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I've just purchased a second hand Olympus Quick Flash AFL-S and on the front it reads 'ISO Auto Set' and I was wondering how film cameras detected what ISO a film was?

I normally use fully manual cameras, so as my first point and shoot I'm guessing it sets aperture and shutter speed automatically, but I'm confused about how it would detect the ISO of a film.

Also, if anyone has any links to the camera manual online that would be greatly appreciated.

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There is a pattern of blocks on the side of a 135 film cartridge that some cameras can "read" to determine the ISO of the film. The system is called DX encoding. It was introduced by Kodak in March 1983, so older cameras certainly won't have the feature.

Quoting Wikipedia...

The outside of film cartridges are marked with a DX Camera Auto Sensing (CAS) code readable by many cameras. Cameras can then automatically determine the film speed, number of exposures and exposure tolerance.

The DX Camera Auto Sensing code takes the form of a grid of contact points on the side of the metal cartridge surface that are either conductive or non-conductive. Electrical contacts in the camera read the bit pattern. Most cameras read only part of the code; typically, only the film speed is read, and some cameras aimed at the consumer market only read enough bits to tell apart the most common film speeds. For example, 100, 200, 400, and 800 can be detected by reading only S1 and S2 and ground.

As pointed out in a comment and explained on Wikipedia, the full DX Camera Auto Sensing system uses 5 bits of information, but even by checking only the first 2 bits, it's still possible to distinguish between ISO 100, 200, 400, and 800 film. Some camera manufacturers took advantage of this to keep production costs down, and so the implementation in lower-end cameras may not read the full pattern. If a DX-coded film of any other speed is inserted, the ISO determined by the camera will be incorrect.

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  • This is true but should be qualified. Some cameras (many compacts) will only read ISO values within a certain range and/or will only recognize films whose box speeds are stated in full stops (100, 200, 400, etc.). Check your manual to understand how it reads the DX codes. – bvy Jul 17 '17 at 3:01

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