Exposure settings

With 'passive' photography (no artificial lighting used) such as weather and landscape photography, there are three variables that determine how much light reaches the film (or digital sensor) to yield a properly exposed photo. These are the lens aperture, shutter speed, and film or sensor sensitivity.

1. Lens aperture

   The lens aperture controls how much light per unit time reaches the film/sensor.

   A camera lens consists of several groups of glass lenses, each group containing one or more elements. There is a group at the front of the lens, where the light comes in, and a group at the back, which mounts to the camera. Between these, all light rays travel parallel to eachother, and this is the location of the aperture (diaphragm) of the lens.

   The aperture measures the diameter of this part of the lens, where light travels parallel, and the lens' aperture is typically indicated as a so-called f/ratio. For example, a 28mm lens with a maximal aperture of f/2.8 has an aperture of 10mm (28mm focal length divided by 2.8). The bigger the aperture, the lower the f/ratio number, and the more light the lens will 'catch'. For many types of weather photography the aperture should be as large as possible (f/ratio number as small as possible), because you usually want as much light as possible.

   Most lenses have adjustable aperture, commonly from f/2.8 to f/22 or so. When you turn the aperture ring on a lens while looking through it from its front, you can actually see the aperture as a set of blades being adjusted. Most lenses allow you to change the aperture by half stops, to have finer control of exposure. A stop difference in exposure is usually rather coarse.

   f/ratio numbers on a lens are given in factors of 1.4, the square root of 2. E.g. a typical lens may have adjustable apertures f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22. Usually the f/ part is omitted, and just the f/ratio numbers are given. Each step is called one stop, and is a factor of 2 difference in light throughput. (This is because the aperture setting is a diameter, and the light throughput is proportional to the aperture surface area which is proportional to the diameter squared.)

   Another thing that the f/ratio determines is the depth of field, or depth of focus. At smaller apertures (higher f/ratios) the lens acts more like a pinhole, and both objects relatively close and far away from the lens will appear sharp. At larger apertures, the depth of focus decreases, and everything except objects within a narrow range of distance will be unsharp.

2. Shutter speed

   The shutter speed determines how long a certain amount of light will reach the film/sensor.

   With daytime photography, shutter speeds are usually very fast, of the order of 1/1000 second (1 millisecond). Shutter speeds on older cameras differ approximately by factors of two, starting maybe from 1 second down to 1/1000 second or faster: 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500 and 1/1000 second. The 1/ part is usually omitted, with the understanding that the number corresponds to the fraction of a second.

   Shutter speeds on older manual cameras can be adjusted (usually) by a dial or wheel on top or front of the camera with these numbers printed on it. Modern automatic cameras usually don't have such a shutter dial, and the shutter speed may be adjustable electronically. The newer digital cameras and automatic film cameras will also have shutter speeds much longer than 1 second, such as 2, 4, 8, 16 and 30 seconds.

   In addition to a range of shutter speeds, most SLR (single-lens reflex) cameras have a B shutter speed, which stands for bulb, a manual shutter control. In B mode, the shutter will stay open as long as you keep the shutter release depressed. This is only practical (but extremely useful) for long exposure times, longer than about one second.

   All shutter speeds that differ by a factor of two differ by one stop. This means that an exposure of 1/500 second at f/8 gives the same amount of integrated light as an exposure of 1/250 second (double the amount of time) at f/11 (half the amount of light per unit time).

3. Film or sensor sensitivity

   The film sensitivity determines how quickly a film or sensor will respond to a certain amount of light to become properly exposed.

   With film, the most common sensitivities are 100 and 200 ISO (also called ASA). The lower the ISO number, the less sensitive a film is. Most slide and print films have sensitivities ranging from 50 to 3200 ISO in factors of 2: 50, 100, 200, 400, 800, 1600 and 3200 ISO. All these factors correspond to one stop difference, so an exposure of 1/500 second at f/8 on 100 ISO film should yield the same amount of exposure as an exposure of 1/500 second at f/11 on 200 ISO film, or 1/250 second at f/8 on 50 ISO film.¹ But 'slower' films have finer resolution than 'faster' films, thus low ISO films are desired unless the amount of available light is very limited.

   Digital sensors traditionally have similar sensitivities, also given in ISO numbers such as 100, 200 and so forth. This is especially helpful for those of us that were already used to film. The major difference with digital sensors is that they don't suffer from reciprocity errors, so at long exposures the sensitivities may seem much different (more sensitive) than expected based on previous experience with film.

   Another designation for film sensitivies is the DIN number (from Deutsche Industrie Norm). A 100 ISO film corresponds to DIN 21, 200 ISO to DIN 24, 400 ISO to DIN 27 and so on. Thus, a difference of three in DIN number corresponds to one stop difference in sensitivity. The DIN number is usually printed on film next to the ISO number.

Trade-off in exposure settings

It would be wonderful if all three exposure variables could be adjusted freely without affecting other things such as the quality of the photo. But everything comes with a price.

• Large apertures (small f/ratio numbers) are subject to visible lens errors on the photo, such as aberration, coma and unsharpness. Some lenses are worse than others at full aperture, but as a rule you should use a lens at apertures of 2 or 3 stops smaller than wide open, if the available light permits.

• Small apertures (large f/ratio numbers) will cause unsharpness in the photo as well, due to diffraction of light. You should avoid the smallest apertures of a lens if possible.

• Also, as noted above, the aperture determines the depth of focus. With weather photography this is usually not an issue since most subjects are at infinity, so the depth of focus is irrelevant. But for macro photography, for example, it is very important.

• Long exposure times require a tripod, and will usually blur the photo if you photograph moving subjects. With film, long exposure times will also suffer from reciprocity errors. Short exposure times however are not always possible because the light may be too low.

• Low film sensitivities (low ISO number) require longer exposures, which is not always possible, and high film sensitivities suffer from coarse film grain (or, with digital sensors, from thermal noise).

It depends entirely on the type of phenomenon and the amount of light available which of the three exposure variables will be most important to you. E.g. for photos of lightning, the aperture and film speed are more important than the exposure time (in fact, during the night the exposure time is determined by the lightning flash itself and not by you), while exposure time is more important than film speed if you photograph something transient at low light such as Aurora or noctilucent clouds, and when macro-photographing snow crystals the aperture is the most important variable. Deciding exposure settings requires some practice and experience. Several combinations of settings will yield the same exposure, but with different trade-off in quality.

I say 'should' here because in practice, the sensitivity of film also depends on exposure time, film becoming less sensitive over time. But this only starts to matter for longer exposure times over 1 second or so (the so-called reciprocity error).