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02-19-2009, 06:33 PM   #1
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Tamron Adaptall Lens Question

Hey fellow Pentaxians,
I just put one of my old Tamron Adaptall lenses on my K10d. It is the 35-80mm 2.8-3.8 SP CF. I was using in Apature Prioraty mode. I set the f stop to 2.8 and I was able to shoot at 80mm at 2.8. Why didn't it move to 3.8? Not that I'm conplaining, just curiouse. Thanks.

02-19-2009, 06:38 PM   #2
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QuoteOriginally posted by Paulypentax Quote
Hey fellow Pentaxians,
I just put one of my old Tamron Adaptall lenses on my K10d. It is the 35-80mm 2.8-3.8 SP CF. I was using in Apature Prioraty mode. I set the f stop to 2.8 and I was able to shoot at 80mm at 2.8. Why didn't it move to 3.8? Not that I'm conplaining, just curiouse. Thanks.
That is a good question. I suspect that there is no mechanism to communicate the variable max aperture to the camera body. I suspect that this is even the case with the PKA adapter. (Definitely true for the PK adapter...)

Steve
02-19-2009, 06:46 PM   #3
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on the Adaptall mounts for PKA there is an internal switch that is set by the two "tabs" on the mount to the correct encoded value on the apature pins to define the lens for maximum and minimum apature.


For variable apature zooms, especially early ones, there was no compensation for focal length and the impact on apature. This came about int he early 1990's


The cameras at the time did not care, because they used the light on the sensor to determine exposure, and these meters were accurate.


It would not matter today, except the desire for super bright viewfinders has led to viewing screens that produce non linear metering (specifcially K10D and K20D). These bodies use actual apature to correct for metering non linearities.
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