what was blurred in those areas simply moves the area away, like a cat chasing its tail. Thus the second element of accommodation must (I assume) be to cease to notice this blurriness, to make no attempt to focus on it (or else to feel permanently drunk or drugged).
Given that these two accommodations pull in opposing directions (notice blurriness and correct versus ignore blurriness and do nothing), I do not know how I will ever learn to use vari-focal lenses.