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yes basically what malcolm says ryan regarding astigmatism
astigmatism is the killer
if he has greater than 1 dioptre the image would start diminishing rapidly
any more than 1.5 dioptres you would need spectacles and a long eye relief eyepiece or the dioptrix
some people have much more astigmatism in one to the other so just use the better eye in such a case
if he has a spherical correction only it would not make any difference at all it is just a refocus
pat
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