I’d been aiming for a cylinder free pair of glasses to match my cylinder free contacts, and was frustrated by the limitation on acuity I was running into. So today I tried doing a trial refraction for my differentials over contacts with cylinder. My target was a near vision card held at arm’s length. I started at +3, and right away introduced the stenopaeic slit. Found a clearer axis, then started going towards 0.
With each step I rechecked the axis by wiggling the slit around until i found the clearest position. As I changed the power, the axis kept changing. At first it seemed to be drifting in one direction with the power, but then reversed around when I stopped gaining acuity.
The theory of stenopaeic slit says that you turn it 90 degrees from the axis, everything goes blurry again, and you continue the refraction until everything clears again. But i didn’t get blur, I got distinct doubling of everything. Even the sides of the slit were doubled, making for one enormous streak across my vision. More power didn’t do anything to resolve this. I could take it further, but I got pretty frustrated and just set it down for the night.
Tomorrow I should take that all the way to 0 and see what happens…