Visible crew/equipment: At the beginning when R2-D2 is in the escape pod and C-3PO is arguing with him there is a shot of C-3PO from the side and you can see the reflection of the camera in his head.
Visible crew/equipment: Just after the Falcon takes off from Mos Eisley, there's a shot of the Falcon with Star Destroyers chasing. The next shot is of Han Solo alone in the cockpit. Watch carefully behind him in the doorway, and you'll see the shoulder of a man in a green shirt quickly move out of shot. In fact, this is Alec Guinness and the shot is an insert from the slightly later scene where Obi-Wan and Luke are in the cockpit. After Han tells them to strap in for their jump to light speed, this shot is seen again with the pair retreating down the ship's corridor. The earlier shot reuses the final frames of the later footage and the edge of Guinness' shoulder, his costume reflecting the green set lights, is visible. (01:01:05)
Visible crew/equipment: In the original widescreen version, during a long shot of a loading bay in the Death Star, you can see a crew member walk just into shot on one side of the screen, stop, look up, and back out again.
Visible crew/equipment: When the Falcon is in the Death Star hangar, a scanning crew is sent aboard to find passengers, one of the stormtroopers walks straight past the camera and its shadow is visible on his leg. (01:01:55)
Visible crew/equipment: In the original widescreen version, when Luke pulls his landspeeder up to the cantina and jumps out, you can see the microphone boom at the upper left half of the screen. This has been corrected in the DVD release.
Visible crew/equipment: You can see the shadow of the crew on the stormtroopers after Luke and Leia have passed over the Death Star's canyon (the shot where you see Obi-Wan).
Visible crew/equipment: After the heroes have escaped the trash compactor and we see them arguing outside of it. As they walk away and Han says "No reward is worth this," the shadow of the camera covers his whole face.
Visible crew/equipment: After the attack on the Death Star, when the pilots return to the base, the camera crew is visible on C-3PO's body.
Visible crew/equipment: When Luke and Leia are stuck on the bridge, just before the stormtroopers arrive on the opposite platform Luke remembers the cable and sees the thruster like things over the top of him and Leia, when we get a view of them a small shadow bobs up and down at the centre bottom of the screen.
Visible crew/equipment: When the Blockade Runner is being attacked, just before C-3PO says something like: 'Do you hear that?' to R2-D2, he turns his head and the camera crew is visible on his temple.
Visible crew/equipment: When Luke, Han, Leia, and Chewbacca are in the trash compactor, there is a shot of Chewbacca knocking on the door. If you look to the right side of him, you can see the reflection of the blue stage lights on the metal part of the wall.
Visible crew/equipment: When Leia and the others come back to the rebel base, she says "there's no time for sorrows." The camera is moving during this shot and its shadow is cast on the actors.
Answer: It is heavily implied that one who becomes a Force-Ghost achieves an untold level of power upon entering the state. Given that they have become a pure entity of the Force, it seems to back up the statement. Obi-Wan also becomes free to assist Luke in any case.
Darius Angel
In addition to this answer, I think Obi-wan also became powerful because he "let go." He did not feel a need to defeat Vader on this occasion - he was willing to surrender the fight and "move on" to a new state. Something that Vader might not currently understand.