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.