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Photography Question 
Jacqueline Rechter
 

ISO


ISO


why are there different iso ratings for films


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3/30/2008 4:15:15 AM

 
Alan N. Marcus   Hi Jacqueline,

Films are carefully prepared by blending different chemicals together. The process is much like making soup only the broth used is gelatin. Finally, the chemical laden gelatin (emulsion) will be coated onto clear plastic sheets or rolls of film (sometimes sheets of glass).

Film makers follow a recipe. We make color films both negative and positive; we make black & white films; we make films for medical imaging like X-ray. We make films for the movies so you can see Disney pictures. We make films that will look through microscopes and telescopes. In other words there is a myriad of film applications.

Additionally, even when we follow the same recipe, ever so carefully, each batch will have a slightly different speed (ISO). However, most times the differences are intentional. There is a blend to make high speed (high ISO) films that will be used for picture taking in feeble light. There is a blend for low speed (low ISO) for films to be used under bright light conditions. As a general rule, low speed films are better, giving more accurate images. Further, low speed films tolerate being blown-up better. Photographers who wish to make large display prints choose slow films. Sports photography generally requires the camera to be set to a high shutter speed to freeze action. Fast shutter speeds reduce the time allotted for the exposure to take place. Thus a sports photographer would choose a fast film as would photographers who shoot picture at night or in a dimly lighted theater. In astronomy exposures are often long, sometimes hours long.

Just as your mom’s kitchen cooks up different and delicious dishes, film makers cook up various film types.

Alan Marcus (marginal technical gobbledygook)
ammarcus@earthlink.net


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3/30/2008 7:03:35 AM

 
Christopher A. Walrath
BetterPhoto Member Since: 4/25/2006
  There are different rating numbers and also different numeric ratings systems. Three ratings systems come to the forefront. ISO (International Standards Organization) employs a geometric (increases made by multplication or decreases made by division) series of numbers based on the constant that was the cube root of 2 (1.26). ISO ratings in third stop increments are as follows . . . 25, 32, 40, 50, 64, 80, 100, 125, 150, 200, 250, 320, 400, 500, 640, 800 and so on. A film with an ISO rating of 00 is twice as sensitive to light as a film with an ISO rating of 200. You may also have seen the term ASA100 in regards to film speed. ASA (American Standards Association) is the progenetor of the ANSI (American Standards Institute) and used the same geometric series of film speed indicators. ISO=ASA. Film producers also provided DIN numbers. DIN stands for Deutsch Industrie Numbers and are used throughout Europe on German and older cameras though not as prevalently as before hence DIN is virtually obsolete. It employed a numeric (series that increases by addition or decreases by subtraction) series of numbers that reflected a film's sensitivity to light. The series went along the lines of 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33 and so on. A film with a DIN rating of 20 was half as sensitive to light as a film with a DIN rating of 23.
Generally you slow speed films are ISO 50 and lower, good for high light source still life and highly lighted portraiture. ISO 100 to ISO 200 are mid speed films good for portraiture and scenic ladscapes. ISO 400 to 800 are good for some motion and lower light photography such as golden hour photography. ISO 1600 and faster is great for night photography and for sports and action as it requires much lower amounts of exposure.

Hope this helps. (Hope I made you proud, Alan)

Thanks
Chris


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3/31/2008 4:32:12 PM

 
Christopher A. Walrath
BetterPhoto Member Since: 4/25/2006
  "A film with an ISO rating of 00 is twice as sensitive to light as a film with an ISO rating of 200." Sorry, 4 key sticks. ISO400 is twice as sensitive to light as ISO200. Oopsies.


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3/31/2008 4:34:12 PM

 
Alan N. Marcus   I am extremely proud of you Chris! Your narrative showed great insight and was explained well. You deserve a tip of the hat.

Allow me to add a little boring history. This emulsion rating thing started in England in 1880 when Messrs. Hurter and Driffield started the science we now call sensitometry; they investigated the characteristics of emulsions. They patented an exposure calculator revolving around the temperature of air in sunlight. The resulting value was used to calculate exposures. In 1932 Western Electric marketed an electric light meter called “Weston” and film speed rating also called Weston. General Electric followed suit. Now we had the Weston and the GE film speed method. Kodak labs under Loyd Jones established a system that eventually would be known as ASA.

Alan Marcus (marginal technical gobbledygook)


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3/31/2008 8:43:05 PM

 
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