Yes, camera equipment is duty free, but it's not tax free. If you import something from the US, you'll pay provincial and federal sales tax on its list value when it crosses the border. Here in Quebec, the amount payable for a $300 lens would be about $40 in tax, plus a $5 "brokerage fee". They charge the tax on the Canadian dollar value, not the US dollar value, so you need to take that into account too. It wasn't so bad when the Canadian dollar was at or near par, but these days it makes a difference.
If importing, you also need to be careful how the lens is shipped. The United States post office service (USPS) only charges $5 brokerage fee, but some of the other carriers like UPS or Fedex will charge a lot more - up to $40 in some cases.
A few months ago, when the dollar was at par, it was well worth buying in the US, but these days the cross-border taxes and exchange rate loss has destroyed most if not all of the benefit.