Adapter Ring to convert Pentax K to M42 (yes that's right)

Kingman · 09-08-2021, 02:51 PM

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Anyone ever hear of an adapter than converts a Pentax K lens to fit into a M42 body or accessory like the Pentax Bellows II. No, I do not have to focus at infinity and yes, that is correct, an adapter ring that converts K to M42, not the other way (I already have plenty of those adapter rings)

**pres589** · 09-08-2021, 03:01 PM

Thinking about a K to M42 adapter ring, like the one from Pentax (but it does not matter the brand) the k-mount flange has to be of a larger diameter than the M42 mount's diameter. You would have to remove material from the k-mount lens to get it inside the bore of the M42 mount.

**seventhdr** · 09-08-2021, 03:01 PM

That is difficult, as the diameter of the K-mount is bigger than the M42 mount, so a K-mount lens will not fit inside the mount of a M42 screw mount camera.

The reverse works well, that is M42 to K-mount as the smaller M42 mount fits inside the K-mount opening.

Regards

Chris

**pres589** · 09-08-2021, 03:04 PM

In theory, with money and design tools and talent, I would think you could take the mount flange off of the lens, model it up in some 3d modeling software, then change the k-mount flange aspects to be an M42 screw. Then you send the file out to an online job shop that does custom machined parts based on CAD files. Since the distance from flange to sensor/film is the same between K and M42 that dimension shouldn't change or need tweaking to make the lens work to infinity.

You would only want to do this with a lens that has an aperture ring, or you'd be shooting wide open at all times. But this should be doable. It's a bit like a Leitax mount adapter.

**UncleVanya** · 09-08-2021, 03:26 PM - **1 Like**

Originally posted by Kingman

Anyone ever hear of an adapter than converts a Pentax K lens to fit into a M42 body or accessory like the Pentax Bellows II. No, I do not have to focus at infinity and yes, that is correct, an adapter ring that converts K to M42, not the other way (I already have plenty of those adapter rings)

Pushing the lens out as far as it would have to be would likely make it macro only. Is that sufficient? There are some k Mount extension tubes that consist of a front and rear mount and tubes that screw together. When you remove the front Mount ring you could work out a thread adapter to connect that part to the m42 Mount I’d guess. The minimum amount of extension isn’t clear but I’d guess more than 12mm.

**pepperberry farm** · 09-08-2021, 03:27 PM - **2 Likes**

or the unasked question: why?

**UncleVanya** · 09-08-2021, 03:35 PM - **1 Like**

A set like this is what I think would work to form the k Mount (lens) side of the adapter.

Macro Extension Tube Ring Set for All Pentax PK K Mount K-5II K-5IIs DSLR Camera | eBay

---------- Post added 09-08-21 at 06:35 PM ----------

Originally posted by pepperberry farm

or the unasked question: why?

I can think of one use; M42 bellows.

**Just1MoreDave** · 09-08-2021, 03:36 PM

Maybe a short M42 extension tube and the mount from a broken K-mount camera? You'll probably need a third piece because the extension tube diameter won't be big enough to mount the camera mount piece to it. The third piece could be plastic (like UHMW) which could be made with simple tools, even careful filing.

**pepperberry farm** · 09-08-2021, 03:36 PM

Originally posted by UncleVanya

I can think of one use; M42 bellows.

gotcha....

I was thinking of what lens would need this set up....?

stevebrot · 09-08-2021, 04:07 PM

Originally posted by Kingman

Anyone ever hear of an adapter than converts a Pentax K lens to fit into a M42 body or accessory like the Pentax Bellows II. No, I do not have to focus at infinity and yes, that is correct, an adapter ring that converts K to M42, not the other way (I already have plenty of those adapter rings)

No, I have not, but you are not the first to ask. To be honest, I have not run across such a converter.

What, precisely, is your use case? It helps if we know exactly what you are trying to do and why using a K-mount lens is compelling. After all, very high quality M42 lenses are readily available at accessible price points.

Steve

Not a Number · 09-08-2021, 04:14 PM - **1 Like**

There isn't such an adapter available off the shelf but PrecisionParts.com will custom machine one for you at $208 USD plus shipping. Minimum effective length would be 16mm. You may need to go longer for the aperture lever clearance (at additional cost).

The price has gone up. Last time this question came up I think it was ~$150 USD.

**TwoUptons** · 09-08-2021, 04:15 PM

As I recall, there used to be such an adapter sold back in the day... it was for, wait for it, mounting K-mount lenses on M42 bellows..

I've seen them for sale once or twice. The first time I saw one I thought "who would ever need that?"
Then, when I wanted to mount a K-Mount lens on a M42 bellows, I couldn't find one.

At some point, I gave up looking when I found a good deal on a K-mount bellows... and promptly saw another one online... :/

You could work one up in resin with a 3D printer. I've had decent luck with M42 and K adapters (not together, for other things) in resin like that.

-Eric

Kingman · 09-08-2021, 05:03 PM

While I have a vast library of M42 Pentax lenses and the Super Takumar Bellows 100mm/f4.0 M42, I want to be able also to mount any of my library of Pentax K lenses on my Pentax Bellow Unit II (M42) for macro work and experimentation.

I like the idea of sacrificing the shorter length extension rings of the M42 and K mount sets and perhaps gluing/super glue them together. I'll have to dig out and find my Pentax K mount extension tube set and check it out. I have plenty of the Pentax M42 extension tube sets.

**K-Three** · 09-08-2021, 06:12 PM

use the K/M lens reversed?
Get an M42 to 49 or 52mm filter thread adaptor, and then step up or step down rings as needed for the lens you want to use?

**UncleVanya** · 09-08-2021, 07:23 PM

Originally posted by K-Three

use the K/M lens reversed?
Get an M42 to 49 or 52mm filter thread adaptor, and then step up or step down rings as needed for the lens you want to use?

That’s a better option honestly.

---------- Post added 09-08-21 at 10:26 PM ----------

Originally posted by Kingman

I like the idea of sacrificing the shorter length extension rings of the M42 and K mount sets and perhaps gluing/super glue them together. I'll have to dig out and find my Pentax K mount extension tube set and check it out. I have plenty of the Pentax M42 extension tube sets.

The specific ones I mentioned have the easier to assemble option. Another option is to make an adapter that goes into the bellows - my bellows has removable mounts rings.

09-08-2021, 02:51 PM	#1
Kingman Senior Member Join Date: Feb 2017 Location: North Texas Posts: 199	Adapter Ring to convert Pentax K to M42 (yes that's right) Staff note: This post may contain affiliate links, which means Pentax Forums may earn a small commission if a visitor clicks through and makes a purchase. If you would like to support the forum directly, you may also make a donation here. Anyone ever hear of an adapter than converts a Pentax K lens to fit into a M42 body or accessory like the Pentax Bellows II. No, I do not have to focus at infinity and yes, that is correct, an adapter ring that converts K to M42, not the other way (I already have plenty of those adapter rings)

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09-08-2021, 03:01 PM	#2
pres589 Site Supporter Join Date: Jan 2014 Location: Wichita, KS Photos: Albums Posts: 4,529	Thinking about a K to M42 adapter ring, like the one from Pentax (but it does not matter the brand) the k-mount flange has to be of a larger diameter than the M42 mount's diameter. You would have to remove material from the k-mount lens to get it inside the bore of the M42 mount.

09-08-2021, 03:01 PM	#3
seventhdr Site Supporter Join Date: Mar 2007 Location: Christchurch, New Zealand Photos: Gallery Posts: 1,440	That is difficult, as the diameter of the K-mount is bigger than the M42 mount, so a K-mount lens will not fit inside the mount of a M42 screw mount camera. The reverse works well, that is M42 to K-mount as the smaller M42 mount fits inside the K-mount opening. Regards Chris

09-08-2021, 03:04 PM	#4
pres589 Site Supporter Join Date: Jan 2014 Location: Wichita, KS Photos: Albums Posts: 4,529	In theory, with money and design tools and talent, I would think you could take the mount flange off of the lens, model it up in some 3d modeling software, then change the k-mount flange aspects to be an M42 screw. Then you send the file out to an online job shop that does custom machined parts based on CAD files. Since the distance from flange to sensor/film is the same between K and M42 that dimension shouldn't change or need tweaking to make the lens work to infinity. You would only want to do this with a lens that has an aperture ring, or you'd be shooting wide open at all times. But this should be doable. It's a bit like a Leitax mount adapter.

09-08-2021, 03:26 PM - 1 Like	#5
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,401	Originally posted by Kingman Anyone ever hear of an adapter than converts a Pentax K lens to fit into a M42 body or accessory like the Pentax Bellows II. No, I do not have to focus at infinity and yes, that is correct, an adapter ring that converts K to M42, not the other way (I already have plenty of those adapter rings) Pushing the lens out as far as it would have to be would likely make it macro only. Is that sufficient? There are some k Mount extension tubes that consist of a front and rear mount and tubes that screw together. When you remove the front Mount ring you could work out a thread adapter to connect that part to the m42 Mount I’d guess. The minimum amount of extension isn’t clear but I’d guess more than 12mm.

09-08-2021, 03:27 PM - 2 Likes	#6
pepperberry farm amateur dirt farmer Join Date: Dec 2014 Location: probably out in a field somewhere... Photos: Gallery \| Albums Posts: 41,660	or the unasked question: why?

09-08-2021, 03:35 PM - 1 Like	#7
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,401	A set like this is what I think would work to form the k Mount (lens) side of the adapter. Macro Extension Tube Ring Set for All Pentax PK K Mount K-5II K-5IIs DSLR Camera \| eBay ---------- Post added 09-08-21 at 06:35 PM ---------- Originally posted by pepperberry farm or the unasked question: why? I can think of one use; M42 bellows.

09-08-2021, 03:36 PM	#8
Just1MoreDave Site Supporter Join Date: Feb 2007 Location: Aurora, CO Photos: Gallery \| Albums Posts: 6,340	Maybe a short M42 extension tube and the mount from a broken K-mount camera? You'll probably need a third piece because the extension tube diameter won't be big enough to mount the camera mount piece to it. The third piece could be plastic (like UHMW) which could be made with simple tools, even careful filing.

09-08-2021, 03:36 PM	#9
pepperberry farm amateur dirt farmer Join Date: Dec 2014 Location: probably out in a field somewhere... Photos: Gallery \| Albums Posts: 41,660	Originally posted by UncleVanya I can think of one use; M42 bellows. gotcha.... I was thinking of what lens would need this set up....?

09-08-2021, 04:07 PM	#10
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by Kingman Anyone ever hear of an adapter than converts a Pentax K lens to fit into a M42 body or accessory like the Pentax Bellows II. No, I do not have to focus at infinity and yes, that is correct, an adapter ring that converts K to M42, not the other way (I already have plenty of those adapter rings) No, I have not, but you are not the first to ask. To be honest, I have not run across such a converter. What, precisely, is your use case? It helps if we know exactly what you are trying to do and why using a K-mount lens is compelling. After all, very high quality M42 lenses are readily available at accessible price points. Steve

09-08-2021, 04:14 PM - 1 Like	#11
Not a Number Moderator Join Date: Mar 2012 Location: Venice, CA Posts: 10,526	There isn't such an adapter available off the shelf but PrecisionParts.com will custom machine one for you at $208 USD plus shipping. Minimum effective length would be 16mm. You may need to go longer for the aperture lever clearance (at additional cost). The price has gone up. Last time this question came up I think it was ~$150 USD.

09-08-2021, 04:15 PM	#12
TwoUptons Loyal Site Supporter Join Date: Nov 2015 Photos: Albums Posts: 4,213	As I recall, there used to be such an adapter sold back in the day... it was for, wait for it, mounting K-mount lenses on M42 bellows.. I've seen them for sale once or twice. The first time I saw one I thought "who would ever need that?" Then, when I wanted to mount a K-Mount lens on a M42 bellows, I couldn't find one. At some point, I gave up looking when I found a good deal on a K-mount bellows... and promptly saw another one online... :/ You could work one up in resin with a 3D printer. I've had decent luck with M42 and K adapters (not together, for other things) in resin like that. -Eric

09-08-2021, 05:03 PM	#13
Kingman Senior Member Join Date: Feb 2017 Location: North Texas Posts: 199 Original Poster	While I have a vast library of M42 Pentax lenses and the Super Takumar Bellows 100mm/f4.0 M42, I want to be able also to mount any of my library of Pentax K lenses on my Pentax Bellow Unit II (M42) for macro work and experimentation. I like the idea of sacrificing the shorter length extension rings of the M42 and K mount sets and perhaps gluing/super glue them together. I'll have to dig out and find my Pentax K mount extension tube set and check it out. I have plenty of the Pentax M42 extension tube sets.

09-08-2021, 06:12 PM	#14
K-Three Loyal Site Supporter Join Date: May 2014 Location: Pugetopolis, WA Photos: Gallery Posts: 938	use the K/M lens reversed? Get an M42 to 49 or 52mm filter thread adaptor, and then step up or step down rings as needed for the lens you want to use?

09-08-2021, 07:23 PM	#15
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,401	Originally posted by K-Three use the K/M lens reversed? Get an M42 to 49 or 52mm filter thread adaptor, and then step up or step down rings as needed for the lens you want to use? That’s a better option honestly. ---------- Post added 09-08-21 at 10:26 PM ---------- Originally posted by Kingman I like the idea of sacrificing the shorter length extension rings of the M42 and K mount sets and perhaps gluing/super glue them together. I'll have to dig out and find my Pentax K mount extension tube set and check it out. I have plenty of the Pentax M42 extension tube sets. The specific ones I mentioned have the easier to assemble option. Another option is to make an adapter that goes into the bellows - my bellows has removable mounts rings.