What does the auto switch do on older lenses?

I have a hole in my lens knowledge (probably more than one!). On an old lens with an auto switch what does that auto switch do? Is it only useful with older cameras (M42 fit)?

If anyone can let me know than you have my thanks.

spbatt

I presume you mean the A setting on the aperture ring, this enables a suitable camera to automatically select an appropriate aperture for the shutter speed you have chosen.
In the case of a program camera, such as the Super A, the camera will select both the aperture and shutter speed for the type of photo you wish to take ie. action or landscape.
With the lens set to A, modern cameras base the 'decision' not only on the program setting and lighting conditions but also on the image characteristics of the lens fitted (the camera knows which lens you're using and the focal length), which is one reason why it is advisable to use own brand lenses!
Such cameras usually allow you to manually select the aperture/shutter speeds from within the program menu or you can change from the A setting on the lens itself and totally override the auto functions.
Manual only lenses work perfectly well on such cameras but the program features of the camera are then more limited.

I'm not sure what features were available when M42 was the standard but I would think the same would apply.

On an M42 fit lens the auto/manual switch is a different thing altogether - it relates to the automatic operation of the diaphragm on Spotmatic cameras, something that modern photographers will take totally for granted!

When set to auto, the lens will be at full aperture for viewing and focusing and will stop down automatically when the shutter release is depressed.

When set on manual, the lens will be at the working aperture all the time, the screen will be darker, but depth of field can be roughly judged. On cameras with no automatic diaphragm mechanism (no plate at the bottom of the mirror box to activate the mechanism) the manual setting can be used.

AS I recall, Super Takumar and later M42 lenses mostly have automatic diaphragms (there will be excpetions).

Thanks for clarifying that John, like I said, I wasn't too sure about M42 having never used that set up.

Thanks John and Kimbo.

That cleared that query up nicely and now I understand a bit more. I love camera education

spbatt