Agfa Optima I/II/III

How is auto-exposure achieved without batteries?
I know there will be some currents from the meter cell but it is puzzling whether it is sufficient to drive what ever the mechanism behind setting the speeds and aperture value automatically.
Will film advance have any contribution to the auto-exposure mechanism?

Auto-exposure without batteries is achieved with a Selenium photo-cell (analogous to a Silicon solar cell) that generates a tiny current, of order 1 mA or less. This causes the needle of a galvanometer to swing according to the current (and therefore amount of light). The galvanometer needle moves between two closely spaced bars. When the shutter release is half-pressed, one of these bars is pressed against the other, catching and freezing the position of the galvanometer needle, that can be used as a firm mechanical reference to close or open the diaphragm according to the amount of light. Only then is the shutter actuated when the release is fully depressed. In some variants, the same trapped needle is also used to set the shutter speed, e.g. in the Olympus trip. This scheme is often called "trapped needle". It is much easier to understand by actually watching the system in operation.

Many thanks. Hope I can find a nice video on this, some day.

bernard_L said:

The galvanometer needle moves between two closely spaced bars. When the shutter release is half-pressed, one of these bars is pressed against the other, catching and freezing the position of the galvanometer needle,...

Click to expand...

These bars are serrated, to catch the needle.
Of course more mechanics have to be added to achieve a program mode.

Agfa Optima was the first program autoexposure camera.