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2 Days Ago   #1
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Pentax 645z Aperture closing fully

Hi!

When taking a shot at, lets say f/5.6, the aperture first goes to f/5.6, the shot is taken, the aperture then closes fully before opening fully.

This also happens when you check the depth of field by twisting the ON/OFF switch past ON.

Is it normal that it closes fully like that? My K-1 or any other camera I have does not do this...

See video: (1080 120p)




Last edited by Tjompen1968; 2 Days Ago at 02:28 PM.
1 Day Ago   #2
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QuoteOriginally posted by Tjompen1968 Quote
Hi!

When taking a shot at, lets say f/5.6, the aperture first goes to f/5.6, the shot is taken, the aperture then closes fully before opening fully.

...
My guess is that the iris is controlled by a stepper motor, and it is always helpful in such control circuits to remeasure step counts from one limit to the other. Proving that the limits can be reached can also be a form of self test.
1 Day Ago   #3
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QuoteOriginally posted by kaseki Quote
My guess is that the iris is controlled by a stepper motor, and it is always helpful in such control circuits to remeasure step counts from one limit to the other. Proving that the limits can be reached can also be a form of self test.
But every shot? Ok.

Does yours do the same?
1 Day Ago   #4
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I just checked mine. Exactly the same, so I would presume it's normal

1 Day Ago   #5
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QuoteOriginally posted by Sandy Hancock Quote
I just checked mine. Exactly the same, so I would presume it's normal
Thank you. Makes me relaxed again =)
1 Day Ago   #6
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A theory (and only a theory).......
The aperture is stopped down by the motor against the tension of the return spring. Allowing it to open fully again after the shot is simply a matter of removing the force applied by the motor. By fully stopping down after the shot, the spring is tensioned to its maximum and this may ensure that the aperture snaps back to fully open. Smaller format lenses may not require the same return forces.
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There is definitely a difference with the 645 aperture actuation. The FA645 lenses, when unmounted and in the "A" position are wide open, while K mount lenses from the A series onwards are fully closed.
1 Day Ago   #8
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QuoteOriginally posted by Sandy Hancock Quote
There is definitely a difference with the 645 aperture actuation. The FA645 lenses, when unmounted and in the "A" position are wide open, while K mount lenses from the A series onwards are fully closed.
I did not notice that... Must check =)

What I do have found is that I can use the aperture ring to set the aperture and still have fully open aperture before shooting and also have the feedback to the camera which aperture I have choosen on the lens.

I guess this has something to do with why it closes before opening.

1 Day Ago   #9
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So I took my A 35mm and put on f/8 on the aperture ring. Now it acts exactly as I had expected, closing to f/8, taking the shot and then opening fully without this weird closing down act.
9 Hours Ago   #10
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Talked to the workshop, the behavior is normal. The K-1 has a separate step motor for the aperture that goes in two directions. The 645z has a motor that only goes in one direction and that also used by the shutter.

All things good.
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