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08-30-2018, 08:42 AM   #31
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QuoteOriginally posted by wtlwdwgn Quote
Andrea, I sent two images to you by PM. I just think your copy of the A 20 needs to be serviced. Eric is the go to guy we like to send our gear to.
I have seen. Thank you.

I will ask to Pentax Europe if it is serviceable.


Kind Regards

---------- Post added 30-08-18 at 18:06 ----------

QuoteOriginally posted by stevebrot Quote
OK, which opening is the "real" f/8? (neither?)
crop @ 100% of the hyperfocal with aperture selected by dial:


crop @ 100% of the hyperfocal with aperture selected by ring:


IMHO more "real" by ring.

08-30-2018, 01:15 PM   #32
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QuoteOriginally posted by Not a Number Quote
You are "snapping" the lever. On my K10D with the lens removed and the DOF preview lever actuated you can see the lever move from wide open to the fully closed position (see video). By counting the number of frames it takes about 1/6th or 0.1667 seconds*. WIthout a lens mounted the K10D operates the same as with a non-"A" or ring off the "A" position. WIth an "A" enabled lens the lever would only move as far as it needs to set the desired f-stop. Again, less travel less force to overcome any stickiness/slowness of the iris.
The camera lever distance to travel isn't relevant here, is it? When the camera closes the aperture to take a photo, it doesn't *push* the lens lever -- it stops holding it open; the spring moves the lever. In both cases, the aperture will only close down to the selected f-stop; it wont't travel any further unless you have a different f-stop set. If the aperture blades are sticky, it will likely just take longer to close it down to a smaller aperture, it won't alter the speed of the movement due to using more or less force, because it isn't using more or less force.
08-30-2018, 01:18 PM   #33
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QuoteOriginally posted by leekil Quote
The camera lever distance to travel isn't relevant here, is it? When the camera closes the aperture to take a photo, it doesn't *push* the lens lever -- it stops holding it open; the spring moves the lever. In both cases, the aperture will only close down to the selected f-stop; it wont't travel any further unless you have a different f-stop set. If the aperture blades are sticky, it will likely just take longer to close it down to a smaller aperture, it won't alter the speed of the movement due to using more or less force, because it isn't using more or less force.
In A mode the distance the arm moves is controlled by the camera and approximates the f/stop. In Manual mode using the aperture ring - it just smashes it all the way open. I assume this will result in different spring force.
08-30-2018, 02:23 PM   #34
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Look at it this way: drop a brick on your foot from say 1 cm vs 1 meter. Which hurts more or causes more damage?

I might be mixing up Force vs Momentum vs Inertia vs Velocity and so forth. It's been over forty years since my college physics classes. But the example above should illustrate the basic idea, granted I exaggerated greatly the difference in distance. In the case of the brick gravity supplies the acceleration. In the case of the lens iris it is the spring.

Mind you I could also be totally wrong depending on the design of the lens. Some lenses the lever moves the entire distance no matter what the ring setting. Some move only as far as to the ring setting. That's probably why the "A" position is past the smallest aperture and not the widest

08-30-2018, 02:33 PM   #35
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QuoteOriginally posted by leekil Quote
When the camera closes the aperture to take a photo, it doesn't *push* the lens lever -- it stops holding it open; the spring moves the lever. In both cases, the aperture will only close down to the selected f-stop; it wont't travel any further unless you have a different f-stop set.
This ^ ^ ^ The actuator lever passively follows the body coupling.

QuoteOriginally posted by leekil Quote
If the aperture blades are sticky, it will likely just take longer to close it down to a smaller aperture, it won't alter the speed of the movement due to using more or less force, because it isn't using more or less force.
It takes longer, but the body will trigger mirror-up and shutter open immediately after the coupling lever finishes its travel. That is why it is important to rule out lazy aperture before doing any other forensics.

QuoteOriginally posted by UncleVanya Quote
In Manual mode using the aperture ring - it just smashes it all the way open. I assume this will result in different spring force.
The coupling tab in the body goes full travel, and the actuator lever follows passively. Remember, resting state for the lens is fully stopped down and is spring-loaded in that state. Also as noted...

QuoteOriginally posted by Not a Number Quote
That's probably why the "A" position is past the smallest aperture and not the widest

Steve
08-30-2018, 03:14 PM   #36
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QuoteOriginally posted by stevebrot Quote
The coupling tab in the body goes full travel, and the actuator lever follows passively. Remember, resting state for the lens is fully stopped down and is spring-loaded in that state. Also as noted...

Steve
Steve - my bad if I move the body coupling tab halfway, and only halfway, the spring is half released not fully released. The tension starts at max and decreases as I move allowing the iris to close. I gotcha.
09-02-2018, 12:09 AM   #37
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I would not worry about this small discrepancy. A long time ago during the film-only days, Popular Photography magazine used to test cameras and lenses with exposure accuracy ratings. The exposure settings from cameras requiring use of the mechanical aperture ring tended to be less accurate compared to those from cameras using electronic aperture control. Also, in the case of variable-aperture zoom lenses, a camera capable of electronic aperture control with the lens set to "A" could correctly maintain the aperture that was set, unless for wide open aperture, while the lens was zoomed to various focal lengths, but if the lens aperture ring was used instead, of course the aperture would change to a different value than that which was set, as the lens was zoomed to various focal lengths.

As to the color difference, a reduction of exposure tends to saturate color more.
09-04-2018, 06:31 AM   #38
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QuoteOriginally posted by mikesbike Quote
I would not worry about this small discrepancy. A long time ago during the film-only days, Popular Photography magazine used to test cameras and lenses with exposure accuracy ratings. The exposure settings from cameras requiring use of the mechanical aperture ring tended to be less accurate compared to those from cameras using electronic aperture control. Also, in the case of variable-aperture zoom lenses, a camera capable of electronic aperture control with the lens set to "A" could correctly maintain the aperture that was set, unless for wide open aperture, while the lens was zoomed to various focal lengths, but if the lens aperture ring was used instead, of course the aperture would change to a different value than that which was set, as the lens was zoomed to various focal lengths.

As to the color difference, a reduction of exposure tends to saturate color more.
Only to recap:
  1. in my smc-A 20mm f/2,8 I obtain the correct iris if I set the aperture by the ring, checked with hyperfocal
  1. the different color rendition is due to a bug of the AWB when the aperture is by the ring


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