Rudy Garcia wrote in message ...
>In article <349e150f.0@news1.ibm.net>, naoki@ibm.net wrote:

>> Ai-S: Before Ai-S, moving the aperture lever of the lens by, say, 1mm meant
>> different things lens by lens. For one lens, it meant you are opening
>aperture by
>> 1 step, for another lens, it meant opeing aperture by half step and so
>on so forth.
>> This was NOT good for program auto exposure and shutter priority
>exposure, etc.
>> Still Nikon FG could give you accurate exposure without having Ai-S lenses
>> attached to it, but then, the reading and the actual aperture used was
>> different. With Ai-S lenses, moving the aperture levers of the each single
>> lens meant the same thing. Thus reading and the actual aperture used
>> matched.

>Hmm, I believe the "linear" f-stop ring in AI-S mounts is a story that
>has been going around for a while.
>
>AI-S just added that "half moon" shaped index mark, milled on the rear of
>the lens flange, so that cameras like the FA could modify their program
>based on the lens focal lenght. An AI-S tele lens would be sensed by the
>FA and it would modify its internal program to favor maintaining a higher
>shutter speed, rather than DOF.
>
>The FA (and other Nikon post AI-S cameras) would take a "delayed" meter
>reading, after the lens stopped down to the taking aperture and before the
>shutter tripped, to make the final exposure determination.
>
>If pre AI-S lenses had this supposedly non-linear f-stop ring, then pre
>AI-S Nikon cameras would not have been able to provide the excellent
>exposure they did, by through the lens metering, or by using an external
>hand held meter. This would have hardly made them the Pro camera of
>choice back then.

Hmmm, I think a bit of confusion exists here - and the original poster
was essentially right (the rear lens diaphragm actuator lever on AIS
lenses can be used to set f-stops on the lens when using compatible
Nikon bodies with program and shutter-priority modes) - it is not
the aperture ring itself that is non-linear in pre-AIS lenses,
but the movement of the actuator lever for controling f-stops used
in program modes (you are right that those bodies check exposure as
the lens is stopped down, to correct for the inevitable slight error
in this system - and that check can also be used with non-AIS lenses
to cover their larger aperture errors when used in P and S modes).
BTW, if you open up AIS and AI lenses to compare the aperture close-down
mechanisms, you will find that AIS lenses have a cam added which makes
it possible for the camera body to select an f-stop using the actuator
lever position alone.
--
David Ruether
http://www.fcinet.com/ruether
ruether@fcinet.com