Toil and Trouble

One of the first symptoms of my tumor was double vision, since it was putting pressure on my optic nerve.  My doctors had hoped that once the tumor had disappeared, my vision would return to normal, but so far things have been about the same.  Last week, I visited a neuro-ophthalmologist, who prescribed me prisms for my glasses.  I just got them today, and while I'm still on the fence about how much of an improvement they are, I have been enjoying thinking about their physical properties.

 

Notice the faint lines on my lenses.  These are the temporary stick-on Fresnel prisms that were added to my glasses.  If I decided to keep them, they can be ground into my next pair of lenses.  The strength of eyeglass prisms is measured in prism dioptres (Δ).  My prescription is 2 Δ out, and 2 Δ down, meaning an object 100 cm away would appear shifted by 2 cm horizontally and 2 cm vertically.

I have noticed some interesting effects, aside from the intended image shift.  When we were in a grocery store, the strong overhead lights sometimes would reflect only from one section of lens, creating a bright stripe.  I've also found I get some distortion from the lines themselves, and I need to turn my head slightly to avoid looking through one.

I'll spend plenty more time with them before I make up my mind, but my initial reaction is to be a bit disappointed.  I guess I'm just used to the extensive development of normal eyeglass technology, whereas eyeglass prisms are so rarely needed, little improvement has been made.