Pentax Kx and Manual Lens


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Aug 19, 2005
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#1
To Pentaxian Seniors,

I recently converted from 4/3 side to the Pentax camp with the Kx, I also recently got an M42 to PK mount converter to use with my Helios 135mm. I noticed, the camera only allowed me to operate in Av and Manual mode.

I notice when using the viewfinder to compose and take the pictures, the shutter speed set by the camera is too fast and images always turn out under exposed. Where when using Live View, the shutter speed was able to correctly set the shutter speed for correct exposure.

Is this normal for all DSLR, that the metering can be so different in LV and using viewfinder?
 

elavan

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Sep 19, 2009
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AMK
#2
Hi, warm welcome :)

do you tap the green button before taking the scene? it's required for M42 lenses
 

Ben Loke

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Aug 26, 2004
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#3
With the K20D, it works with AV & Green Button.
However, I prefer AV with +0.5 or +1.0 compensation.
 

Aug 19, 2005
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#4
Hmmm, my green button is tied to digital filters.....what function should my green button be tied to?
 

Last edited:

elavan

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Sep 19, 2009
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#5
try Tv shift, if you have that option..
 

Aug 19, 2005
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#6
Yes, I have that option, but it seems the most effective method is to play with the EV compensation. Thanks for the suggestions and advice so far.
 

Jan 16, 2009
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#7
I believe green button stop-down metering only works for K or M lenses. M42 lenses do not have the aperture coupling to allow the DSLR to stop down the lens, therefore the best mode to use with M42 lenses is Av mode. You should use M mode + green button for K or M lenses.

If the M42 lens has a M or A setting, make sure that it is set to M so that when you turn the aperture ring, the aperture actually changes. While A allows you to do stop-down metering, this is not supported through the adapter, so you will end up shooting at max aperture no matter what you turn the ring to.

More details: http://www.pentaxforums.com/forums/...80739-k-x-stop-down-metering-m-42-lenses.html
 

ishou80

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Mar 9, 2008
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#8
hmmmm base on my experience the green button does work for m42 but normal it's will be under expose by a little so need up the EV like Ben said.
 

Jan 16, 2009
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#9
Well, it works because the green button returns you to Program line, and since it's not an A lense, the aperture size cannot be controlled and the green button does the same thing as Av mode. The M42 lens aperture isn't actually stopped down when you press the green button. You should be able to hear the aperture blades close and open again if stop-down metering is actually being done. If stop-down metering is not being done, you should just use Av instead of M+green and save yourself the trouble of having to press the green button .
 

azul123

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Dec 4, 2004
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#10
In LV mode, does Kx not have option to view histogram on screen?

../azul123
 

andros_lee

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Aug 19, 2005
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#11
Hi Pentaxian Seniors,

Thanks for the advice so far, I have bought two Helios M42 lenses so far and am very happy with the results and quality. After playing with EV compensation a few times, I am getting used to it.

The strange thing is that I actually find it easier to use the M42 on a GF1....erm is mentioning of panny products taboo here? Sorry not really a pentax purist yet, forgive me.:D
 

Baracus

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Mar 24, 2008
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#12
The strange thing is that I actually find it easier to use the M42 on a GF1....erm is mentioning of panny products taboo here? Sorry not really a pentax purist yet, forgive me.:D
Nah. Some of us have "mistresses" as well. :bsmilie:

Just use whatever system that fits your shooting style. ;)
 

hamustar

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May 23, 2007
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#13
Actually green button(on K20D which i had, i assume it's same throughout) works in 'M' mode for M42 lenses as well. The only difference is that there is no "shzz shzz" sound coz there is no stop-down mechanism unlike Pentax-K lenses(m42 u stop down manually using the aperture ring on the lens), but the camera takes an accurate metering all the same.
 

andros_lee

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Aug 19, 2005
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#15
Actually green button(on K20D which i had, i assume it's same throughout) works in 'M' mode for M42 lenses as well. The only difference is that there is no "shzz shzz" sound coz there is no stop-down mechanism unlike Pentax-K lenses(m42 u stop down manually using the aperture ring on the lens), but the camera takes an accurate metering all the same.
In manual mode I press the button. Nothing happens. Maybe it's my settings?
 

Jan 16, 2009
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#16
The purpose of M mode with K/M lens is for the stop-down metering. Sure, you can use M mode without stop-down metering for M42 lenses, but it's just a waste of time and should be using Av mode instead.
 

andros_lee

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Aug 19, 2005
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#18
I dun have any nice pics to show. Its stuck at f1.8 as there is a catch to allow stop down which i dun have a compatible body to activate.
 

Jan 16, 2009
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#19
Ok, let me try to explain what's going on with the Av and M modes when you meter with K, M or M42 lenses.

When you set the aperture ring on a K or M lens, or M42 lens in 'Auto' mode, the physical aperture of the lens remains at maximum aperture (e.g. f/1.4 for a f/1.4 lens), and is not immediately stopped down. There is a mechanism for the camera to tell the lens to stop down, and this is done in two cases: When you release the shutter to take the shot, and in M mode when you press the green button to meter.

So what happens if you use a K or M lens in Av mode? The camera will meter at maximum aperture, but when you take the shot, the camera will stop down the lens aperture down to whatever is set on the aperture ring (e.g. to f/2.8), before taking the shot. You would expect this to lead to the shot being underexposed because the camera is expecting more light (from an f/1.4 aperture) to come in but it's only getting 25% of the light because the aperture gets reduced to f/2.8... however I discovered something strange from my testings. The shots are correctly exposed AND I can see through the lens that the aperture has been stopped down AND the shutter speed is correct for the lens being wide open. My conclusion is that in Av mode, the shutter is actually released BEFORE the aperture is stopped down (or after it has reopened), which means that in Av mode, no matter what you set the aperture ring to, you will ALWAYS be shooting wide open.

If you use a K/M lens in M mode, the camera does not meter at all, until you press the green button. When you press the green button, the camera then stops the lens aperture down (to f/2.8) and then take a meter reading, then sets (and locks) the shutter speed and ISO. When you shoot, the aperture will be stopped down to f/2.8 and the shot should be correctly exposed.

If you use an 'Auto' M42 lens in whichever mode, no matter what you do the lens aperture will always be stuck at f/1.4 because the camera is not an M42 camera and doesn't have the necessary mechanism to stop down the lens aperture.

If you use a 'Manual' M42 lens, then whenever you turn the aperture ring, the lens aperture will open or close depending on the setting on the aperture ring. If you set to Av mode, then the camera will continously meter correctly all the time and adjust the ISO and shutter speed. In M mode, the camera will not meter at all until you press the green button, then a meter reading will be taken to set and lock the ISO and shutter speed. No stopping down of the aperture is done so you will not hear the "shzz shzz" sound. In effect the green button acts like the AE-L button.


Conclusion: If you are using K or M lens, use M mode with the green button. If using M42 lens, use Av mode.
 

Last edited:

sebianos

New Member
Aug 23, 2005
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#20
Ok, let me try to explain what's going on with the Av and M modes when you meter with K, M or M42 lenses.

When you set the aperture ring on a K or M lens, or M42 lens in 'Auto' mode, the physical aperture of the lens remains at maximum aperture (e.g. f/1.4 for a f/1.4 lens), and is not immediately stopped down. There is a mechanism for the camera to tell the lens to stop down, and this is done in two cases: When you release the shutter to take the shot, and in M mode when you press the green button to meter.

So what happens if you use a K or M lens in Av mode? The camera will meter at maximum aperture, but when you take the shot, the camera will stop down the lens aperture down to whatever is set on the aperture ring (e.g. to f/2.8), before taking the shot. You would expect this to lead to the shot being underexposed because the camera is expecting more light (from an f/1.4 aperture) to come in but it's only getting 25% of the light because the aperture gets reduced to f/2.8... however I discovered something strange from my testings. The shots are correctly exposed AND I can see through the lens that the aperture has been stopped down AND the shutter speed is correct for the lens being wide open. My conclusion is that in Av mode, the shutter is actually released BEFORE the aperture is stopped down (or after it has reopened), which means that in Av mode, no matter what you set the aperture ring to, you will ALWAYS be shooting wide open.

If you use a K/M lens in M mode, the camera does not meter at all, until you press the green button. When you press the green button, the camera then stops the lens aperture down (to f/2.8) and then take a meter reading, then sets (and locks) the shutter speed and ISO. When you shoot, the aperture will be stopped down to f/2.8 and the shot should be correctly exposed.

If you use an 'Auto' M42 lens in whichever mode, no matter what you do the lens aperture will always be stuck at f/1.4 because the camera is not an M42 camera and doesn't have the necessary mechanism to stop down the lens aperture.

If you use a 'Manual' M42 lens, then whenever you turn the aperture ring, the lens aperture will open or close depending on the setting on the aperture ring. If you set to Av mode, then the camera will continously meter correctly all the time and adjust the ISO and shutter speed. In M mode, the camera will not meter at all until you press the green button, then a meter reading will be taken to set and lock the ISO and shutter speed. No stopping down of the aperture is done so you will not hear the "shzz shzz" sound. In effect the green button acts like the AE-L button.


Conclusion: If you are using K or M lens, use M mode with the green button. If using M42 lens, use Av mode.
Thank you, Wabbit ! I'm going to print this out tomorrow, burn it and add to water to drink. Should be an expert by friday night. :bsmilie:

Need some time to digest this and test it in practical.
 

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