241 a
Apollo 9 was the only mission with a spacewalk conducted from the LM. On the fourth day of the mission, McDivitt and Schweickart entered the LM Spider in preparation of the EVA.
Because of space sickness, Schweickart’s scheduled two hour EVA to simulate external transfer rescue techniques was scrubbed and then rescheduled when Schweickart felt better.
Schweickart floated out through the LM’s side hatch onto the front porch (feet first and face out instead of feet first and face down as astronauts would do on the Moon, for easier egress in the zero g environment) for a 37.5 minute EVA 140 miles over the Earth. With his PLLS (Portable Life Support System) on the back, he placed his feet into specialized gold-painted foot restraints affixed to the front porch dubbed the “golden slippers”. Scott took this picture as was himself performing a stand-up EVA in the open hatch of the CSM Gumdrop; observing, filming and photographing the EVA. McDivitt stayed inside the “Spider”.

From the mission transcript during the EVA:

073:07:30 Schweickart (PLSS): Do you have your camera on there, CMP (Command Module Pilot)?
073:07:32 Scott (Gumdrop): It’s ready.
073:07:34 Schweickart (PLSS): Okay. Proceeding on out. [...]
073:08:05 Schweickart (PLSS): Okay, in the golden Slippers.
073:08:12 Schweickart (PLSS): Hello, there.
073:08:15 Scott (Gumdrop): Hello, there. That looks comfortable.

241 b
A very famous photograph showing Scott taking photographs with the Hasselblad in the open hatch of the CM Gumdrop during his stand-up EVA. Schweickart took the photograph from the porch of the LM during his own EVA.

The first two-man EVA was performed by cosmonauts Khrunov and Yeliseyev, conducting a spacecraft transfer from Soyuz 5 to Soyuz 4 two months before in January 1969.

“I took this shot of Dave Scott taking a picture of me at the beginning of my EVA on Apollo 9,” said Schweickart. “It captures just a bit of the fantastic beauty of the Earth juxtaposed against the infinite black of space. In the foreground is that amazing combination of human and machine that is enabling us to emerge into the universe out of the womb of Earth” (Jacobs, p. 42).