Just a quick post showing the effect of aperture (F/stop) on the look of a portrait by changing the depth of field.
click on the image for a larger version
The mannequin was photographed with a Canon 5D Mark II with the 85mm f/1.8 lens. Focus was on the right eye (camera left). The background is about 32 inches behind the mannequin head. Light is from the modeling lamps in two strobe heads. One is in a Photek 46" Softlighter II (model SL-5000) on camera right and a 2x2 square Interfit softbox on the opposite side as a kicker.
I used the modeling lamps because I would not be able to power down the strobes enough to get the large apertures. This is not practical for normal portraiture because the exposure times at ISO 100 was 10-seconds for the f/22 shot. Even the f/1.8 version was slow at around 1/15 second. You would need to bump up the ISO or use brighter lights to get usable shutter speeds for portraits. It is all about trade-offs. I could get camera strobes to work at f/1.8, but would lose the modeling lamp and possibly have to deal with large dilated pupils. If I switched to studio strobes I could probably get the aperture down to around f/5.6, maybe f/4. To get to f/2 or wider I'd have to use neutral density filters. With a 1000 watt tungsten light I'd have to deal with heat and the comfort of the model (luckily mannequins don't complain). I could get the wide apertures, but would not be able to stop down all that much without running into slow shutter speeds. I don't like to go below 1/60 when doing head shots with continuous lights. A 150-watt fluroescent lamp would give a similar amount of light as the tungsten lamp without the heat, but the bulb is too large to fit into the Softlighter, so I'd have to change to a different modifier for the main light.
So, there's a sampler of the thought process around selecting which lights to use for a project.
You may also ask if using a longer lens would give more control over the depth of field. The answer to that is no. To maintain the head size on the sensor with a longer lens I would have to move back and the depth of field would be the same at the same aperture. The difference would be a narrower angle of view, so we would see less of the background (the pattern would be larger, what is sometimes erroneously called telephoto compression). But the DOF would remain the same. If I left the camera at the same distance from the subject yes there would be less depth of field. But I'd be cutting off the chin and forehead of the subject.