What does the ISO setting for mechanical 35mm film cameras actually do?

Question

I am wondering if the ISO setting affects the aperture or does it just makes some shutter speeds unavailable to be used?

Edit: I have a Konica Autoreflex T with a broken light meter.

Answer 1

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.

Answer 2

The function of an ISO setting completely depends on the camera:

• Fully manual, no meter camera (Early rangefinders/TLR's): May have had a dial or slot for you to store part of the film box as a reminder for you to remember what you loaded. Exposure calculation would be done with a handheld meter or would be assumed based on rules like Sunny-16. Cards were often issued or pages printed in the manual that had varying lighting conditions and exposure settings to use. Keep in mind that film has incredible exposure latitude and consumers want photos, not perfection, so the guessing exposure was just fine. This is still how disposable point and shoots work (the film is over-exposed in most settings by default because it tolerates it, allowing the end user to simply get.the.shot)
• Fully manual, meter (Pentax K1000, other early SLR's): The camera has a meter in the window that takes into account the set ISO and shutter speed and then points a matchstick (or some other method) to the f/stop to use for a correct exposure based on the light in the scene the camera is being pointed at.
• Auto-Exposure Modes (Canon AE-1 Program, other early manual focus SLR's): Just like the previous example, except that in this case, if you are using that Program mode then the camera will automatically set the aperture for the shot. Early program modes function like Shutter Priority mode (Tv) today, where the ISO and shutter speed are selected on camera and the lens aperture dial is set to A (Auto) and left up to the camera to calculate shot by shot.
• Auto-Exposure Modes (Canon EOS, other later autofocus SLR's): By this time, cameras have modes like Shutter Priority (Tv), Aperture Priority (Av), and maybe even a host of scene-specific auto modes. Just like the above, however, is the fact that the ISO relates to the film and serves as the base from which a nominal shutter speed and aperture combo is chosen. Meters in these cameras often show an arrow at nominal and then go off into -2, -1 or +1, +2 territory. If using manual, your shutter speed and aperture combo would calculate with the given ISO and the meter would show you how close to nominal you were, given the scene. One would then adjust either aperture or shutter speed accordingly to get a nominal exposure. Or, in the case of auto modes, the camera would pick for you.

In a fully manual, meter available camera, the ISO dial is needed to be set accurately in order for the camera to recommend the nominal aperture value to use. Obviously, changing the ISO will change the recommended f/stop, producing a new baseline/nominal exposure.

In an Auto-Exposure camera, the ISO is used to calculate the baseline exposure. Additional Exposure Compensation changes would then add or subtract exposure based on the EC setting. But, if one didn't have an EC dial, the same addition or subtraction could be applied by adjusting the ISO. That being said, one must remember to change the ISO back as EC is usually a shot or scene dependent setting while ISO is per-roll.

I am wondering if the ISO setting affects the aperture or does it just makes some shutter speeds unavailable to be used?

As you can see above, in any example, all of your shutter speeds will remain available. However, that doesn't mean that any of your shutter speeds and ISO combos will give you an available exposure.

Let's assume that you're set to ISO100 and shutter speed of 1/250 and your matchstick needle says that f/2.8 is the aperture value to use. You may very well be using a lens like the 70-200 f/4, where f/4 is your maximum aperture. The film in your camera is what it is, so your only option would be to change your shutter speed to 1/125 so that you can use f/4.

Let's take this example up. This time, assume you are using ISO3200 film and want to use 1/1000 and your meter is telling you that f/32 is needed for a nominal exposure...except your lens maxes out at f/22 physically. So, you open up a stop and go to set your shutter speed value to 1/2000 only to note that you camera maxes at 1/1000! (Note that some old cameras max at 1/250 or 1/500 shutter speed values, further constraining your options).

So, the ISO doesn't necessarily limit your shutter speed options on camera - but it does affect what the baseline exposure is and the available aperture/shutter speed combos for the scene. With film, one cannot simply change the ISO dial in order to get a new ISO as this is related to the physical film you've loaded in the camera. It's for this reason that some medium format cameras designed their film holders to be swapped out mid-roll. Unfortunately, I don't know of any 35mm cameras with this functionality so, if one needs to change the ISO mid-roll, then the options are:

• Rewind the film early. Try to keep track of where you were at so that you can reload and get back to shooting the rest of the roll later.
• Rewind the film early. Sacrifice the rest of the roll and simply develop as-is
• Understand that the film is too fast or too slow for the Aperture/Shutter speed combo you want to use and shoot it anyway. This will under or over expose it and that's a problem you'll have to deal with later. Bear in mind which direction you chose to go so that you can start salvaging the film in the development phase.

Answer 3

Agree with mattdm, but I wanted to spin it another way.

It's where you tell the camera the ISO rating of the film you've put in it.

In practice it works like exposure compensation and tells the light meter (or automatic exposure) how it should vary the aperture and shutter settings for correct exposure, but many people shooting 35mm film would just think "I've put in 400 ISO/ASA film, so I set the dial to "400"." Normally they would leave it at the same setting until they changed the film (and perhaps not even then, if they used the same rated film).

It's probably less true now, as more people who shoot film would be enthusiasts, but when a lot of casual photography used 35mm film, you'd buy film for its ISO/ASA rating, and if you did the same sort of photography there was little need to change from what you were used to using.

Answer 4

What does the ISO setting for mechanical 35mm film cameras actually do?

In the simplest of terms, it is a setting that calibrates the meter.

Let's assume we are shooting under "Sunny 16" conditions where a proper exposure of ISO/ASA 100 film would be 1/100 second at f/16.

• If we set the aperture to f/16 and meter the scene, the meter will show a recommended exposure time of 1/100 second when the ISO is set to "100". Or the meter will be centered when we set a shutter time of 1/100 second.
• If we change the ISO setting to "200", the camera calculates that the film will be properly exposed in one-half the time that ISO/ASA 100 film would be and will recommend an exposure time of 1/200 second or it will be centered when an exposure time of 1/200 second is selected.
• If we change the ISO/ASA setting to "400", the camera calculates that the film will be exposed properly in one-fourth the time that ISO/ASA 100 film would be and will recommend an exposure time of 1/400 second.
• And so on...

If the camera has no meter, then if an ISO/ASA setting is even there it is to remind the user what speed film is in the camera when the user consults an external meter or calculates exposure based on "rules of thumb" or experience. In such a case it does nothing that directly affects the camera's operation with regard to shutter time or aperture.

Answer 5

Note that in theory the camera could transfer ISO settings (and possibly aperture) to the flash hot shoe even when not interpreting them on its own. I think Canon did this sort of transfer comparatively early in their hot shoes. This does not require either TTL metering (which would actually render it unnecessary to inform the flash about anything but "that was enough light, thank you") or powered electronics since the earliest extensions of hot shoes transferred information with analog signals, typically by switching resistors, for the sake of "thyristor" flashes (which has very little to do with thyristors except that they happened to be the circuit elements used for extinguishing a flash before the flash capacitor was completely discharged).