What does the ISO/ASA dial do on a mechanical 35mm film camera?
Asked 8/6/2019
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On a mechanical 35mm film camera, what does the ISO/ASA setting actually control? Does changing it affect the aperture or shutter speeds directly, or is it mainly for the light meter? For example, on a Konica Autoreflex T with a broken light meter, does the ISO dial do anything at all?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
6y ago
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Usually, it does not affect or limit the aperture or shutter speed at all. Rather, it tells the exposure meter where the center is. In some ways, it's exactly like exposure compensation dials.
If the camera has a program mode, it's essential information for getting exposure right. If it doesn't, like the Pentax K1000, it just shifts the exposure needle — if for a certain scene the needle would be centered at a given shutter speed and f/2.8 with ISO 100 film, if you switch to ISO 200 (whether you actually change the film or not!) the needle would now be centered at f/4. (One stop faster ISO, one stop slower aperture.)
If the camera doesn't even have a meter, it's just there as a reminder for you.
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
6y ago
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Generated from our catalog & community — verify before relying on it.
On most mechanical 35mm film cameras, the ISO/ASA dial does not directly change the lens aperture or mechanically block shutter speeds. Its main job is to tell the camera’s exposure meter what film speed is loaded.
That changes where the meter indicates “correct” exposure. For example, setting ISO 200 instead of ISO 100 shifts the meter by one stop, so it will suggest one stop less exposure.
How much it matters depends on the camera:
- Manual camera with meter: it only affects the meter reading/needle.
- Camera with automatic exposure or program mode: it affects the camera’s exposure decisions.
- Camera with no meter, or a broken meter: it does nothing practical except serve as a reminder of the film speed loaded.
So on your Konica Autoreflex T with a broken light meter, the ISO dial won’t affect exposure by itself. You would set exposure using a handheld meter, a phone meter app, or a rule like Sunny 16, and the ISO dial is just a reminder unless the meter is repaired.
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