You shouldn't have to visit an ophthalmologist too often. So perhaps we can excuse you when you spell the word incorrectly.
Ophthalmologist picture from Shutterstock
I was reminded of the difficulties inherent in ophthalmology when Kotaku editor Mark Serrels mentioned in an online chat that he was going to visit an "optomologist". Since you normally visit an ophthalmologist (who specialises in eye diseases and anatomy) after visiting an optometrist (who specialises in assessing your vision), it's an easy mistake to make.
As well, these two are the only vaguely common words in English beginning with the sequence ophth- (others with the same root include ophthalm, ophthalmia, ophthalmic, ophthalmitis, ophthalmoscope and ophthalmoscopy, but those are all rarities). Under the circumstances, it's no wonder people get it wrong.
If you're putting a note in your diary, you can put "eye guy" and save yourself the trouble. If you're writing in a professional setting, this is one time when a spell checker is definitely your best friend. Accuracy matters.
Lifehacker's Mind Your Language column offers bossy advice on improving your writing.