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Is there a condition where myopic power changes with ambient brightness. When I drive in the morning in bright sunlight my power is -4.0; however when I wear the same -4.0 glass in the evening (dusk time and night) I cant see properly. The images are blurred. I have to change to -5.0 glass. However the same situation does not happen in my right eye which has a constant -5.0 power in both daylight and night. I have observed this for many years (15+).

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NIGHT MYOPIA - is a tendency for eyes to become near-sighted in dim illumination. There are multiple theories explaining the phenomenon, most of them are related to accommodation or chromatic shift in scotopic light conditions when to light is focused before the fovea thus worsening the existing myopia.


In dark adaptation the eye becomes more sensitive to shorter wave lengths (Purkinje shift- In the light-adapted eye the region of maximal brightness is in the yellow; in the dark adapted eye, the region of maximal brightness is in the green),and visual acuity depends on parafoveal blue cones. Shorter wavelengths come into focus in front of the retina, and this chromatic aberration accounts for some of the relative myopia that a normal eye experiences at night; much of the remainder is due to an increase in accommodative tone in the dark. It is also called dark focus of accommodation, or tonic accommodation, or resting state of accommodation. Consequently, a measurement of the refraction (i.e. the determination of appropriate optical glasses to obtain good vision on these distant objects) gives the result that the eye is myopic. Night myopia can reach values of up to about - 4.0 D. Night myopia decreases with age31 and therefore it can be of special importance for young drivers at night. In one experiment with people aged 16 to 25 years, 38% had night myopia of - 0.75 D or more, and 4% had – 2.50 D or more. Cause of Night Myopia:

Spherical aberration (The optical effect of the larger pupil decreases the depth of focus) Chromatic aberration. Parkinje shift Ciliary spasm At low light levels there is a change of the biochemistry of imaging on the retina: in bright light the receptors in the retina are cones with higher image acuity, and imaging of colors, whereas in low light the receptors in the retina are rods with reduced image acuity, and imaging of black and white only. The transition between both states, i.e. to get maximum sensitivity when moving from the bright to the dark, doesn't happen immediately, but takes some minutes33. This is a problem e.g. when driving a car in sunlight and entering a tunnel. The low light imaging by rods in the retina can be further reduced by a lack of vitamin A and zinc Symptoms: 1. Blur vision only in low luminance. 2. Feeling discomfort while maintaining fixation in low illumination level. 3. Difficulty in night driving. 4. Hallows around light.

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While this link may answer the question, it is better to include the essential parts of the answer here and provide the link for reference. Link-only answers can become invalid if the linked page changes. – Satwik Pasani Mar 2 '14 at 17:10
@SatwikPasani thanks for your constructive comment. The answer changed appropriately. – Ilan Mar 2 '14 at 18:45

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