I pronounce ophthalmology with an F, consistent with my pronunciation of other PHTH words, but I’ve since thought of an exception (in my case).
Phthalo green is a dark slightly bluish green artist’s paint colour. (Wikipedia tells me that the pigment is Phthalocyanine Green G.) I pronounce it as ‘thalo’ because that’s the way my art teacher pronounced it. (My younger daughter, on the other hand says ‘phalo’, because she has trouble with TH.)
There’s a surprising number of PHTH words out there once you start thinking about them.