The f-stop is indicative of the size of the hole that light enters the camera through. If you're to imagine a camera as an artificial eye, the f-stop tells you the size of the pupil. As a result, adjusting your camera's f-stop to correspond with a larger hole (or aperture) will make your photos brighter, a fact that is sometimes a godsend when a scene doesn't allow for slow shutter speeds or a tripod isn't immediately at hand. More than that, though, expanding a camera's aperture allows light from a greater variety of directions to enter its chamber. In other words, a relatively incoherent barrage of light rays is able to pass through the wider hole, mildly distorting whatever is not explicitly the focus of an image. In fact, this phenomenon is what allows photographers to take those photos where all but certain key objects fade into a sort of fuzzy blur. These photos would be referred to as having a shallow depth of field.
At this point, most of the next two days would normally be spent in the dim of the dark room, greeted at every turn by the sight of these enlargers. It was with these metal monstrosities that we turned our film negatives into enlarged prints.
The first thing to do was usually the creation of a contact sheet. Made by overlaying a sheet of photosensitive paper with the sleeve containing our negatives and exposing it to an enlarger's light, these were mostly used for checking the approximate focus and exposure of our photos. Exposure time (at f-8) tended towards 10-15 seconds.
With the speed easel (the small yellow platform) positioned such that it encompassed the image cast by the enlarger, we used a knob adjacent to the size-adjusting handle to fine-tune the picture's focus. This procedure was aided by the use of "grain focusers"—optical devices that magnified a reflected version of our picture so we could see how well the sand-like grains of pigment on our film were in focus. We also placed trash photo paper underneath the grain-focusers so minor discrepancies in height wouldn't disrupt the focus and changed the enlarger f-stop to 2.8 to achieve better detail.
When a print didn't come out with the contrast we hoped for, however, there were usually two ways to deal with it.