Why did pentax do away with auto aperture with introduction of K mount?

geekette · 05-31-2011, 01:47 PM

.. I mean, they had auto aperture with the M42 lenses.. Why weren't the Pentax-M and Pentax K auto aperture capable as well?

dgaies · 05-31-2011, 01:52 PM

Originally posted by geekette

.. I mean, they had auto aperture with the M42 lenses.. Why weren't the Pentax-M and Pentax K auto aperture capable as well?

I could be wrong, but I think they are so long as you use them on a non-crippled mount.

Blue · 05-31-2011, 02:00 PM

Originally posted by geekette

.. I mean, they had auto aperture with the M42 lenses.. Why weren't the Pentax-M and Pentax K auto aperture capable as well?

The M and k were auto aperture. They worked via the lever rather than the pin used on the m42. In fact, they work with open aperture metering on pentax k-mount bodies. The meaning of A on A series lens has a different meaning in that it allows for bodies to set the aperture when in shutter priority or program mode, and of course require an A series body to take advantage of this.

Features and Operation of the Ka Mount

http://kmp.bdimitrov.de/technology/K-mount/K.html

icywindow · 05-31-2011, 02:05 PM

I'm under the impression that "Auto" on M42 lenses dealt with being able to see what you were looking at to frame an object without the darkening of a closed aperture, then the camera actually closing the aperture when you took the shot. If that's what "Auto" indeed did mean on M42 mounting, I believe K mount lenses are by definition, "Auto".

Someone chime in to correct me if I'm wrong.

Blue · 05-31-2011, 02:10 PM

Originally posted by icywindow

I'm under the impression that "Auto" on M42 lenses dealt with being able to see what you were looking at to frame an object without the darkening of a closed aperture, then the camera actually closing the aperture when you took the shot. If that's what "Auto" indeed did mean on M42 mounting, I believe K mount lenses are by definition, "Auto".

Someone chime in to correct me if I'm wrong.

That is basically accurate. None of the pre-spotmatic bodies could do open aperture metering and in fact, only the Elecro Spotmatic, ES, ESII and Spotmatic F could do open aperture metering if they were used with a compatible lens.

**nater** · 05-31-2011, 02:53 PM

Originally posted by geekette

.. I mean, they had auto aperture with the M42 lenses.. Why weren't the Pentax-M and Pentax K auto aperture capable as well?

M42 auto aperture and KA-mount auto aperture are different things. K-mount lenses like the M and K series actually work just like auto M42 auto lenses with regards to aperture (as I understand things). For detail on how KA-mount works (and differs from all previous mounts), Bojidar Dimitrov has a good summary in his "Features and Operation of the KA Mount".

Incidentally I noticed that he didn't refer to an auto mode for KA-mount lenses (just an 'A' setting) on that page, so I went and checked the Pentax Super Program manual and they do call it an Auto setting, i.e. from page 3: "Rotate the aperture ring on your 'A' series lens to the Auto position (indexed in green), while depressing the aperture auto lock button."

I can understand how such similar terminology could be confusing, but as you can see, KA-mount adds a significant amount of additional functionality that is not present in the older K-mount or M42 auto mount.

geekette · 05-31-2011, 03:16 PM

Ok I understand, auto in the sense that you can look through the lens wide open while at a higher f stop setting.. the camera automatically closes up the iris, snaps it and opens it back up.

Thanks for clearing that up for me.

So the "AUTO" on my M42 Tokina 400mm lens (switchable between A and M) means it has capability to automatically have supported camera close up the iris when snapping and opening it back to wide open. My K-X doesn't support this AUTO since it can't control the pin so I guess I am limited to: manual M42 lenses, switchable, or if I get an AUTO only lens, I have to keep that pin pressed in somehow.

So in the old days, all SLR's were dim through the viewfinder the higher the f-stop. Hey if they could get by doing that then, so can I with m42 lens on my K-X I guess

Last edited by geekette; 05-31-2011 at 03:28 PM.

geekette · 05-31-2011, 03:30 PM

One thing I notice though, when I use a manual M42 on my K-X in Av mode, it underexposes for some reason. I don't understand since it should be able to meter accurately since it's all through the lens.

SOldBear · 05-31-2011, 03:35 PM

Originally posted by geekette

...when I use a manual M42 on my K-X in Av mode, it underexposes for some reason.

The metering problem with manual lenses (pK and M42) of Pentax K-series DSLRs is well-known.

Depending on the lens, it can be over- or underexposure.

You have to learn the metering character of each lens and compensate accordingly.

Read this recent thread on the topic.

**boriscleto** · 05-31-2011, 03:40 PM

Originally posted by SOldBear

The metering problem with manual lenses (pK and M42) of Pentax K-series DSLRs is well-known.

Depending on the lens, it can be over- or underexposure.

You have to learn the metering character of each lens and compensate accordingly.

Or use it in M mode and use the AV+- button.

mtansley · 05-31-2011, 04:51 PM

Originally posted by geekette

So in the old days, all SLR's were dim through the viewfinder the higher the f-stop. Hey if they could get by doing that then, so can I with m42 lens on my K-X I guess

Actually, I have a 400mm Lentar M42 lens and it has two aperture dials, one has click stops and right behind it is another one that is smooth running. You'd set your aperture on the click stop one and you could still open the aperture wide for focusing by turning the other dial fully open, once you'd focused you just had to remember to turn the smooth dial back to where it would stop, at that point the aperture would be back to where you'd set it for exposure.

If that makes sense!!

Blue · 05-31-2011, 05:00 PM

Originally posted by mtansley

Actually, I have a 400mm Lentar M42 lens and it has two aperture dials, one has click stops and right behind it is another one that is smooth running. You'd set your aperture on the click stop one and you could still open the aperture wide for focusing by turning the other dial fully open, once you'd focused you just had to remember to turn the smooth dial back to where it would stop, at that point the aperture would be back to where you'd set it for exposure.

If that makes sense!!

That type of aperture diaphragm is referred to as a pre-set diaphragm, hence preset lens.

mtansley · 05-31-2011, 05:04 PM

Originally posted by Blue

That type of aperture diaphragm is referred to as a pre-set diaphragm, hence preset lens.

Now that I didn't know.

I imagine they date from quite a few years ago.

I haven't a clue how old my lens is. The IQ isn't great so I imagine that brand was probably quite a cheap brand.

RioRico · 05-31-2011, 05:18 PM

Originally posted by mtansley

Actually, I have a 400mm Lentar M42 lens and it has two aperture dials, one has click stops and right behind it is another one that is smooth running.

This type of aperture is called a PRESET. Pentax made a few presets, as did others. Many presets also have many aperture blades, for smooth bokeh, and these are what I lust after now. My two-ring presets are:

Mir-1 37/2.8 (M42, only 8 iris blades)
Macro-Takumar 50/4 (1:1, M42, 8 blades)
Jupiter-9 85/2 (M39, 15 iris blades)
Opticam 135/2.8 (T2, 10 iris blades)
Hanimar Preset 135/3.5 (M42, 13 blades)
Jupiter-11 135/4 (M39, 11 iris blades)
Tele-Takumar 200/5.6 (M42, 10 blades)

I have sold exactly one preset, because of too much PF. Knowing what I do now, I should have kept it:

Sunset Telephoto 135/2.8 (M42, 8 iris blades)

As for exposure variance: some non-A-type manual lenses meter erratically on my K20D, and some don't. It's a crap-shoot. And some meter erratically due to Operator Error, ie I focked up and let the meter-select switch slide between positions. Oops.

geekette · 05-31-2011, 06:15 PM

Originally posted by boriscleto

Or use it in M mode and use the AV+- button.

With m42 lens, in manual mode, the +/- button doesn't meter. If a Pentax-M or Pentax K lens is in it and I press the +/- it does stop down metering fine. I cannot get the camera to meter through the m42 lens in manual mode at all. So I make a guess and snap and look at histo.. it's wasteful but the best I can do atm.

05-31-2011, 01:47 PM	#1
geekette Veteran Member Join Date: Apr 2011 Location: Texas Photos: Gallery \| Albums Posts: 528	Why did pentax do away with auto aperture with introduction of K mount? .. I mean, they had auto aperture with the M42 lenses.. Why weren't the Pentax-M and Pentax K auto aperture capable as well?

05-31-2011, 02:00 PM	#3
Blue Moderator Site Supporter Join Date: Jun 2008 Location: Florida Hill Country Photos: Gallery \| Albums Posts: 17,377	Originally posted by geekette .. I mean, they had auto aperture with the M42 lenses.. Why weren't the Pentax-M and Pentax K auto aperture capable as well? The M and k were auto aperture. They worked via the lever rather than the pin used on the m42. In fact, they work with open aperture metering on pentax k-mount bodies. The meaning of A on A series lens has a different meaning in that it allows for bodies to set the aperture when in shutter priority or program mode, and of course require an A series body to take advantage of this. Features and Operation of the Ka Mount http://kmp.bdimitrov.de/technology/K-mount/K.html Last edited by Blue; 05-31-2011 at 02:08 PM.

05-31-2011, 03:35 PM	#9
SOldBear Veteran Member Join Date: Feb 2009 Location: Alameda, CA Posts: 3,206	Originally posted by geekette ...when I use a manual M42 on my K-X in Av mode, it underexposes for some reason. The metering problem with manual lenses (pK and M42) of Pentax K-series DSLRs is well-known. Depending on the lens, it can be over- or underexposure. You have to learn the metering character of each lens and compensate accordingly. Read this recent thread on the topic. Last edited by SOldBear; 05-31-2011 at 03:42 PM.

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05-31-2011, 01:52 PM	#2
dgaies Veteran Member Join Date: Oct 2009 Location: Maryland / Washington DC Posts: 3,917	Originally posted by geekette .. I mean, they had auto aperture with the M42 lenses.. Why weren't the Pentax-M and Pentax K auto aperture capable as well? I could be wrong, but I think they are so long as you use them on a non-crippled mount.

05-31-2011, 02:05 PM	#4
icywindow Veteran Member Join Date: Mar 2011 Photos: Gallery \| Albums Posts: 303	I'm under the impression that "Auto" on M42 lenses dealt with being able to see what you were looking at to frame an object without the darkening of a closed aperture, then the camera actually closing the aperture when you took the shot. If that's what "Auto" indeed did mean on M42 mounting, I believe K mount lenses are by definition, "Auto". Someone chime in to correct me if I'm wrong.

05-31-2011, 02:10 PM	#5
Blue Moderator Site Supporter Join Date: Jun 2008 Location: Florida Hill Country Photos: Gallery \| Albums Posts: 17,377	Originally posted by icywindow I'm under the impression that "Auto" on M42 lenses dealt with being able to see what you were looking at to frame an object without the darkening of a closed aperture, then the camera actually closing the aperture when you took the shot. If that's what "Auto" indeed did mean on M42 mounting, I believe K mount lenses are by definition, "Auto". Someone chime in to correct me if I'm wrong. That is basically accurate. None of the pre-spotmatic bodies could do open aperture metering and in fact, only the Elecro Spotmatic, ES, ESII and Spotmatic F could do open aperture metering if they were used with a compatible lens.

05-31-2011, 02:53 PM	#6
nater Loyal Site Supporter Join Date: Aug 2009 Location: New England Photos: Albums Posts: 706	Originally posted by geekette .. I mean, they had auto aperture with the M42 lenses.. Why weren't the Pentax-M and Pentax K auto aperture capable as well? M42 auto aperture and KA-mount auto aperture are different things. K-mount lenses like the M and K series actually work just like auto M42 auto lenses with regards to aperture (as I understand things). For detail on how KA-mount works (and differs from all previous mounts), Bojidar Dimitrov has a good summary in his "Features and Operation of the KA Mount". Incidentally I noticed that he didn't refer to an auto mode for KA-mount lenses (just an 'A' setting) on that page, so I went and checked the Pentax Super Program manual and they do call it an Auto setting, i.e. from page 3: "Rotate the aperture ring on your 'A' series lens to the Auto position (indexed in green), while depressing the aperture auto lock button." I can understand how such similar terminology could be confusing, but as you can see, KA-mount adds a significant amount of additional functionality that is not present in the older K-mount or M42 auto mount.

05-31-2011, 03:16 PM	#7
geekette Veteran Member Join Date: Apr 2011 Location: Texas Photos: Gallery \| Albums Posts: 528 Original Poster	Ok I understand, auto in the sense that you can look through the lens wide open while at a higher f stop setting.. the camera automatically closes up the iris, snaps it and opens it back up. Thanks for clearing that up for me. So the "AUTO" on my M42 Tokina 400mm lens (switchable between A and M) means it has capability to automatically have supported camera close up the iris when snapping and opening it back to wide open. My K-X doesn't support this AUTO since it can't control the pin so I guess I am limited to: manual M42 lenses, switchable, or if I get an AUTO only lens, I have to keep that pin pressed in somehow. So in the old days, all SLR's were dim through the viewfinder the higher the f-stop. Hey if they could get by doing that then, so can I with m42 lens on my K-X I guess Last edited by geekette; 05-31-2011 at 03:28 PM.

05-31-2011, 03:30 PM	#8
geekette Veteran Member Join Date: Apr 2011 Location: Texas Photos: Gallery \| Albums Posts: 528 Original Poster	One thing I notice though, when I use a manual M42 on my K-X in Av mode, it underexposes for some reason. I don't understand since it should be able to meter accurately since it's all through the lens.

05-31-2011, 03:40 PM	#10
boriscleto Loyal Site Supporter Join Date: Dec 2009 Location: North Syracuse, NY Photos: Gallery Posts: 16,475	Originally posted by SOldBear The metering problem with manual lenses (pK and M42) of Pentax K-series DSLRs is well-known. Depending on the lens, it can be over- or underexposure. You have to learn the metering character of each lens and compensate accordingly. Or use it in M mode and use the AV+- button.

05-31-2011, 04:51 PM	#11
mtansley Veteran Member Join Date: Oct 2008 Location: Ajax, Ontario, Canada Photos: Gallery Posts: 1,139	Originally posted by geekette So in the old days, all SLR's were dim through the viewfinder the higher the f-stop. Hey if they could get by doing that then, so can I with m42 lens on my K-X I guess Actually, I have a 400mm Lentar M42 lens and it has two aperture dials, one has click stops and right behind it is another one that is smooth running. You'd set your aperture on the click stop one and you could still open the aperture wide for focusing by turning the other dial fully open, once you'd focused you just had to remember to turn the smooth dial back to where it would stop, at that point the aperture would be back to where you'd set it for exposure. If that makes sense!!

05-31-2011, 05:00 PM	#12
Blue Moderator Site Supporter Join Date: Jun 2008 Location: Florida Hill Country Photos: Gallery \| Albums Posts: 17,377	Originally posted by mtansley Actually, I have a 400mm Lentar M42 lens and it has two aperture dials, one has click stops and right behind it is another one that is smooth running. You'd set your aperture on the click stop one and you could still open the aperture wide for focusing by turning the other dial fully open, once you'd focused you just had to remember to turn the smooth dial back to where it would stop, at that point the aperture would be back to where you'd set it for exposure. If that makes sense!! That type of aperture diaphragm is referred to as a pre-set diaphragm, hence preset lens.

05-31-2011, 05:04 PM	#13
mtansley Veteran Member Join Date: Oct 2008 Location: Ajax, Ontario, Canada Photos: Gallery Posts: 1,139	Originally posted by Blue That type of aperture diaphragm is referred to as a pre-set diaphragm, hence preset lens. Now that I didn't know. I imagine they date from quite a few years ago. I haven't a clue how old my lens is. The IQ isn't great so I imagine that brand was probably quite a cheap brand.

05-31-2011, 05:18 PM	#14
RioRico Veteran Member Join Date: Jun 2008 Location: Limbo, California Posts: 11,263	Originally posted by mtansley Actually, I have a 400mm Lentar M42 lens and it has two aperture dials, one has click stops and right behind it is another one that is smooth running. This type of aperture is called a PRESET. Pentax made a few presets, as did others. Many presets also have many aperture blades, for smooth bokeh, and these are what I lust after now. My two-ring presets are: Mir-1 37/2.8 (M42, only 8 iris blades) Macro-Takumar 50/4 (1:1, M42, 8 blades) Jupiter-9 85/2 (M39, 15 iris blades) Opticam 135/2.8 (T2, 10 iris blades) Hanimar Preset 135/3.5 (M42, 13 blades) Jupiter-11 135/4 (M39, 11 iris blades) Tele-Takumar 200/5.6 (M42, 10 blades) I have sold exactly one preset, because of too much PF. Knowing what I do now, I should have kept it: Sunset Telephoto 135/2.8 (M42, 8 iris blades) As for exposure variance: some non-A-type manual lenses meter erratically on my K20D, and some don't. It's a crap-shoot. And some meter erratically due to Operator Error, ie I focked up and let the meter-select switch slide between positions. Oops.

05-31-2011, 06:15 PM	#15
geekette Veteran Member Join Date: Apr 2011 Location: Texas Photos: Gallery \| Albums Posts: 528 Original Poster	Originally posted by boriscleto Or use it in M mode and use the AV+- button. With m42 lens, in manual mode, the +/- button doesn't meter. If a Pentax-M or Pentax K lens is in it and I press the +/- it does stop down metering fine. I cannot get the camera to meter through the m42 lens in manual mode at all. So I make a guess and snap and look at histo.. it's wasteful but the best I can do atm.