My default reference aperture is f/5.6 because it tends to be where the lens sharpness peaks. I rarely stop down beyond this point but more often than not open up to f/4 and f/2.8 to reduce depth of field. The choice is dependent on my distance from the subject of my image. I find the closer I get to my subject the background detail can become a distraction. Using a wider aperture reduces the focal depth and helps to emphasize the subject. The rendering of the unfocused background is called Bokeh—whether it is soft and luminous, or contrasty and harsh—a characteristic of the lens optics. More importantly certain images demand the background to be out of focus regardless of the lens characteristics.
In close focal distances, a photograph taken with the aperture set to f/5.6 will have more background detail close to the focus point, while an aperture of f/2 the background will soften helping to isolate the subject from the background. Each aperture on the aperture ring represent a factor of 2 difference. Relative to an aperture of f/5.6, f/4 (open up) doubles the light captured and f/8 (stop down) reduces the light captured by half. I find f/2.8 and f/4 the apertures I use the most on my 50mm lens when I’m isolating the subject from the background. To get to those apertures is a balance of controlling the lighting conditions (Aperture/Shutter Speed) and the film speed (ISO). In general I need to be able to control 2-stops in my photography.
Modern digital cameras offer shutter speeds beyond 1/1000 to 1/4000 sec. (2-stops) which makes it fairly easy to open up the lens to f/2.8 under normal lighting. However, my old Leica M3 film camera is limited to 1/1000 sec. In sunlight with film ISO 100 a normal exposure can be f/5.6 at 1/1000 sec. To get to f/2.8 I need to use a filter with a 4X factor which in optics is the exponential reference (22=4) for 2 aperture stops. Likewise a filter factor of 8 is 3-stops (23=8). A Multi-Coated Neutral Density 4X filter is an important part of my kit.
Up to now I have been talking about working in sunlight. Once I move into the shade I can begin to use slower shutter speeds to open up the lens. As the light diminishes I either can use a tripod (not always practical) or select a faster film such as Tri-X ISO 400, which is two stops faster than ISO 100. Ultimately the choice for me is do I load my camera with ISO 100 or 400 film.