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04-24-2020, 03:17 AM   #1
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Difference in aperture setting on on DFA* 50 versus DFA* 70-200 on non-Pentax cameras

I have used both these lenses for videography, using the lens with a lens mount adapter and a non-Pentax camera capable of 4k shooting.
I expected no aperture control and planned to only shoot wide open, potentially using a ND filter to control exposure given that the shutter speed would be 1/50s.
However, and this surprised me, while the DFA*50mm did perform like this, the DFA*70-200 appeared to be at a much smaller aperture, and needed about 5000 ISO to expose correctly.
Can someone explain to me why this is?

04-24-2020, 03:30 AM   #2
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The DFA*70-200 has a KAF3 mount, while the DFA*50 is KAF4. I suspect the zoom might be at f/22.

Might that be the difference?
04-24-2020, 03:41 AM   #3
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Sandy is right. KAF 3 lenses are stopped down completely when off the camera.
04-24-2020, 05:08 AM   #4
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Great thanks, I guessed as much. So I guess it's effectively an arbitrary consequence of the design and electronics that they work this way?

In bright sun, you can get correct expose at around ISO 500, (1/50 + f22).

04-24-2020, 08:46 AM   #5
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You can probably find an adapter that has an aperture setting ring. It won't have markings for F-stops so you'll have to approximate via the metering system on the camera.
04-24-2020, 09:05 AM   #6
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QuoteOriginally posted by Not a Number Quote
You can probably find an adapter that has an aperture setting ring. It won't have markings for F-stops so you'll have to approximate via the metering system on the camera.
Like Novoflex adapters.
04-24-2020, 09:53 AM   #7
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QuoteOriginally posted by Dericali Quote
Can someone explain to me why this is?
KAF4 (the D* FA 50/1.4) are wide open by default and cannot be stopped down mechanically. The zoom can be stopped down with an appropriate adapter, but will not be calibrated to a particular f-number.


Steve
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