i spend a month a year in bear country,two weeks in the spring,two in the fall.this is in the wilderness canoe area in northern minnesota.i have two sprayers,both are police style because the ones marked for bear use are too expensive and IMO not any better.one is a large foggier and it's carried in a day bag and them set out near the cooking area in camp and then in the tent at night.the small belt size one also is carried in the day bag inside the canoe but on "iffy" portages i put it my pocket."iffy" is hard to nail down but you get an idea of places where you might run into a bear after a while.
the only bears i have ever seen in 25 years were all running away.
i stay away from busy areas where there are well used campsites and try and get off in more remote places,i call them hidden camps,the fire places sometimes have seedlings growing in them.
now about how to use one.i assume that the black bears i might run into are going to be camp bears,some places you have no choice but to camp in a used area.so the bear is not going to come charging out at you,even on the portage i don't think a ambush is at all likely.so you will have a chance to prepare your shot.
first put you arm across your nose and mouth,pull down you hat if you have one to cover more of you face and just before you shoot,if thats the right word,close one eye so if you get blow back one eye will still work.don't aim for the bears head,its too small rather fog the ground in front of it.
bears are like getting hit by lighting,it could,it can and it does, but so seldom where i canoe and camp it's not something i worry about but am ready for.i know it's another story out West.

We got fogged with OC, when it was first issued to us for work. Worked fine on us.

I sure wouldn't use it as an area prophylactic, or elect to spray it on myself--unless I were already in the bear's grip.

^^^^ This! Except in Yellowstone, where they ham it up by the side of the road hoping for (illegal) hand-outs. It might help that my dog is almost always with me. You would think that a 300# bear wouldn't be afraid of sweet old 85# shepherd mutt, but in 13 years that's been the case (either that or I need to change deodorant brands).

I still carry the spray, which kinda stinks because it IS expensive and has a 2-year shelf life. I wouldn't trust smaller canisters made for defense against human aggressors. Supposedly the larger cans have longer range. And as other bears might be attracted to the smell, you might need to defend against multiple targets. I figure if I'm going to spend $45 every two years and put up with the darn thing on my hip it better work if I need it.

Amazing how many animals dont want to mix it up with dogs.Why my mutts are the A#1 defensive item we own,and if your dog messes up a perp its an insurance problem usually,shoot em and you may face serious jail time,and the deterrent factor of mutts,again danged good.

The spray we have puts out a massive cloud,far different from the little key chain style things,whole different critter.I still take the bear spray camping,hey one time we faced a potential attacker/robber,scary,the bear spray coulda stopped him if he hadnt backed out.This was in a state camp ground,secluded (well only the 2 camps,his and buddy and ours at the time)

I was caught flatfooted outside motorhome,wife was prepared to shoot him from inside MH,thank God he decided not to go for it,that they were looking for an easy score and were idiots and not pro's (tweekers).Would have been a disaster,this happened at dusk at Whiskey Creek for you Calif folks.

2 small dogs,....now 2 BIG DOGS.ALWAYS.

Ive learned,danger lurks and you cant be sure what kind,be prepared.With multiple increasing force defensive layers.Bear spray isnt just for Bears if your life is on the line.