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FILM SPEED - What do the numbers 100, 200, 400 really mean?

Written by "Vector"
Managing Director, the Vector Trust
One of three founding members of the Photos@VectorTrust.com team.

   Just like everything else in life film is "rated" or ranked. The most common performance standard for film is its speed. All films have a speed rating. This is the often refered to ( and advertised) ISO rating, the 100, 200, 400, etc number that is printed on the film cassette and packaging. All film sold has an ISO number. ( If it doesnít, walk away from that retailer, quickly.) No matter what film format, 35mm, 3x5, 4x6, the new small APS or 110 and 120, the film cannister will show an ISO number, but what does that number mean?

   First a bit of background information. The ISO numeric scale was established by the International Organization for Standardization.  This universal rating number combined/replaced the older DIN, ASA, or ANSI systems older photographers may remember from many years ago. The ISO speeds run from (slow) 20, 25, 32, 40, 50, 64, 80, 100, 125, 160, 200, 250, 320, 400,(fast) 800, 1000, ( very fast) 1200, 1600, 3200 and up to 6400. Not all of these films are available widely. Most film users have only seen 100, 200, 400 and maybe 800 speeds at most. These are the most commonly sold and used consumer films worldwide. L to R, FujiFilm Color Superia X-tra, Kodak ExtraPress 1600, Kodak Ektar 25, AGFA HDC 200, and Kodak Professional B/W T MAX 400

   In practical terms, photographic film is measured in two ways Let me start with the ISO numbers, then Iíll mention the other measurement scale called "Film grain".
   An ISO speed is simply a measurement of how efficiently it will react when exposed to light. Films are made for different applications, so a more sensitive film has a higher ISO number or rating. As an example, a consumer ISO 200 speed film is twice as sensitive to light as the ISO 100 film ( from the same manufacturer ) If you use ISO 200 film, it requires half as much light to make a correct exposure as ISO 100 film. Photographers refer to film as being slow (under ISO 200), fast (ISO 400, 800 and 1000), and very fast (over ISO 1000).

   If film is ranked by its light sensitivity, then a "slower film" is more suited to use where there is lots of light. The FujiFilm web site suggest consumer ISO 100 speed film for family picnics in the park at midday. "Fast" films, with higher ISO numbers are useful for compensating for low, or poor light conditions, like indoor family photos. You may have noticed that Kodak has tied a whole marketing campaign around this issue with its new Kodak MAX labeled film. They are selling it as all around family film, because it will return better pictures in tough situations. Think of the scene at a childís birthday party. The lighting is atrocious. Unlike the picnic example above, the family rooms in most homes never have enough light, so camera flash units are working overtime to light the scene. The "hot spots" from the candles, and Dad using a cheap, "point and shoot" type camera donít help either. When the pictures come back, everyone has red-eye and there are huge shadows on the wall behind the children. No one wants 10 copies of a family picture like that, so Kodak doesnít sell more print paper. The creative solution Kodak came up with was a film product called Kodak MAX, a film that helps the amateur get great family photos with any cheap camera. Why does it work so well? Kodak MAX is a discounted ISO 800 speed film. All film makers get nasty letters, because consumers blame the film for lousy pictures. If Kodak MAX returns nice photos in challenging environments, "Mom" will buy more of that film, without knowing that the higher ISO speed film was the solution to her photography problem all along.

   Film speed is also a "relative" scale, not a linear one. That means that if you compare Kodak , FujiFilm , AGFA and any other "brand" of film, all the ISO 100 films will be "relatively" the same in performance. Any ISO 400 film you buy in Sydney, Australia, Tampa, Florida USA or in Bonn, Germany will give you similar "ISO 400" performance as well. The ISO rating is NOT however an absolute, non-variable number like speed (MPH/kph ) or temperature.(C/F). Each film manufacturer will custom build the light sensitive emulsion on the plastic filmstrip. The only way to know whether you "like" Kodak , FujiFilm , AGFA , ILFORD B/W, or any other "brand" of film is to try a few rolls and compare it to your current favorite. Photos@VectorTrust.com uses films from Kodak and FujiFilm, but which specific film depends on the lighting situation and subject matter.

   So far the average non-professional photographer reading this article would think, "OK, if ISO 200 is better ( more light sensitive ) than ISO 100, and ISO400 is more responsive than ISO 200, Why not just purchase only ISO 1000 or 1600 and every picture will be perfect." I just said that Kodak used that as a marketing feature. Camera manufacturers and Film companies would like one universal all-in-one- film too. It would simplify their inventory problems, if nothing else, but there is a little problem with that plan. Itís called "physics". Remember those science classes you hated in school. "Mr Easley" told you that you would need to know some of these "Laws of Physics" someday. Check the calendar, because "someday" is every day for a photographer. What "Mr Easley" was trying to teach you in those lectures about lightwaves and the experiments using prisms, is that light moves at a fixed speed. Admittedly, it is really fast, but the speed of light never changes either. When Film manufacturers make film in several ISO speeds they are giving the photographer one more tool to control the final image. The trade-off that a photographer makes for the faster ISO films mentioned above is the lower quality of the fine detail in a finished photographic print. "Film Grain" is the other measurement of a films performance ability, and there isnít a widely advertised number, standard or ranking for this variable. (There is a "standard", but not in this discussion). Lesser, or "poor grain" in a film revels itself by producing prints with reduced sharpness and detail, less vivid colors, and increased pixelization of the photo. Choosing the "right" film ISO speed is a constant balance between a fast ISO speed film ( needed to better control shutter speed ) and a filmís grain quality ( which determines the sharpness and fine detail.in the final photographic print). Learning how to balance these two competing factors is subject enough for three more articles. You can read more "How To" articles about photography, by visiting the newsletter index.

   By the way.... Any ISO speed film you select will work in your camera. Mechanically, the camera equipment doesnít care. The vast majority of modern film cameras, especially cheaper "point and shoot" cameras, automatically read the ISO code from the film cartridge, and then adjust the camera settings to match. On older, manual cameras, rotating a film dial on the camera's body somewhere will do the same thing. Even the oldest cameras will offer the full range of common ISO numbers from 25 to 1000 or 1600.

   Your new understanging of ISO film speeds applies to digital cameras too. As a control feature of most new digital cameras, the ISO equivalent is set using one of the camera's electronic menus or submenus. Again, some cheaper, consumer digital cameras will set the ISO automatically, selecting a higher ISO equivalent in low light for faster shutter speeds. A high "ISO equivalent" setting provides greater sensitivity to light just as it does with faster ISO film. Remember, just as in film, image quality degrades as the ISO speed is increased. In digital cameras, this lower quality image resolution is called "increased signal noise" or "digital graininess". One other quick point about digital cameras while we are at this point. Your computer monitor only shows 72 dpi resolution of the picture on this page. "dpi" is slang or short-hand for the number of printed pixels or "dots" per inch in a photographic image. Itís an important measure of the completeness or density of your picture. The average, cheap 3x5 prints you receive back from the film lab are over 300 dpi. Be sure to save any digital camera image files as "large" as possible, so the prints you make later wonít be a disappointment. Viewing digital images on a computer screen is mis-leading in this way. You can learn more about "What is a pixel? .... and why digital cameras need so many of them." in our newsletter archive.

   

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