It's not even remotely close. It's their nose. It's their #1 defense mechanism.

I gotta go with smell.Then hearing.Theyre visions not all that good, Ive had em within yards of me out in fairly open ground, a lot of times they see you, know somethings wrong, but they wont run.If I sit and dont move at all while theyre bolwing, snorting, bobbing thier heads, and stamping thier feet Id say 35 maybe 40% of the time theyll calm back down and simply walk off or continue feeding.

If they see you and circle you untill they can smell you its game over every time.Its like they dont trust thier own vision at times and need thier noses to confirm what theyre seeing.Ive probably had a couple hundred deer within bowrange from the ground over the last 10 years and if your halfway concealed and not moving you can fool thier eyes pretty frequently.

Cigarette smoke is a funny thing as well.I smoke and its been my experience you can almost use it as a cover scent itself.They really dont pay all that much attention to it.A lot of places deer come into contact with smoke smells almost on a daily basis.Burning wood, burning garbage, car exhaust, people burning off weed patches, its a common smell really.Not recomending it as a cover scent or anything buteven some Indians even used to smoke thier clothes and stuff before going on a hunt.Deer wont neccassarily freak out over every smell they run across.

With their hearing or vision if they aren't sure about something they will attempt to use another sense to confirm the danger,almost always they are looking to confirm with their nose whether or not they need to flee.When they get a snoot full of you they are out of there,no need for confirmation.It is their sense of smell,the others not even being close.
It would probably depress the heck out of all of us to know the number of big bucks that have slinked away with our odor intheir nostrils,with us never having seen a glimpse of them.

There smell is the best I think

Concerning the deer's sight, they are extremely good at detecting movement, but not so good at making out and object. That's why I think all the debate about different camos is pretty much BS. I've had deer look straight at me many times when they heard a sound, but couldn't make out what I was. If you are on the ground, though, and they spot your movement, they will freeze on you and you are usually busted.

However, if a deer smells you, it doesn't need to see or hear you and it knows which direction the smell came from most of the time.

I'm going with smell....

Yea, I knew it was smell. The poll results are a landslide.

A deers nose is it's best defense! All their sences are good, but the nose has it! Next I'd have to say is their "sixth sence!"
In places that they are used to humans, sometimes you can do just about anything without spooking them, except to click a safety off, or draw back a bow!

Amen to that!

It has always been my understanding that deer see about as well as we do.
They also pick up motion much better because that's what they have been doing their entire life in order to keep surviving.

A deer can hear about 9 times better than we can. That's because a deer's ears are about 9 times larger than ours, plusthey have that ability to turn their ears to focus on suspected noise. They also completely understand what noises are a threat.

A deer can see you and still not know what you are. They can hear you and still be confused, but you can NEVER confuse a deer's nose if he smells a hunter. You don't want to know how many deer you have never seen because a deer has picked up your scent and left the area before you even knew he was there.

About their sense of smell....I believe one thing...it is uncomprehensible to humans to even try to understand how good a deer can smell. They can smell 100-150 times better than a dog.

Ask yourself this...Why does every hunter say hunt the wind? Never hunt with the wind at you back blowing in the direction you are hunting. If you hunt tht way, you might as well stay at home and watch Sports Center.