Monroville said:Uhh, Darth was over Tatooine in the beginning of the movie! Even if he couldn't force sense either Luke nor Obi-Wan due to distance (maybe his long distance Force bill wasn't paid yet or he didn't have any "bars"), he WAS able to force sense Obi on the Death Star (being that he was on the MF just a few yards away). Granted, he may not have recognized it until later, but if he could pick up Obi he could pick up Luke, especially since EMPIRE and JEDI amped the Force sense thing to 100,000 mile+ distances!
Uh, I never saw it like that at all. I saw it as if you know about someone, you can sense their presence in the Force. Since Vader didn't know about his kids, he'd never be able to sense them as individuals, just a couple of lifeforms. It's similar to how Obi-Wan felt the sudden disturbance that was Alderaan getting blown to smithereens and killing all the people (it was probably more the people he felt being killed than the planet being destroyed). He likely felt the sudden death of all those people, but not necessarily anyone in particular unless he knew them.
As far as sensing Obi-Wan, he didn't sense him specifically. He sensed a presence. A strong Force presence. It's quite possible that between the presence of his son (even though he didn't know it was his son) and the presence of Obi-Wan, he sensed something strange. It was similar to Obi-Wan, but not quite right. Obviously once he (Vader) found out about Leia's escape, that was confirmation enough that Obi-Wan was there. And if we assume that Luke wasn't Vader's son during ANH, then it's obvious that Vader simply felt Obi-Wan, but didn't remember it. It had been 20 years since he'd seen his Master, so there's no reason to believe he might forget what his Master's presence felt like.
Once Luke destroyed the Death Star and his name became well known, Vader was then able to reach out with the Force and sense his presence. He could've felt Leia's presence as well, but there was no need. In Jedi, Vader only found out about Leia because Luke wasn't able to bury his thoughts deep enough.
So no, I don't see distance as a factor at all. Vader, I'm sure, could probably sense Obi-Wan if he wanted to. There was no need to though. Obi-Wan, at least as far as Vader was concerned, wasn't a threat anymore even if he was still alive. As for Yoda, according to some of the EU, he was on a planet with a heavy Dark Side presence, which masked his visibility in the Force. But even if he weren't, why worry about some ancient Jedi Master that's hiding? Vader's only concern, once he found out about Luke, was in making sure Luke didn't become a trained Jedi. That's what brought down the Empire anyway.