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Re: Can difference in vision be explained?
| Mike Tyner | 30 Dec 2006 00:27 |
> I have noticed for a long time that my distance vision, which is corrected
> with glasses, is degraded when driving at night. Is the above the
> explanation for that? If so how can one compensate for the problem -
> different prescriptions for day and night use?
You could, but a precise prescription should work for both.
In your circumstances, it's often that the glasses are a little off but it's
only evident when the pupils are large and depth-of-field is short.
-MT
|
| VicTek | 29 Dec 2006 23:25 |
> are too weak or when they don't have them. Similarly, flip on the
> overhead lighting and the pupils constrict, giving better acuity. Turn
> all lights off, and they do worse.
I have noticed for a long time that my distance vision, which is corrected
with glasses, is degraded when driving at night. Is the above the
explanation for that? If so how can one compensate for the problem -
different prescriptions for day and night use?
|
| William Stacy | 29 Dec 2006 22:58 |
Oh, and this effect can be (and often is) utilized to distort acuity
measurements. Allow the patient to squint (squeeze the lids together),
and the reduction in the "aperture" will allow him to see smaller
letters. This is why you see myopes squinting a lot when their glasses
are too weak or when they don't have them. Similarly, flip on the
overhead lighting and the pupils constrict, giving better acuity. Turn
all lights off, and they do worse. The other big difference is
psychological. Some people are far more profoundly bothered by blur
than others. Some will feel completely happy in a -3 fog while others
will feel like a fish out of water.
w.stacy, o.d.
> Take the pinhole away and they'd be lucky to get 20/200. |
| William Stacy | 29 Dec 2006 22:31 |
All other things being equal, pupil size is far and away the biggest
determiner of unaided acuity between two myopes. Just like a camera, if
you set the focus at a particular distance, changing the f-stop will
profoundly affect the photo. That's why cheap cameras with tiny
apertures don't really need any focusing mechanism, while expensive ones
with large lenses and variable f-stops need such fine focusing. In the
extreme example, take the -3 patient and have them look through a
pinhole. They will be close to 20/20 without any lens at all. Take the
pinhole away and they'd be lucky to get 20/200.
w.stacy, o.d.
>Is there a scientific explanation for why two people with the same
>prescription appear to have different levels of vision? For example, I've
[quoted text clipped - 3 lines]
>
> |
| katy | 29 Dec 2006 07:22 |
Is there a scientific explanation for why two people with the same
prescription appear to have different levels of vision? For example, I've
read somewhere that two people who are -3 won't necessarily see the same
without glasses, one might see better/worse than the other. What factors
influence this?
|
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