If you define "normal" as "average", then no, a -8.00 in a six month old is not normal. It is very unusual. I think the first thing to do is obtain a second opinion from a doctor who sees infants who can do an exam to verify the result. This is not to say anything was the matter with the first exam, but second opinions are always nice when results are unusual.
IF the results are true, then eliminating possible eye health issues as a cause is the most important next step. Unusual results may be due to unusual eye conditions. All sorts of issues could be involved that are too numerous to just start listing.
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