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.
The lens aperture controls how much light per unit time reaches
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.
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
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
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 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
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.