Long/short designs

[Pogu]

I only have about 3.5 years or so under my belt thus far. I'm also only 26 and don't wear glasses at all, much less a PAL. So, not suprisingly there are some things I'm having trouble grasping.

The Fist shop I worked at, we had a house PAL (image) and a 'premium' PAL (AO Compact). You got the Image unless you wanted a frame that put you below 18mm or wanted to 'upgrade'. The place I work now, the patient pays for certain levels of PAL(normal range is the Precise or Hoya summit, premium is diditally surfaced, cheapskates get an Image or a Navigator). Hyperopes are always in 'long' progressives at no less then 20mm. Myopes stronger than -2.00 or so all get short designs, regardless of frame choice, though they do get more freedom in frame selection.

I have never caught wind of this scheme of long/short choice from anyone else, though I doubt they made it up. Is this a normal concept? If so can someone explain the reason to me?

[YrahG]

I only have about 3.5 years or so under my belt thus far. I'm also only 26 and don't wear glasses at all, much less a PAL. So, not suprisingly there are some things I'm having trouble grasping.

The Fist shop I worked at, we had a house PAL (image) and a 'premium' PAL (AO Compact). You got the Image unless you wanted a frame that put you below 18mm or wanted to 'upgrade'. The place I work now, the patient pays for certain levels of PAL(normal range is the Precise or Hoya summit, premium is diditally surfaced, cheapskates get an Image or a Navigator). Hyperopes are always in 'long' progressives at no less then 20mm. Myopes stronger than -2.00 or so all get short designs, regardless of frame choice, though they do get more freedom in frame selection.

I have never caught wind of this scheme of long/short choice from anyone else, though I doubt they made it up. Is this a normal concept? If so can someone explain the reason to me?

More than likely due to the effect of the power on the corridor, The real field of view which takes into consideration the power of the lens not only the position and size of corridor will show that the hyperope will have a narrower corridor then the myope. To offset this I believe they are going with a longer corridor to help widen the corridor and coversly in the myope going with short since their is already a widening effect with the power. For an example:

+10.00 hyperope fit 25mm from center of the eyes rotation, corridor 10mm wide.

tan(x) = 0.005*(40D - 10D)
tan(x) = 0.15
x = 8.53

The real field of view is 17.06 degrees

-10.00 myope fit 25mm from center of the eyes rotation, corridor 10mm wide.

tan(x) = 0.005*(40D + 10D)
tan(x) = 0.25
x = 14.04

The real field of view is 28.08 degrees

You can see that the real field of view for the hyperope compared to the myope is a reduction of roughly 40% in the scenario presented above or roughly a 6mm corridor fromt he original 10mm. That is why lenses today are multidesign to take into consideration the effects of power on the design.