Tag Archives: vision

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. Continue reading

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.

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Pulse Width Modulation & Eyestrain

In my last post I mentioned problems with PWM-dimmed monitors as an aside but that’s not an accurate reflection of the negative effects I think PWM can have on some people.

Pulse Width Modulation dimming involves varying the frequency of the LED backlight pulse to give the impression that the monitor is less bright. TFT central explain the technique here

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