Digital Photography vs Traditional Photography
film speed - shutter - f-stops

Now let''s return to traditional photography and how that technology works. If you remember that is where we began this discussion. So what is the traditional way to expose a photograph?



ISO - ( International Organization for Standardization)

ASA - (American Standard Association)

DIN - ( Deutsche Industrie Normen )

Table 1.

To capture the proverbial "perfect" image, traditional photography relies on three things:

1.) Film Characteristics

2.) Shutter Speed

3.) Aperture (f-stop) (aka .. light intensity)


Film is rated according to it sensitivity to light. Faster film is more sensitive to light, and slower film is less sensitive to light. There are two film speed scales in use today, and they are illustrated in table 1.

The ISO (ASA) system is based on the concept of film speed doubling or halving. Each speed is either twice as fast as the previous one, or exactly half as fast as the next one. It is an arithmetic system.

The DIN system is an older German system that is logarithmic. Both the ASA and DIN systems are giving way to the ISO system today. According to the acronym ISO should be IOS, but they adopted ISO based on the Greek stem "isos" meaning equal.

Film speed is a function of light sensitivity, but there are two other important characteristics also based on the speed and film base. Resolution (I thought we were finished with that!) is related to film speed. As a general rule, faster films use more granular chemistry and thus have lower resolving power (resolution) than slower films.

Film shows an non-linear response to the various colors of light. Hence film can be balanced to produce more accurate colors based on the type of light to which it is exposed. You can purchase Daylight film or Tungsten film based on the type of lighting you will use to take your photographs.

Film is purchased based on speed (speed is directly related to resolution), and color sensitivity.



f / 1.4
f / 2.0
f / 2.8
f / 4.0
f / 5.6
f / 8
f / 11
f / 16
f / 22

Table 2.a

Table 2.b


There are two mechanical functions that combine to determine the correct exposure of film. Cameras use a shutter mechanism to control the duration of exposure, and an iris to control the intensity of the exposure.

Most 35 mm SLR cameras use a focal plane shutter to control the time of exposure. A focal plane shutter is simply a curtain that crosses in front of the film for a fixed duration of time. Like most things in traditional photography, shutter exposure times are based on a doubling scale. Each time is one half of the next. Typically they are expressed in fractions of a second, but may exceed one second in low light situations.

Table 2.a shows some typical exposure times. From a short exposure of about 1/1000 of a second (0.001 seconds) to a long exposure of 1 second. Each value in the series is roughly 1/2 of the next or twice the previous.

How a Focal Plane Shutter Works

Focal plane shutters work like stage curtains only they can operate independently. When the curtains are closed they are to one side of the film and overlapping. First one curtain begins to open exposing the film, then the second one moves at some time behind the first cutting of the exposure.

If exposure time permits then the two curtains making up a focal plane shutter move independently of each other. One curtain opens starting the exposure of the entire piece of film, and then the second one closes stopping the exposure. This is the method that must be used when synchronising flash. The entire film must be exposed by the flash burst all at the same time.

Shutter closed
first curtain
Completely open
second curtain
Shutter closed

When exposure time must be faster, then both curtains move simultaneously creating a moving slit across the film surface.

Shutter closed
first curtain
Both Curtain move
Both Curtain move
Both Curtain move
Shutter closed

The second column (Table 2.b) shows a series of typical f-stops. An iris either opens or closes to control the intensity of light reaching the film. The f - number is the ratio between the size of the aperture and the focal length of the lens. Since lenses are round and the iris is round, the area covered is a function of pi x radius squared. The values in the table each represent either twice the size or half the size of the iris. Hence each represent either two times the light intensity transmitted or half the light intensity transmitted.

IRIS Mechanism or Diaphram
used to set f-stop

IRIS between lenses
f / 5.6 more light
f /16 less light