Spectacles - tilted for another prescription?

[Mr Red]

So what is the mechanism?

OK when I bought el cheapo pair (because I have a habit of loosing them) I found that 2.50 diopters was (were?) the nearest for distance.

However if I tilt them a bit I can get another prescription, in this case nearer my eyesight's need.
And for viewing the PC monitor (used as a TV full screen) tiliting more achieves almost perfect clarity in a small area needed to see the CRT (yes I still use a CRT for contrast ratio, colour rendering, and speed)

When I read in bed I can see the advantage because I can get the lenses of my reading glasses (correct?? prescription) normal to the line of sight between the New Scientist and eyeballs. Much as John Spencer used to do when he used hinged glasses to play snooker, tilting for the shot and returning vertical for surveying the table.
And it also allows for minor adjustments to the prescription if my eyes have changed.

[speakertoanimals]

This is known as pantoscopic tilt.

I think it works because as we all know, things don't look quite the same looking through any part of your lenses. Your prescription is going to be set up so that you get the clearest vision looking straight ahead and level, which is why they do all that stuff measuring the distance between your pupils, I assume.

Okay, so when looking at a snooker table, you're looking through the top part of your lenses, not the central part where vision is clearest. Hence the tilt means you know bring the central part of the lens in the right position for you to view the balls through that.

Optical centre, I think that is the phrase. And ditto for other sports like archery, where you'd be viewing through the side of your lenses, without special glasses.

[carnyx]

Looks like you are applying the 'Scheimpflug correction'. Basically, with the centre of the New Scientist page normal to the axis of your eyeball, it wil most likely be held at a tilt ... the top being further away than the bottom, and so the top and bottom will be out of focus.

But if the lens is tilted by approx half of the page tilt angle, the whole page ought to be in focus come into focus,


Here it is;


en.wikipedia.org/wiki/Scheimpflug_principle

[Mr Red]

Yes I had the New Scientist reading wheeze clocked. 'Scheimpflug correction' gives me a name. Ta

Not sure about the pantoscopic tilt because the sites I see (pun intended) say that it introduces astigmatism, maybe I have a bit, and maybe it improves the average prescription. Dennis Taylor's glasses were designed to give a prescription with a part of the lense more or less normal to the line of sight. I guess Toxologists would have the same on the side.

But obviously it alters the angle the light enters and leaves and with the curved wedge section it would alter the prescription, in this case, in my favour. Albeit in a smaller usable area. But they are cheap.