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Changing the ASA Setting
There is another way to correct exposure which can be used whether you are in the manual or the AE mode. That is to change the ASA film speed setting on the camera. A film with an ASA rating twice that of another film requires only half the amount of light for correct exposure as compared with the other film. For instance, if you have an ASA 200 film loaded, you can make an exposure correction equal to closing the aperture one f/stop or raising the shutter speed one step by changing the ASA setting on the camera to ASA 400.
Other useful tricks can be performed by changing the ASA setting. You may be stuck, for instance, indoors without flash. Even with ASA 400 film loaded in your camera, you may be forced to use undesirably slow shutter speeds. You may solve this by “pushing” the ASA setting on the
camera to a higher value. Usually this is done by multiplying the normal ASA rating by some multiple of two, say by two or four.
In | this | case, | the | whole roll | of |
film | must | be shot | at | the “pushed” | ASA |
or you will have to sacrifice one or the other of a whole series of frames. Also, do not forget to inform your developing lab of the change or the whole film will be incorrectly developed. This is only an introduction to this technique. Not all films can be “pushed” and not all developing labs will handle them. Read more about “pushing” the ASA in photographic literature and know what you're doing before you try it.
This trick should not be confused with the above which involves intentional exposure correction for only one frame after which the ASA dial should be reset to its original value.