On Sat, 31 Oct 2009 00:38:02 +0000, Tony Houghton <h
...@realh.co.uk> wrote:
>I noticed a problem with my new glasses:- things looked out of focus
>when I was looking downwards. If I tilted them (top away from my face,
>bottom towards it) that improved it a lot. I bent the angle between the
>arms and frame as much as seemed reasonable, and it's improved the
>vision in the bottom portion of the lenses, but at the expense of vision
>looking upwards, which is now quite distorted, and the downwards vision
>still isn't as good as my old glasses. I also noticed that the effect
>seems more pronounced in one eye than the other.
>I already had the lenses changed because I knew they weren't right and
>they were giving me headaches, but I couldn't work out what was wrong,
>so I downgraded the lens material from 1.6 to 1.5 plastic thinking it
>might be due to too much dispersion (although I've had 1.6 before which
>were OK).
>What are likely causes? I'm guessing it's mainly because the lenses are
>too flat, but I wondered seeing as it's worse in one eye than the other,
>could it be they're miscorrecting my astigmatism, or would the
>differences between each eye's prescription cause a slight difference in
>the experience of too-flat lenses? The prescription is -6.25 -0.75 10
>and -6.75 -0.5 37, and it's the first of those which seems a tad worse.
>The only differences from my old prescription are that the axes have
>changed from 15 and 42.5.
CR39 (traditional plastic) will most likely be a spherical design, so look for a
base curve of about +2.50 ± .50 D for best form (minimal off-axis marginal
astigmatism and power error). If aspheric, the BC will be somewhat flatter,
depending on the degree of asphericity. Higher index of refraction lenses will
have slightly steeper BCs for best form optics, although many of these will be
aspheric, hence flatter BCs.
Have an experienced optician dot the OC and draw horizontal and vertical lines
through that point. The vertical line should be ±1mm center pupil for each eye.
Raise your chin so that the lens is perpendicular to the floor. Look straight
ahead- the horizontal line should be ±1mm center pupil (both eyes). If not,
adjust the panto tilt until it is, although the final tilt should be in the six
to twelve degree range. The above assumes no precribed prism.
The dihedral tilt (face form or wrap) should be about +5 degrees.
Vertex distance should be as little as possible without the lenses hitting the
lashes or face. 12mm to 14mm is typical. Longer distances are not desirable but
sometimes unavoidable due to individual facial structures. Regardless, it should
be ±2mm at this power compared to the doctor's vertex distance. However, it's
unlikely that the doctor measured the Vtx so assume about 13mm.
Inspect lenses for waves and warpage (no more than 1 D for the latter).
A visit to the doctor is required if all of the above checks out.
Robert Martellaro
~~~~~~~~~~~~~~~~~~
Optician/Owner
Roberts Optical
Wauwatosa Wi.
~~~~~~~~~~~~~~~~~~
"Science is a way of trying not to fool yourself."
- Richard Feynman
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