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10-14-2013, 02:05 AM   #1
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Aperture ring with manual lens on K-30

Interesting times with my K-30 and manual Pentax lens this morning. I felt mostly sorted, that the aperture ring does control the aperture, and that in manual mode the green button does stop-down metering to set the exposure parameters (hardly manual then, but ok). So I had the manual lens on it, got off the train, shot off some test shots - which swung wildly between completely blown-out, and looking like midnight. Some fiddling and I got that mostly sorted. So. Feeling good.

Then I switched to my B&W 'user' mode and snapped away ... WAT happened to the exposures??? Oh, no stop-down metering, because not in manual mode any longer. Ok, fine, can deal... Wait, what? I'm sure that depth of field isn't, well, deep. Decided to test ...

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Yeah I'm pretty much 100% sure that's not f/11. xP

So as far as I can tell, in any mode other than manual, the aperture ring does nothing, and the lens stays wide open at f/2. With the camera set to manual exposure, the aperture ring on the lens works fine. Confusing to me.

10-14-2013, 02:08 AM   #2
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It is only supposed to work in M mode, to work in Av or other modes you need a lens with an A position on the aperture ring.
10-14-2013, 02:13 AM   #3
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QuoteOriginally posted by elliott Quote
It is only supposed to work in M mode, to work in Av or other modes you need a lens with an A position on the aperture ring.
Yeah, I misunderstood what I read in the manual and online: I just thought that was about the camera being able to control the aperture or not; I guess the aperture ring just feels so tactile I assumed it was a physical connection/control of aperture - so the electronics wouldn't affect it.

Now I know.
10-14-2013, 02:30 AM   #4
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It is a direct control of the aperture, though the lever in the back of the lens overrides it and will hold it wide open, as soon as the lever is released the lens goes to whatever the aperture ring is set to. That is why you can open and close it with the ring when the lens is not mounted.

In Av mode without A contacts on the lens the camera does not release that lever because I believe it assumes you are using an adapted M42 lens and there is no lever. An M42 lens works reasonably well in Av mode, though it will tend to overexpose at smaller apertures.

10-14-2013, 03:21 AM   #5
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QuoteOriginally posted by elliott Quote
It is a direct control of the aperture, though the lever in the back of the lens overrides it and will hold it wide open, as soon as the lever is released the lens goes to whatever the aperture ring is set to. That is why you can open and close it with the ring when the lens is not mounted.

In Av mode without A contacts on the lens the camera does not release that lever because I believe it assumes you are using an adapted M42 lens and there is no lever. An M42 lens works reasonably well in Av mode, though it will tend to overexpose at smaller apertures.
Ahhh I see. Thank you for that.
10-14-2013, 03:50 AM   #6
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Oh, and this is the old lens on the k-30:

Like a lens only smaller by -Occasionally Focused-

Like a lens, only smaller.
11-03-2013, 04:05 AM   #7
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I have a lens Pentax-A 50mm 1.7. In any opening in the lens ring, the machine does not accept this openness, both in the M position as in AV.

Then I discovered that the ideal is to put the lens in position A, and then in any exhibition, P, M, AV, then choose the wheel on the machine and choose the aperture I want and shooting.

Only then get the depths of field I want, if you press the green button and it breaks back at 2.0.

With the lens in manual position, the apertures of the lens barrel does not work in M or AV.

Even with that setting the Pentax advises TVShift in P-Line the apertures do not work in manual mode.

In P mode the machine operates the Pentax-A (position A) as if a lens DA, until the flash works ok.
11-03-2013, 06:15 AM   #8
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Pentax film cameras, and perhaps early digital units, had a mechanical means of detecting the aperture position of older non-A lenses. There is a petition thread asking Pentax to restore this capability. I personally don't think it will happen for marketing reasons, but hope springs eternal.

K-mount lens have springs that pull the aperture blades to the smallest opening. When you mount these lenses, the body pushes against these springs to open the blades to their largest opening. If the lens has an aperture ring, there are mechanical stops that prevent the lens from closing smaller than the stop set on the ring. When the camera's shutter is released, the body allows the lens to close to this mechanical stop.

K-mount lenses with 'A' capability (A-series and newer, including all those lenses without an aperture ring) are detected by the camera body. The body is also able to detect the aperture range for these lenses. When these lenses are in the 'A' position (lenses without aperture rings are always in the 'A' position), the camera body doesn't just let the lens close to its mechanical aperture stop, but knows how far to let the aperture controlling lever move to achieve a specific stop.

To summarize, with non-A lenses, the lens determines where to stop the lens aperture movement, but with A lenses, the body determines where to stop the lens aperture movement.

The issue with using older non-A lenses on digital camera bodies has to do with exposure metering. Because the lens is held wide open when the scene is metered, and the body has no idea how much the exposure will be reduced by the aperture ring setting, Pentax has added a step using the green button whereby the lens is allowed to close the aperture to its set value before the meter is activated. Thus the term 'stop down metering'.

There IS a way to non-destructively modify a non-A k-mount lens and the camera body to approximate the operation of an A-series lens. Exposure will typically be within 1 stop, and the error will always be on the over exposure side.

Look at the post in this link, then scroll down a couple posts if you want to see pictures. While I am using a KA 50mm f/1.7 lens, I lose all 'A' features when I mount the lens on my macro converter (or my bellows unit). It effectively becomes an M-series lens. These posts show how I get around that limitation.
PK/A extension tubes? - PentaxForums.com

11-03-2013, 02:18 PM   #9
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Very interesting. Thanks for that. I'm rather enjoying the challenge of it at the moment, but will remember this for possible later use.
11-03-2013, 02:52 PM   #10
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That's why I prefer manual lenses to be PK/A or KR compatible just to have more control and options. I too learned the hard way.
11-26-2013, 02:01 AM   #11
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By the way does the mirror flip when metering with the green button?
11-30-2013, 09:08 PM   #12
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QuoteOriginally posted by angevoi Quote
By the way does the mirror flip when metering with the green button?
I don't know, but I suspect not; with other lenses the camera meters with the shutter closed/mirror down.
11-30-2013, 09:19 PM   #13
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QuoteOriginally posted by angevoi Quote
By the way does the mirror flip when metering with the green button?
No. If it did, it would not meter as the metering circuit is in the Pentaprism.
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