Difference in aperture setting on on DFA* 50 versus DFA* 70-200 on non-Pentax cameras

Dericali · 04-24-2020, 03:17 AM

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?

Sandy Hancock · 04-24-2020, 03:30 AM

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?

**Rondec** · 04-24-2020, 03:41 AM

Sandy is right. KAF 3 lenses are stopped down completely when off the camera.

Dericali · 04-24-2020, 05:08 AM

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).

Not a Number · 04-24-2020, 08:46 AM

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.

zapp · 04-24-2020, 09:05 AM

Originally posted by Not a Number

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.

stevebrot · 04-24-2020, 09:53 AM

Originally posted by Dericali

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

04-24-2020, 03:17 AM	#1
Dericali Senior Member Join Date: Jul 2016 Location: Berlin Posts: 298	*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 DFA50mm did perform like this, the DFA70-200 appeared to be at a much smaller aperture, and needed about 5000 ISO to expose correctly. Can someone explain to me why this is?

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04-24-2020, 03:30 AM	#2
Sandy Hancock Moderator Site Supporter Join Date: Feb 2012 Location: Adelaide Hills, South Australia Photos: Gallery \| Albums Posts: 11,272	The DFA70-200 has a KAF3 mount, while the DFA50 is KAF4. I suspect the zoom might be at f/22. Might that be the difference?

04-24-2020, 03:41 AM	#3
Rondec Loyal Site Supporter Join Date: Mar 2009 Location: Gladys, Virginia Photos: Gallery Posts: 27,653	Sandy is right. KAF 3 lenses are stopped down completely when off the camera.

04-24-2020, 05:08 AM	#4
Dericali Senior Member Join Date: Jul 2016 Location: Berlin Posts: 298 Original Poster	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
Not a Number Moderator Join Date: Mar 2012 Location: Venice, CA Posts: 10,526	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
zapp Pentaxian Join Date: Jul 2011 Posts: 2,385	Originally posted by Not a Number 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
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by Dericali 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