Q What role do ASAs play in applying the Zone System to my photography

A: Most photographers think of ASA as simply a rating of a given film's sensitivity. This is true as far as it goes, but there is more to it than that. Earlier I said that ASA numbers can be related to f/stops and shutter speeds in the following way: As the ASA number gets smaller, the amount of exposure needed increases. Keeping in mind that the amount of exposure determines the negative's shadow density, we can state the above rule in another way: The lower the ASA number you use for a given film, the more density and detail you will get in the shadow areas of your negatives and prints.

Taking this into account, you can see that the important question is what ASA should I use with my film to get the best shadow detail? Many photographers have discovered from experience that the manufacturer's recommended ASA doesn't give them the amount of shadow detail they need in their work. In other words, after carefully placing a shadow reading on Zone III using ASA 400, you may find that the negative is underexposed by one stop (Zone II). This is not because the placement is incorrect, but rather because the photographer overestimated the speed of the film. The remedy is to shoot the film at ASA 200 instead. This would give the film one stop more exposure every time and guarantee that Zone III will always print with the detail you expect.

There are, of course, many situations where you need to use a higher ASA than the manufacturer recommends. This is called pushing the film, and it is often necessary in low-light situations. Do this with films that are relatively fast, such as Kodak Tri-X or Ilford HP-5+. Faster films generally have more exposure latitude than slower films. The only way to determine what ASA will suit your needs is to test your film and developer.

I should make one last point about the effect of changing a film's ASA. Using a lower ASA will improve your shadow detail, but it will also add density to the highlight areas of the negative. To compensate for this, you must establish a Normal Development Time that is shorter than the manufacturer recommends. The reverse is also true. If you push the ASA of a film, you must increase the film's Normal Development Time.

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