DX coding (the black and white squares) on 35mm film cassettes make it unnecessary to manually set the film speed (ISO) on an auto-exposure camera that can read the code. Some of these cameras, including my Nikon N65, have no other way of setting the film speed in the camera.They typically default to a setting of ISO 100 with an uncoded cassette.
Not all current 35mm film cassettes have DX coding. The Ferrania film above is an example. And most bulk loading cassettes also have no coding. Does that mean that the only uncoded cassettes that can be used in a camera like the N65 are those with ISO 100 film? Not necessarily.
Like many auto-exposure film cameras, the N65 has an exposure compensation control. The exposure can be changed all the way from minus three stops to plus three stops in steps of half a stop. In effect this is the same as changing the camera’s ISO setting.
For example, setting the camera ISO to 100 and setting the exposure compensation to minus 1.0 halves the exposure, which is the same as setting the ISO to 200. This means I can put an uncoded cassette in the N65 and effectively set the auto-exposure ISO to anything from ISO 12.5 to ISO 800 as follows:
Setting the exposure compensation to -3.0 equals ISO 800, -2.5 equals ISO 560, -2.0 equals ISO 400, -1.5 equals ISO 280, -1.0 equals ISO 200, -0.5 equals ISO 140, 0.0 equals ISO 100, +0.5 equals ISO 70, +1.0 equals ISO 50, +1.5 equals ISO 35, +2.0 equals ISO 25, +2.5 equals ISO 18, +3.0 equals ISO 12.5.
Note: The intermediate ISO values above are not exact. They are the nearest values shown on a typical exposure meter.