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Or - why you don't set the film speed on old cameras.
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One question we get asked a lot is how to set the film speed (ASA/ISO/DIN) on an older style camera like the Agfa Isolette or Russian Lubitel. The important thing to note is that the majority of these cameras have no built in light meter so they have no way of using the film speed anyway. The film speed indicator (where present) is purely as a memory aid to the photographer - as its easy to forget what speed of film you have loaded if you don’t use it up quickly.
The place to set the film speed is on the light meter - or if you are using another camera to give readings - then on that. Obviously if the old camera you are using does have a built in light meter then you do indeed need to set the speed on that. This is really only applicable to better quality rollfilm cameras as the Lubitels never had light meters.
Otherwise if you have no light meter at all - you sometimes get little exposure charts printed in the inside of film packets. eg:
ASA 100, Bright weather conditions would be 1/250 at F/11.
ASA 100, Normal weather conditions would be 1/125 at F/11.
ASA 100, Cloudy/Dull weather conditions would be 1/60 at F/11.
Note - the film speed is built into the table. If the film was 200 ASA, the table would look like:
ASA 200, Bright weather conditions would be 1/500 at F/11.
ASA 200, Normal weather conditions would be 1/250 at F/11.
ASA 200, Cloudy/Dull weather conditions would be 1/125 at F/11.
An example using the Agfa Isolette II and a separate light meter:
Assuming I've loaded 100 ASA film in a Isolette II.
I'll use my Minolta Dynax 7000i SLR as a light meter - so I set 100 ASA on that.
Point it at the subject and take a reading (say F/8 at 1/125).
Then I set F/8 and 1/125 on the Isolette II and take the picture. The Isolette II has no understanding of why F/8 and 1/125 - it relies on me 100%.
If for instance I then want to take another photo of a different scene - I'll point my Minolta 7000i SLR at that and take another reading. If its lighter - then the 7000i might give me a reading of F/11 at 1/125. It knows it would have to let in less light because the scene is lighter. However of course I'm not using the 7000i to take a picture so I just transfer the settings F/11 and 1/125 onto the Isolette and the exposure will be right. Again the Isolette II has no understanding of the film speed. It is relying on me 100% to get the shutter and aperture settings right.
The same is true if I then change the film in the Isolette II to 200 ASA. I can take another picture of this second scene with F/11 at 1/250. The Isolette II has no understanding that the film speed has changed - just that I've asked it to let in half the amount of light for this exposure (because *I* know that the film is twice as sensitive).
Obviously if I changed the film in the 7000i to 200 ASA, the 7000i would understand what I had done and tell me to use F/11 at 1/250. This is because the 7000i can read the film speed automatically. This feature combined with its built light meter ensure its exposure is always correct.
In the long term a small hand held light meter is easier to use than a SLR - but thats extra cost. There are quite a lot of 'Leningrad' Russian light meters 2nd hand for very little money and these seem to work fine.
Here are some great links on exposure which should help you understand all this much more:
http://www.photo.net/making-photographs/exposure
http://www.photonhead.com/exposure/exposure.htm
http://www.photonhead.com/exposure/simcam.htm
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