It can be uncorrected refractive error
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I am Dr Dadapeer, an ophthalmologist and I answer health problems related to eye.
I reviewed your history. The complaint you have mentioned is commonly seen in clinical practice where person will not be aware of the reduced vision
in one eye as one eye compensates for the eye while we are seeing with both the eyes open.
This is usually noticed only when seeing by closing one eye as during eye examination.
The common problems are refractive errors retinal problems like retinal scars
The commonest cause is uncorrected refractive error. If the condition is not corrected it may make one eye dull called as lazy eye or ambylopia.
The other common problem is retinal scars usually because of infections seen in childhood like toxoplasma scars involving the macula...
Hence first thing to be done is visual assessment, refraction
examination of retina by a ophthalmologist.
The next treatment depends on the cause. If it is uncorrected refractive error the ttreatment available is spectacles, contact lenses or laser
refractive surgeries. If it has caused ambylopia not many options are available as you are now 26 years but still refractive error correction can improve vision up to some extent.
The retinal problems need to evaluated and if active require treatment.
Hence you need to get complete examination to find out the cause by a ophthalmologist. If you can get back to me with the results of vision assessment, refraction and retinal examination I can guide you further.
Hole I have answered your question and the information is helpful to you.