Three linked settings
The exposure triangle is a way to think about three linked settings: aperture, shutter speed, and film speed, usually marked ISO. A meter suggests combinations that give similar brightness, but each setting changes something else in the picture. With film, ISO is mostly chosen when you load a roll; unlike aperture and shutter speed, it is not a control you freely alter from frame to frame.
Treat one stop as a doubling or halving of light. If a reading is f/8 at 1/125 second, opening to f/5.6 gives one stop more light; changing instead to 1/60 second gives about the same increase. Close the aperture to f/11 or use 1/250 second, and exposure falls by a stop. These paired changes are the triangle in practice.
Aperture and motion
Aperture describes the opening inside the lens. The smaller the f number, the wider the opening: f/2 lets in more light than f/8. A wide setting also makes the sharply focused area thinner, which can separate a face from a busy background in film portraits. At f/8, more of a nearby group may stay acceptably sharp, though focus distance matters too.
Shutter speed is the length of time the film receives light. A quick setting such as 1/500 second can hold a running dog more sharply; 1/30 second may blur its legs or show a small tremor from your hands. Choose based on movement as well as brightness. When you need a quick shutter speed in dim light, a wider aperture may preserve the exposure without making movement soft.
Film speed
Film speed describes how sensitive the emulsion is to light. Set the camera’s ISO dial to the number printed on the roll’s box if you want the exposure meter to read it as rated. A faster film speed can make a dim scene easier to photograph, but film is fixed for the whole roll; the dial changes the meter’s advice, not the emulsion already in the camera.
Faster films often show more visible grain structure, while a slower film may have a smoother appearance; the result also depends on exposure and processing. If you are photographing a scene with deep shadows, a little extra exposure can help retain detail, but too much light can wash out bright areas. Make changes deliberately rather than spinning the dial between pictures.
Use the reading
Start by setting the film speed, then use the meter’s reading as a starting point — your intended motion and focus decide which equivalent setting is useful. In steady daylight a reading might offer f/8 at 1/250 second; if you want a softer background, open to f/4 and raise the shutter speed to 1/1000 second to keep the same exposure. The numbers should change in matching stops.
Try a roll with one purpose in mind, such as keeping moving subjects sharp, and note the settings after each picture. If a camera that divides each exposure into two smaller pictures gives you 72 exposures per roll, the same triangle still applies to every shot. A pale stripe may point to light leaks rather than a poor exposure choice. The meter’s “correct” reading is only a useful beginning… compare the negatives and decide what you wanted the light to do.
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