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01-09-2014, 03:29 PM   #16
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Then just set the camera at Aperture Priority and you are done. no need to press any button other than shutter release button to get the camera metering for you.

01-09-2014, 03:34 PM   #17
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QuoteOriginally posted by grahame Quote
Tak 300/3.5 is a preset lens. There is nothing on the read end controls aperture. The 2nd ring on the front does that. Pushing green button won't do anything to the exposure metering or aperture stop down. Camera meters before you press the shutter button.
This is how it works in other than M mode (camera defaults to Av mode). Unfortunately, Av mode stop-down metering is unreliable on the K-3 when compared to M mode using the green button. If you want consistent results at all apertures, M mode using the green button to set the shutter speed is the way to go.


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01-09-2014, 03:37 PM   #18
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QuoteOriginally posted by grahame Quote
Then just set the camera at Aperture Priority and you are done. no need to press any button other than shutter release button to get the camera metering for you.
See my comment immediately above. Compare the values for the two modes using a faster lens (say the ST 55/1.8) for the full range of f/stops. I think you will find about 1-2 stops under-exposure wide open using Av mode and fairly reasonable metering using M mode. The problem is less severe at moderate apertures for most lenses. On older bodies such as the K10D and K20D, stop-down metering is flaky regardless of mode. The issue was addressed with the release of the K-7 but the fix is limited to M mode on that and all subsequent bodies.


Steve

Last edited by stevebrot; 01-09-2014 at 03:56 PM.
01-09-2014, 03:41 PM   #19
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Don't you just set M or A and adjust the speed. There is no stopping the lens other than manually selecting it's aperture.

01-09-2014, 03:44 PM   #20
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What you talkin bout, Willis?

This is all news to me since I use m42s all the time, but the camera goes straight to Av mode when I do.

However, just tried it on the K-01 and the green button meters fine, in fact it will meter even with no lens mounted at all.
01-09-2014, 03:48 PM   #21
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QuoteOriginally posted by stevebrot Quote
Unfortunately, Av mode stop-down metering is unreliable on the K-3 when compared to M mode using the green button.
I see. I usually shoot close to wide open with 200/3.5, and most other M42 lenses. Only very occasionally I stop them down to F5.6 or 8. I haven't notice much problem on K10D and K5.
but still I don;t understand why K3 or k50 has this problem while older models are doing better. The mount on K3 and K100D and the same.
Is there anything on the lens touches the aperture lever bar on k3?
01-09-2014, 04:02 PM   #22
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QuoteOriginally posted by stevebrot Quote
See my comment immediately above. Compare the values for the two modes using a faster lens (say the ST 55/1.8) for the full range of f/stops. I think you will find about 1-2 stops under-exposure wide open using Av mode and fairly reasonable metering using M mode. The problem is less severe at moderate apertures for most lenses. On older bodies such as the K10D and K20D, stop-down metering is flaky regardless of mode. The issue was addressed with the release of the K-7 but the fix is limited to M mode on that and all subsequent bodies.
This I see on the K-5 but not the K-01. On the K-01 I get identical readings either way. On the K-5, there is often a difference, but only like 1/3 of stop, never 1 or 2 stops. So I don't worry about it. I have noticed in live view (on the K-5), if you stop an m42 way down with a low iso so the shutter speed in starting to get into the seconds instead of fractions that it sometimes seems to give up or something and will not go lower for shutter than a certain value. But if you pump up the iso than it works ok.
01-09-2014, 04:05 PM   #23
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QuoteOriginally posted by grahame Quote
I haven't notice much problem on K10D.
This is a historic and well-documented problem with the K10D with many (ancient) threads on this site discussing the issue. You are correct in that there is no aperture coupling with the M-42 lenses. You must always move the A/M switch to the M position with those lenses unless you want to shoot wide open. That being said, the aperture coupling is unrelated to metering on any K-mount camera. Adapted lenses must always use stop-down metering with the lens manually stopped down.

One would think that Av mode and M mode should behave the same and they do with the older cameras. Unfortunately stop-down metering on the older cameras is unreliable and this fact is noted in the user manuals. With the K-7 and new bodies the issue was resolved for M mode only. Using a M42 lens stopped down in Av mode will give the same mixed results as with the older cameras. The solution is to:
  • Shoot in M mode
  • Meter by stopping the lens down manually and pressing the green button to get the shutter speed
  • Shoot using those settings until either the subject or light changes


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01-09-2014, 04:17 PM   #24
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QuoteOriginally posted by stevebrot Quote
One would think that Av mode and M mode should behave the same and they do with the older cameras. Unfortunately stop-down metering on the older cameras is unreliable and this fact is noted in the user manuals. With the K-7 and new bodies the issue was resolved for M mode only. Using a M42 lens stopped down in Av mode will give the same mixed results as with the older cameras.
Can't speak for other models, but am absolutely not seeing this on the K-01 or K-5. I'm sitting here trying to get different readings (more than 1/3 stop) from Av to M from f/1.4 to f/16 and can't do it. Time to break out the K10D and see how it is -- wasn't using m42s back then.

(And for the record find green-button metering with all-manual K-mount lenses more unreliable than m42 stop-down metering in Av mode.)

Last edited by vonBaloney; 01-09-2014 at 04:24 PM.
01-09-2014, 04:50 PM   #25
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QuoteOriginally posted by vonBaloney Quote
On the K-5, there is often a difference, but only like 1/3 of stop, never 1 or 2 stops.
It might depend on the lens*. I don't have a new body, but was able to document the difference with the K-7 when I demo'ed that camera. Take a look in your K-5 manual (Appendix, p 11) and note the lens/mode combinations with the text "but an exposure
error may occur".

Again...this is not Live View, but stop-down metering off the focus screen.


Steve

* I find that at apertures f/5.6 and narrower, the metering is generally pretty close and with my slower lenses the K10D just pretty well.

(Stop-down metering with the Pentax-M 50/1.7 vs. Pentax-A 50/1.7 wide open is a full stop difference on the K10D...just measured it.)

Last edited by stevebrot; 01-09-2014 at 05:01 PM.
01-09-2014, 04:57 PM   #26
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QuoteOriginally posted by vonBaloney Quote

(And for the record find green-button metering with all-manual K-mount lenses more unreliable than m42 stop-down metering in Av mode.)
Truly. This is contrary to the owner's manual for your K-5. With my old K10D, I get the same (wrong) readings for both Av and M mode for equivalent lenses regardless of mount.

Can you provide documentation? I would find this quite surprising.

BTW...what is your definition of a manual lens? Almost all K-mount lenses have automatic apertures, though only those with A contacts have automated aperture control.


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01-09-2014, 05:08 PM   #27
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QuoteOriginally posted by stevebrot Quote
Truly. This is contrary to the owner's manual for your K-5. With my old K10D, I get the same (wrong) readings for both Av and M mode for equivalent lenses regardless of mount.

Can you provide documentation? I would find this quite surprising.

BTW...what is your definition of a manual lens? Almost all K-mount lenses have automatic apertures, though only those with A contacts have automated aperture control.
That's what I mean -- non-A bayonet lenses where the only choice is M mode/green-button...

Will do some more m42 tests, but I do shoot with them all the time (in Av, mostly K-5 in non-LV) and I just don't have any major exposure problems...
01-09-2014, 06:06 PM   #28
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QuoteOriginally posted by vonBaloney Quote
non-A bayonet lenses where the only choice is M mode/green-button
Or block the connector on the camera using a tap. Camera will think it is a M42-type lens and stop the aperture coupling mechanism. That way, you get the stop-down metering lens like m42.
I once did that on M40 lens, since the green button on K01 is hard to reach.
01-09-2014, 06:13 PM   #29
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QuoteOriginally posted by vonBaloney Quote
I just don't have any major exposure problems...
You might not. I shoot my Tamron 70-150/3.5 with the regular PK adapter and get acceptable exposures most of the time too. Ditto with my slower M42 lenses. It is with fast lenses that things go weird at the wide end. My Jupiter-9 85/2 will underexpose (no kidding) 3 stops at f/2.


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01-09-2014, 06:24 PM   #30
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The metering is of course never as good as with an A-lens because it will only do center or spot...will keep an eye out.
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