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Manual cameras / ASA setting

charles swanson , Jan 24, 2011; 09:08 p.m.

I cant seem to find the answer to this one. What exactly, mechanically, happens to the camera when you adjust the ASA dial on a mechanical camera?


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Lorne Sunley , Jan 24, 2011; 09:09 p.m.

Usually it moves a do-hickey so that the meter reads a different exposure..... of course the do-hickey and the linkage are different for every brand/model of camera.

charles swanson , Jan 24, 2011; 09:14 p.m.

So it dosnt really do anything but fool the user into changing an exposure control? Then its worthless for exposure compensation in an all-manual old camera?

Dennis W , Jan 24, 2011; 09:25 p.m.

The way I see it, changing the ISO is the same as changing aperture or shutter speed (except it won't change depth of field or stop motion). If you change ISO from 200 to 400, you've effectively changed exposure by one stop. If you change from 100 to 200, you've again changed one full stop. Correct me if I'm wrong, please.

Clay James , Jan 24, 2011; 09:31 p.m.

Dennis, yup you are right.

charles Not true, you can use it for exposure compensation, what do you think you are doing when you move the compensation dial on something like the Pentax ME Super? You can do the same thing without fiddling with the ASA dial, set the camera so that the meter shows that it is proper exposed and then shift the aperture or shutter speed 1 or 2 stops depending on what you are trying to do. Done.

Tom Scott , Jan 24, 2011; 09:32 p.m.

It simply allows either more or less light to reach the metering cell depending on which direction you move it. Many of the leaf shutter fixed lens rangefinders actually do this mechanically. As you turn the ASA dial, it is moving a plate with different sized holes to eiither a larger or smaller hole that corresponds with the position of the metering cell.

Matthew Currie , Jan 24, 2011; 09:51 p.m.

If we're talking here about the ASA dial on a meterless, manual camera, it's just a reminder dial to keep track of what film you've put in it. It's coupled to nothing at all. You'll find one, for example, on the base of a Nikon F., or under the rewind crank on a Miranda F. It's entirely unrelated to the dial on a meter head.

Rob Holz , Jan 24, 2011; 10:00 p.m.

Mechanically, nothing really happens (in an SLR). If your camera has a built in meter, then the ASA dial adjusts a variable resistor which allows the voltage generated by the cds cell to raise or lower the metering needle according to the sensitivity that you've set it for. If there is no meter, then it is as has been said, a reminder.

John Shriver , Jan 24, 2011; 10:47 p.m.

In a metered manual camera, in some way it adjusts the metering circuit to expose properly for that speed of film. It may be a switched resistor, it may be a variable resistor, it may make a mechanical adjustment (on many match needle or center-needle cameras). For instance, on the Topcon Super D, it changes the anchor point of the chain that "adds" the shutter speed and aperture to rotate the "fixed" part of the meter. On most classic Pentax cameras, it is a variable resistor.
I don't know, but on the fancy pattern metered cameras like the Nikon FA, it may be digitally encoded (say in gray code) to go to a small microprocessor via some number of input pins.

charles swanson , Jan 24, 2011; 11:02 p.m.

Thanks to all. My question is adequately answered. I should have explained why I ask. I am using an old Canon TX for fun, with max shutter of 1/500, so I was curious about exposure compensation if over-exposed (and no faster shutter speed to help out). So now I know it would do nothing to change the ASA dial in my case. Again, thanks for the responses. :)

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