Cylinder axis explanation video

Is this correct about the cylinder power being like a periscope, 90-degrees from the axis?

The astigmatism dial tool has the axis numbers differently.


Will I measure the axis right using the dial or do I have to convert it (90-degrees faces horizontally in the dial versus up like in the video)? The video above suggests that the axis is not the blurry direction. That means 90-deg cylinder/axis is horizontal astigmatism/blurry vision along the 180-deg line. Do I have this correct, or am I thinking something wrong?

And mainly, can the dial numbers be trusted? Has someone measured their axis wrong by accident? Mine were measured by an optician, so I don’t know where exactly they’re supposed to be pointing. So which of the axis-maps is used in lenses?

(Dumb question really, but I thought there was a fault in the math-page in one of those equations as well, but after watching the youtube video associated with it I realized the fault was that I didn’t understand it but after seeing the picture.)

Yes. The curve runs along the axis and pulls light from the area perpendicular to the axis and focuses it along the axis. You can kind of see how this works in this video, here:

For instance, my left eye has axis 180, around -2D (it’s changing, so I’m not being specific). What this means in practice is that unaided, I see vertical lines very clearly, and horizontal lines are washed out. This was really bad a few months ago; it’s how I started to find active focus, actually. I took off my glasses and started looking for horizontal lines–blinds, door frame lintels, sidewalk edges–and trying to “clear them up,” or at least make them as sharp as I could see nearby vertical edges.

So basically, in order to correct this tendency of my eye to pull light from the “left” and “right” into a sharper image in the center, my 180 degree axis lens needs to spread that light back out to the left and right, i.e., along the “horizon.” It does this by being oriented horizontally, 90 degrees off my eye’s natural tendency to do this vertically due to its pathological shape. This cancels out that tendency, at least in theory.

I haven’t used that measuring gizmo, although I went to the trouble to make one–I could not get one to print out with the clarity I really needed for use. I’m thinking, if I recall the instructions, that you’ll see blur with the longer line and arrowhead after you see blur with the shorter line and no arrowhead while moving it away from your eye. The normalized difference between these two figures gives you the cylinder power needed to correct. The angle of the arrowhead line, with zero at the top and the arrow pointing to its strongest, clearest angle, gives you your axis reading. Again, disclaimer: I’m going from memory since I haven’t used that scale in live fire exercises. I just go off what the opto told me for power and axis, and have been reducing (documented elsewhere), in order to correct the astigmatism/cylinder.


Now it makes more sense. And those numbers on the dial are for MINUS notation and on the video is for PLUS notation. Which explains that the axis is rotated by 90-degrees between the notations.

ah, that does make a lot of sense.

I use the minus, since in the US, that’s all I’ve ever seen.