The rule is applied across for board for all kinds of goods, not just lenses. I've bought model kits, car parts, from USA, and Hong Kong, and as long as the declared amount + shipping is more than S$400 in a single parcel/package, GST has to be paid.
Of course, you can get the sender to "under-declare" the value of the goods. Not advisable if you intend to buy insurance for the shipping, as any payout *touchwood* will be based on the under-declared value.
I've not bought/received gifts above S$400, so can't comment whether will it be taxed or not.
A friend of mine got several special accessories for his Harley, directly from German manufacturer. Price was always stated properly, excluding German taxes. (Saving 19% VAT does make a difference!) No GST was claimed, he got the stuff nicely delivered to his hands. Seems there are differences in the way certain goods are handled.
I can't comment on Singapore customs authority, but FYI, Adorama does not ever ship items marked as gifts. If you wanted to go this route you'd need to have it shipped to a friend or relative in the US and ask them to forward it on to you.
NB if you ship within NY or NJ, you'd be liable for US state taxes.
As long as item above 400> you need to pay GST, PERIOD.
@Octarine, maybe your friend got items <400$? If above 400$> and freight company didn't charge him then they have to pay from their own pocket.
For TS, i received my lens on Saturday (B&H) value > 400$ and paid GST to UPS delivery man.