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Lens Base Pin



 
 
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  #1  
Old October 8th 04, 12:41 PM
Ira Mellon
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Default Lens Base Pin

Please tell me the function of the small, retractable (by spring
action) pin at the screw-mount base of the interchangeable lens on my
Pentax Spotmatic 35mm SLR (film-type, not digital). Would it have
anything to do with the lens's AUTO/MANUAL function? If so, if the
pin's spring is broken, can the lens still function only in the MANUAL
setting? Or is it the other way around (function in AUTO only)?
  #3  
Old October 8th 04, 07:20 PM
Jeremy
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"Ira Mellon" wrote in message
m...
Please tell me the function of the small, retractable (by spring
action) pin at the screw-mount base of the interchangeable lens on my
Pentax Spotmatic 35mm SLR (film-type, not digital).


When Asahi released their "Super-Takumar" lanses, they featured automatic
diaphragm operation--new at the time. The diaphragm remained fully open
while the photographer composed and focused his shot--an important feature,
since the viewfinder of the typical SLR was rather dim. Now, on the
previous generation of lenses, the photographer had to manually stop down
the aperture prior to firing the shutter--an extra step that added extra
time to get the shot, and one that had the potential of ruining the shot if
the photographer forgot to manually stop down the lens prior to shooting.

So, you ask, why the "Auto/Manual" switch? The reason is simple: Asahi
Optical Company wanted to make these new lenses backward-compatible with
previous camera bodies, that did not support automatic diaphragm operation.
(Long-term compatibility has always been one of Pentax's strong suits).

If the lens was mounted on an older camera, the "Auto/Manual" switch was
unlocked and could be placed in the manual position, allowing the lens to be
operated in stop-down mode.

It is important to note that the switch can be moved ONLY if the lens is
mounted on an older Pentax camera--do not try to force it if you mount the
lens on a Spotmatic, because you will break it. There is a VERY TINY pin on
the lens mount that must be depressed to unlock the Auto/Manual switch. You
can usually depress it by screwing on a rear lens cap, and that will allow
you to move the switch. Many uninformed users think that there is something
wrong with the switch when they cannot move it.

The rule of thumb is to leave the switch in the Auto position if the lens is
mounted on a Spotmatic or ES camera body, and don't move it anymore. If you
do not have an older pre-Spotmatic model, the switch is irrelevant to your
needs.

All Spotmatics, SP-II, SP-IIa, SP-500, SP-1000, SP-F and the twi ES models
ES and ES-II feature a stop-down switch on the left of the lens mounting
section. That is the way to view in stop-down mode, not by using the
"Auto/Manual" switch. Older Pentax models do not feature the stop-down
switch, and on those bodies, you use the Auto/Manual switch instead. Got
it?

Understand that, if Asahi did not include that Auto/Manual switch on the
Super-Takumar and later SMC Takumar lenses, they could not be used on older
model camera bodoes, except in fully-open aperture--obviously no good. So
Pentax was protecting the investments of their existing customer base by
adding that switch.

And, yes, if the auto-aperture linkage pin is broken, you CAN use the
Auto/Manual switch to operate the lens in manual mode at all times, but that
is rather cumbersome, and it means that you do not get the benefits of a
brighter viewfinder when composing. Might be better to get a replacement
lens on eBay instead.


 




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