Supposedly less than 1% of Canadians travelling to the USA are turned back at the border. But it seems that every day there's another horror story in the news which makes it appear that a lot of people are being stopped.
Here's a CBC story
that puts things in perspective.
In theory, you don't need a visa. You're not going for work or for an extended stay. I'm not aware of anyone needing a visa simply to attend to a conference or convention in the USA. But most folks aren't carrying an armful of camera gear.
You're simply going to a photography workshop which teaches you how to use your cameras so you have to bring your camera gear.
Most border folks don't know about carnets although many TV and film crews use them. Canadian border people are known to ask for a carnet from many commercial and news photographers coming into Canada (e.g. for the Vancouver Olympics).
In theory, you need a BSF407-1
when bringing your gear back into Canada. They're free and you get them before you leave Canada. I've been asked to show it only once or twice in twenty years. But it doesn't hurt to carry it.