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06-19-2015, 09:49 AM   #1
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Pentax Manual Lens ignored aperture in Av mode (green button)

Hi everyone,

Since it took me quite some time to figure out why on one of my manual lenses (not an A lens) (Porst 50mm) the greeen button wouldn't work, and with another lens it would work (Porst 135mm), here for everyone who is desperately googling why two manual lenses behave differently:

For manual lenses the Pentax K-30 (v.1.05) seems to have two modes of operation:

1.: In manual (M) mode you can pre-set the aperture ring and with the green button, brightness is measured. In all other modi (namely Av) the aperture will always be fully open regardless of the settings of the aperture ring. (The camera effectively ignores any aperture ring settings)

2.: in manual (M) mode the green button does absolutely nothing. Aperture for every shot is automatically set to the setting on the aperture ring. this enables the (personally) most comfortable mode for manual lenses for me. You can set the camera to Av mode, and with EV correction you can correct exposure time for different apertures.
e.g.: 50mm lens with 1.7/2.8/4/5.6... The camera always measures exposure for aperture 1.7(maximum) so if you want to shoot with 2.8 add +1EV. this goes for every step. so 4 will be +2EV etc. ISO has to be set to a fixed value, but when you're not changing apertures rapidly, you can use the lens almost as comfortably as an A -lens.

Now the tricky part: Those two modes are mutually exclusive (oh god why pentax). Mode 1. is active if the base of the lens is electrically conductive. Mode 2. is active if the base is electrically isolated. To achieve the later a bit of very thin tape can be put onto the lensbase where all(!) the contacts of the camera are, and voila you can use mode two. For mode 1 I heard tin foil would do the trick.

Hope this helps,
Cheers

P.s.
I'm not completely sure why mode 1 would be preferred to mode 2 if the aperture is not changed rapidly. I guess the measuring could be more accurate, but on the other hand its physics how the exposure/aperture works.
If you have any experience with these differently operating principles for your lenses I'd be happy to hear your thoughts about it :-)

06-19-2015, 10:22 AM   #2
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QuoteOriginally posted by manufocus Quote
Hi everyone,

Since it took me quite some time to figure out why on one of my manual lenses (not an A lens) (Porst 50mm) the greeen button wouldn't work, and with another lens it would work (Porst 135mm), here for everyone who is desperately googling why two manual lenses behave differently:

For manual lenses the Pentax K-30 (v.1.05) seems to have two modes of operation:

1.: In manual (M) mode you can pre-set the aperture ring and with the green button, brightness is measured. In all other modi (namely Av) the aperture will always be fully open regardless of the settings of the aperture ring. (The camera effectively ignores any aperture ring settings)

2.: in manual (M) mode the green button does absolutely nothing. Aperture for every shot is automatically set to the setting on the aperture ring. this enables the (personally) most comfortable mode for manual lenses for me. You can set the camera to Av mode, and with EV correction you can correct exposure time for different apertures.
e.g.: 50mm lens with 1.7/2.8/4/5.6... The camera always measures exposure for aperture 1.7(maximum) so if you want to shoot with 2.8 add +1EV. this goes for every step. so 4 will be +2EV etc. ISO has to be set to a fixed value, but when you're not changing apertures rapidly, you can use the lens almost as comfortably as an A -lens.

Now the tricky part: Those two modes are mutually exclusive (oh god why pentax). Mode 1. is active if the base of the lens is electrically conductive. Mode 2. is active if the base is electrically isolated. To achieve the later a bit of very thin tape can be put onto the lensbase where all(!) the contacts of the camera are, and voila you can use mode two. For mode 1 I heard tin foil would do the trick.

Hope this helps,
Cheers

P.s.
I'm not completely sure why mode 1 would be preferred to mode 2 if the aperture is not changed rapidly. I guess the measuring could be more accurate, but on the other hand its physics how the exposure/aperture works.
If you have any experience with these differently operating principles for your lenses I'd be happy to hear your thoughts about it :-)
You forgot to mention the mount on those two lenses.... are they k-mount (with aperture lever) or m42 mount with or without A/M switch? Also, for the k-mount, some people remove the aperture lever (reversible procedure) so it behaves like m42 mount; therefore, you can use it in Av mode. I suspect that those two lenses are different.
06-19-2015, 10:29 AM   #3
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They are both K-mounts with aperture lever, in design and function identical (except one is non-conductive).
06-19-2015, 10:33 AM   #4
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QuoteOriginally posted by manufocus Quote
They are both K-mounts with aperture lever, in design and function identical (except one is non-conductive).
Just put an aluminium foil to shot the data-pin on the mount, they will be the same. For non-conductive base, the camera does not recognize the lens mounted, I think all Pentax lenses are conductive. Also, with conductive base, you can use the catch-in-focus feature on the camera which is nice. I would prefer k-mount with aperture lever removed and conductive base.... then you can use Av/Tv mode and stop down metering (no need for green button) plus catch-in-focus feature on the camera; and of course, no m42/k adaptor.


Last edited by aleonx3; 06-19-2015 at 10:48 AM.
06-19-2015, 11:30 AM   #5
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This is a well-known phenomenon. Basically what it boils down to is that all Pentax-manufactured K mount lenses have a conductive base, and the camera assumes you are shooting an m42 lens (all of which are non-conductive). The problem is when you run into 3rd party K lenses using non-conductive anodized aluminum mounts.

As a legacy of removing the mechanical aperture coupling in modern digitals (aka the "crippled K"), Pentax uses conductive/nonconductive as a shorthand to determine whether to activate the iris actuation lever prior to shooting and take a meter reading. The problem with m42s is that if this lever actuates, it can get hung up on protruding parts on the back of the lens and damage it or the camera.

When you are shooting K mount lenses in Av mode, since the lens has no way of telling how far to stop down (which it gets through the electrical contacts on A lenses) and assumes that the lens is doing the stopping down for you and it has gotten an accurate reading prior to your pushing the shutter button. It's not that it is ignoring the aperture setting - it simply has no way to meter wide open and make the correct stop-down calculation in the current mode.

Conversely, the reason why when you return to M mode that it won't stop down for nonconductive lenses is that by assuming you are using an m42 lens, it also assumes that the meter reading at the time you push the shutter button is already correct, since you'll have already stopped it down manually before taking your shot.
06-19-2015, 12:13 PM   #6
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Thanks @dcshooter for the detailed explanation of this behavior.
Mode 2 (Av + EV correction) is limited to 5 steps down the aperture setting. Nothing I would miss at the moment since I shoot with quite cheap lenses that get quite terrible at fully open and fully closed. However it would be nice to have the EV-correction mode available for conductive lenses since fiddling around with the green button and afterwards taking a shot seems rather inconvenient for faster objects.

@aleonx3 I think at somepoint I might try removing the lever, do I guess correctly that the aperture won't be controlled with the camera anymore? This would mean that the viewfinder would accurately show DOF, however I think that this could become an issue for focussing since the area of "sharpness" will be longer making it more difficult to land the desired object in the "middle" of the sharpness zone.
06-19-2015, 12:17 PM   #7
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Behaviour depends on the camera model. K10, K20: green button still works normally with non conducting mount (ie stops down or tries to). K-r, K5 - no! However if you check closely you wil see that both of the latter are still metering and setting a shutter speed (try it with an M42 takumar for example, A-M switch on M, changing the f-stop on the aperture ring)

See also my piece on metering of the k-r with non-conducting mounts.

https://www.pentaxforums.com/forums/116-pentax-k-r/271872-tests-plated-vs-pai...r-whats-t.html
06-19-2015, 02:06 PM   #8
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QuoteOriginally posted by manufocus Quote
do I guess correctly that the aperture won't be controlled with the camera anymore? This would mean that the viewfinder would accurately show DOF, however I think that this could become an issue for focussing since the area of "sharpness" will be longer making it more difficult to land the desired object in the "middle" of the sharpness zone.
Correct, this is the same as m42 with A/M switch in M position.... you do the aperture, not the camera. And because of this, metering is done when the shutter is released. Also, as you pointed out, the focussing will be harder as you stopped down the aperture since the viewfinder is darker. With the m42 lens, you can simply flip the A/M switch to keep it wide-open while you are focusing and push to M position when you are ready to trip the shutter. With the k-mount lens (no aperture), you can turn the aperture wide-open when you are focusing and then 'count' to the desired stop before tripping the shutter. In general, if you seldom go above f4.5, it won't matter.

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