Sunday, September 9, 2012

Exposure Fundamentals: ISO Values and Aperture Settings

Margaret Bourke-White (1904 - 1971) on the Chrysler Building, New York.
Photographer:  Oscar Graubner, 1934

Photographs aren’t taken, they are made. I say this because every photo requires some degree of planning, selection of proper tools, and a craftsman’s (or woman’s) execution of the project.  Now occasionally every photographer may need to take a “grab shot”, one without much premeditation.  But even these require some small bit of planning, and the conscious effort to capture a specific small slice of time.

Exposure is at the very heart of the entire process. It is the correct assessment of the amount of existing light, followed by the decision on how we want the camera to render the image. This photographic produce is influenced by our initial selection of three exposure settings: ISO, Lens Aperture and the camera's Shutter Speed. All three factors are related, as we shall see.

ISO Values

ISO used to be called ASA, and was the sensitivity rating for  film. A high ISO value meant that the film was more sensitive to light than a film with a low ISO rating. This roll of Kodachrome has an ISO of 64. Low ASA values were associated with fine detail, while high values tended to add “grain”, irregularities at the image edges that reduced the crispness of the image. Executive summery? Low ASA/ISO, fine detail. High ASA/ISO values degrade detail. Photo: Photographyblog

When shooting, I have four "go to" ISO settings:
  • ISO 200 for static subjects, or when used with powerful (or multiple) electronic flashes.
  • ISO 400 for general shooting outdoors under natural light, or with a Zumbrella used indoors.
  • ISO 800 for indoor bounce flash, or a Zumbrella outdoors.
  • ISO 1600 for available light shots made indoors, or long-range bounce flash.
Aperture Settings
Aperture values are mathematical relationships based on the amount of light that can pass through a lens. The aperture is a small “hole” in the lens that opens and closes much like the pupil in the human eye dilates and constricts. As our pupils constrict in response to bright light, so must our lens apertures close (or stop down) to prevent overexposure.

When used to describe a lens, the aperture value is preceded by the letter “F”, a hold-over from the days when the F value (or F-Stop) represented the ratio between the focal length of the lens and the actual diameter of the aperture.

In this series of photographs, you can see the the largest aperture setting on this lens is 0.95 (lower right), which is slightly less that a value of 1.0. At the upper left, a small aperture (F 16).

While we seldom see lenses marked F 0.95 or  F 1.0, 1.4 is relatively common in high performance (and high priced!) fixed focal length lenses. Photo: Nuyoka & Co.

Now this is important.
  • Each F value represents a ratio between the focal length of the lens and the lens aperture.
  • Low F-Stop Values = High light throughput and shallow depth of field.
  • High F-Stop Values = Low light throughput and greater depth of field.

Here we can see the traditional sequence of aperture values. These values were usually printed on the aperture control ring of manually adjusted lenses of the twentieth century. The short list goes as follows:

1     1.4   2     2.8   4     5.6   8     11    16    22    32    45    64

The aperture ring of this Nikon AI lens has the numbers listed in the reverse sequence. While this particular lens has a maximum aperture (widest setting) of 1.4, many lenses will have an intermediate value such as 1.8 or 3.5. These would fall somewhere between the values I have listed.

Each value on the F-Stop scale represents and increase or decrease in the light transmission by a factor or ½,or 2, depending on whether you go to a smaller aperture (larger F-Number) or a larger aperture (smaller value F-Number).

Current production lens have eliminated the aperture ring altogether. Aperture is set using a command dial on the camera's body. Photo: Nikon Rumors.

Next time: Shutter Speed and the Sunny Sixteen Rule.