FL is the earliest version of Victory series binos. The "FL" stands for fluorite lenses, which is a type of extra low dispersion HD glass.
HT is a newer series that shares much of the same optical design as the FL series binos, but uses what Schott calls their "HT" (high transmission) glass. Supposedly, this offers slightly improved light transmission vs the FL series.
SF is the newest series that has an "open bridge" design (like Swarovski EL series) vs. the standard continuous hinge design of the other two series. It also has greater field of view and supposedly has an "improved" focusing mechanism design in some way.
