Aniseikonia, the serious vision problem nobody seems to know about

I’ve had dry eye for a few years and have posted quite a bit about my problems focussing on images on a computer screen for hours on end.  What I didn’t know is that there is a scientifically accepted reason why some patients can’t adjust properly to glasses but can see significantly better with contact lenses.

The condition is known as Aniseikonia.

“Aniseikonia is an ocular condition where there is a significant difference in the perceived size of images. It can occur as an overall difference between the two eyes, or as a difference in a particular meridian “

There are 2 kinds of aniseikonia, static and dynamic. Static is observable when we focus on an object while dynamic is observed when we try to move our eyes to focus on a moving object or parse text, for instance. See here for more information and some explanatory graphics.

science technology

Lens design

As someone who has had a metric tonne load of problems in seeing clearly, I’ve become quite interested in lens design.

There’s a great course on Opticampus that explains the rationale behind and mathematics of spectacles lens design.  Lens design is a trade-off between the minimisation of different forms of aberrations brought about by practical considerations such as the shape of the lens, the quality of the material and the prescription that must be glazed.

Lens designers measure differences in refraction across tangential and sagittal meridians (orthogonal meridians describing refraction of light at different degrees from the optical centre of the lens).  They must minimise the oblique astigmatism which is the difference in refractive power between the 2. A good way to describe this is that light may be bent more vertically than horizontal leading to a squatter image or vice versa.

They must also compensate for power error which arises from the focal point of a perfect lens as light hits it from its range of lateral and vertical points of incidence   (theoretical and ignoring oblique astigmatism) being different from the Focal Point Sphere which represents the back surface of the eye.  The FPS is generally more curved and hence the lens may focus behind the eye away from the optical centre.


The pleasure of finding things out

R asked me recently to recommend a book on physics and science. I’m not really sure why. I think perhaps she feels she’s missing out on something although, tautologically, she doesn’t actually know what it is. I recommended she read Hawking’s “Brief History of Time” as I felt it had some great attributes. Clarity, reasonable depth, breadth and memorable writing. Everything you want from a great popular science book.
I made the wrong recommendation however. No disrespect at all to Dr. Hawking but the best communicator of scientific ideas was probably Richard Feynman. What he communicates better than any other is the enthusiasm required for great scientific endeavour, or as he aphoristically put it “The Pleasure of Finding Things Out”. I’d find it difficult to recommend anyone who hasn’t studied physics or at least mathematics in college to buy the Feynmann lectures but his collection of short essays “The Pleasure of Finding Things Out” is marvellous for even the uninitiated.
Reading Feynman is an antidote to the general dullness of much of what we now describe as knowledge. His science is as much swashbuckling adventure as dry machination. In particular the observation of Parallel computing pioneer Danny Hillis regarding collaboration with Feynman sticks in my memory. Feynman volunteered his services to Hillis when he was creating the original connection machine. Seeing that Hillis was about to tackle what’s known as a BIG problem and sure they needed some assistance, Feynman showed up for work. Feyman wasn’t a computist so when he was asked to analyse the router design for the Connection Machine 1, he produced a set of differential equations as a result. Hillis account is available here
His seminal essay on what’s now described as nano-tech “There’s room at the Bottom” has inspired a new science which will undoubtedly lead to revolutions, “good” and “bad”.
If you were to compare science to magic most publications are created b y prestidigitation (literally “fast hands”). Some are more profound, the work of conjurers. By contrast Feynman was a magician.

“There are two kinds of geniuses: the ‘ordinary’ and the ‘magicians’. An ordinary genius is a fellow whom you and I would be just as good as, if we were only many times better. There is no mystery as to how his mind works. Once we understand what they’ve done, we feel certain that we, too, could have done it. It is different with the magicians. Even after we understand what they have done it is completely dark. Richard Feynman is a magician of the highest calibre.”Mark Kac, Feynman collaborator and mathematician

Feynman’s work isn’t faceless. Physicists like Paul Dirac or Neils Bohr were brilliant yet impossible for the ordinary joe to empathise with. With Feynman you get an exuberant character who lived life with zest and mixed light anecdote with grave fact, and sometimes vice versa. If reading “Surely you’re joking Mr. Feynman” doesn’t inspire someone to learn more about the world they live in then they’re brain is probably not wired for physical curiosity or skepticism. Now that would be sad!