The primary mission of the crew was to fly the LM within fifty thousand feet of the Moon’s surface in order to inspect landing site 2 on the Sea of Tranquility, tentatively selected as the landing spot for Apollo 11.
This photograph was taken from the LM Snoopy with the 80mm lens looking forward (west) toward the lunar horizon over the 3.8 km-wide Crater Censorinus in the southeast Sea of Tranquility during its descent approach to the future Apollo 11 landing site. (To the northeast of Crater Censorinus is the crater Maskelyne.) The LM thruster is in the left foreground. The very low altitude of the spacecraft over the Moon is clearly noticeable.
“The flight of Apollo 10 permitted man to observe directly features on the lunar surface from an altitude of 50 000 ft, an altitude within the range of high performance aircraft on Earth.”
—Apollo 10 crew observations (NASA SP-232, pg. 1)
From the mission transcript during the LM’s descent approach to the Sea of Tranquility:
100:38:31 Young (Charlie Brown):
Boy! Are they down there among them!
100:38:34 Duke (Mission Control):
Roger. Bet it looks like they’re really hauling the mail.
100:38:39 Cernan (Snoopy):
Yes. We’re doing it. Surprisingly enough, Charlie, it really doesn’t look like we’re moving too fast down here. It’s a very nice pleasant pace. [...]
100:44:57 Cernan (Snoopy):
Hey, I tell you, we are low! We are close, babe! This is, like, it! And it really looks pretty smooth down there, surprisingly enough. [...]
100:45:05 Cernan (Snoopy):
OK. I’ve got Maskelyne out here off my right side. We are coming up on Boot Hill which is very easy to distinguish, and Maskelyne. And I see the craters that are going to lead us right into the - right into the landing site. [...]
100:45:17 Cernan (Snoopy):
We’ve got Duke Island on the left, just past Boot Hill and we are coming up - I’ve got Wash Basin just off my right arm - very easily distinguishable.
100:45:33 Cernan (Snoopy):
Tom, ought to have Sidewinder Rille coming up on the left - Tom, give me that [the Hasselblad]...