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07-31-2011, 04:14 AM   #1
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Praktica B-mount => Pentax K-mount

Hi,

Knowing that such adapter does not exist, I was surprised when I've seen CZJ Sonnar on K mount... Bought from fleebuy at risk.

I received perfectly clean lens in mint condition converted to K and mount was made in very professional manner ... and the original Praktica B came in a small plastic bag...

I did a few test shots and all looks good and infinity is there as well.

For the sake of clarity I tried digging out this subject further, but found almost nothing new.

Any idea, whether someone finally cracked this issue?

Cheers,
Alex

07-31-2011, 05:20 AM   #2
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Any lens also available in M42 mount should be at least possible.

Does the aperature linkage engage on the K mount?
07-31-2011, 05:29 AM   #3
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Hi,
I know abt M42 - isn't an issue and, probably, makes no sense to convert. There are plenty of adapters around...

The aperature linkage doe not engage to the mount.

Cheers
07-31-2011, 08:56 AM   #4
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It can't be done, that's why Praktica B mount lenses are so much cheaper than their m42 counterparts. The registration distance is shorter than either m42 or PK mounts. Here is a link of lens registration distance in descending order: Camera Mounts Sorted by Register

Hope this helps,

07-31-2011, 10:38 AM   #5
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Are you positive that it was not originally an M42 mount and they sent along a b mount with it ? As excanonfd has said b mount should not give infinity although
If the lens was available in Both M42 and B mount it might be possible or if it is a more recent lens they are offered in several mounts with different registration distances.

Last edited by erkie; 07-31-2011 at 10:48 AM.
07-31-2011, 01:21 PM   #6
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Slightly tangential: Adapting Exakta to PK isn't difficult nor destructive. Here's how:

* Remove (and save!) the index screw from the lens base
* Glue a cheap M39-M42 adapter ring around the bayonet -- I use contact cement, which is dissolved by thinner, so the mod is reversible (don't lose the screw!)
* Grind away about 0.6mm from the wide side of a cheap M42-PK adapter -- tape rough sandpaper to a very flat time, use d block of wood as a grip while grinding
* After the glue is well dried, screw-on the ground-down adapter -- keep grinding till you get infinity focus

See, this mod chops the adapter, not the lens. And one ground-down adapter can be used on several glued lenses.
07-31-2011, 02:33 PM   #7
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Hi all and thanks for replies.
It does not look like m42 original??? not sure but at the same time not keen to remove the mount...
Attached some tests taken / all with f wide open:
1. Inf

2. Appx 10 mtrs

3. Appx 1.2 mtrs via glass wall

4. Lens itself


Not a fantastic shots, but taken as tests only.

Cheers,
Alex

Last edited by alex1967; 02-18-2012 at 12:03 AM.
07-31-2011, 02:54 PM   #8
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Looks like a crackerjack of a lens to me. I wish I knew more about the particulars of this lens in Bmount. I certainly don't know this for a fact but perhaps on the sonnar they simply changed length to the lens body at the b mount to work at the new register distance as opposed to completely redesigning the lens. if that is the case it may be possible with some machining ?

Yours seems nice indeed !

Eric

07-31-2011, 03:31 PM   #9
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I've turned a Sigma Superwide in PB fit down in a lathe and fitted a PK mount, with poor results. I need to adjust it to get infinity focus, either my machining was wrong or the difference in register distance that I worked to was wrong. I suspect the latter, I think the dimension I worked to was different to the chart linked above. ( I'm away from home and haven't got the bit of paper I worked to )

It's a lot of work to adjust the register difference properly, perhaps it's easier to wait for the right lens to come along ?
08-01-2011, 03:22 AM   #10
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Hi, some info re this lens is here, but not much:

http://www.praktica-users.com/lens/blenses/czjp3.5_135.html

Mine one so far so good, I will put this lens on further trail...

Out of curiosity I’ve sent a message to the seller trying tofigure out how was it made possible, but received nothing in return so far.



Cheers
08-01-2011, 03:38 AM   #11
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Hi I found something here (for those who might beinteresting):



http://oomz.net/kadapter/



but I would not exercise it and agree M42 is easier andsafer way to go.



Cheers,

Alex
08-06-2011, 11:43 PM   #12
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Hi, I found this one on fleebuy...
CARL ZEISS JENA PRAKTICAR 1:2,4 f=35mm MC - Pentax K | eBay
if someone will be interested...

I found my sonar is fairly good for its age and price...

Attachment 99691

Last edited by alex1967; 02-18-2012 at 12:03 AM.
08-07-2011, 10:30 PM   #13
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QuoteOriginally posted by RioRico Quote
Slightly tangential: Adapting Exakta to PK isn't difficult nor destructive. Here's how:

* Remove (and save!) the index screw from the lens base
* Glue a cheap M39-M42 adapter ring around the bayonet -- I use contact cement, which is dissolved by thinner, so the mod is reversible (don't lose the screw!)
* Grind away about 0.6mm from the wide side of a cheap M42-PK adapter -- tape rough sandpaper to a very flat time, use d block of wood as a grip while grinding
* After the glue is well dried, screw-on the ground-down adapter -- keep grinding till you get infinity focus

See, this mod chops the adapter, not the lens. And one ground-down adapter can be used on several glued lenses.
RioRico,

I've been reading this how to convert Exacta mount to PK for the umpteenth time trying to figure out how you achieved infinity focus.

PK register distance = 45.46mm
Exacta/Topcon register distance = 44.7mm

Difference of 0.76mm

In the modification you have described, you have mounted an m42 to PK adapter on top of the Exacta mount. If you take a cheap, black m42 to PK adapter and grind away all of the flange so that you are left with only the m42 threaded ring that has 3 PK bayonets, and screw this onto the m39 to m42 adapter that's been glued around the Exacta lens' bayonets, the result is an Exacta lens with unchanged register distance but with a PK mount. Am I correct, so far?

Here comes the big BUT... The lens register distance is the distance between the sensor plane and the lens mount plane on the camera body. You may have an Exacta lens with a PK mount but when you mount this lens on a Pentax DSLR, you have effectively extended the lens by 0.76mm, making infinity focus impossible. I cannot see how you can achieve infinity focus without shortening the Exacta lens by 0.76mm at the back of the lens. If the modification as you have described was to be done to either Olympus or Nikon lenses, it makes perfect sense because either of the two brands have longer register distance than Pentax.

It seems obvious that you have done this modification and achieved infinity focus with an Exacta lens, please enlighten me if I have confused myself in my thought process along the way.

Thanks,
08-08-2011, 06:19 AM - 1 Like   #14
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I start with a silver narrow-flanged PK-M42 adapter, a cheap clone of the Official Pentax thingy, NOT with a black (or silver) wide-flanged no-infinity-focus adapter. Because I ground that amount from the narrow flange of the M42-PK adapter, the Exakta lens now fits closer to the sensor plane. I have reduced the extension by that amount.

Ah, the amount: I just found my micrometer, hidden in a guitar case (yes, I measure unknown strings to figure what to replace on my guitars, banjos, dulcimers, mandolins, lutes, etc). The thickness of the unmodified adapter's narrow flange seems to be 1.5mm. The ground-down adapter looks to be 0.5mm thick. (It's a cheap old micrometer that may be off by ~0.1mm.) So I've pulled in the Exakta lens from the ~45.5mm of the Pentax register, down to ~44.5mm from the sensor plane, enough that the 44.7mm register Exakta lens now focuses a smidgeon past infinity. Whew.

NOTE: The ground-down M42-PK adapter works fine on various lenses with removable index screws -- one adapter suffices for them all. But the index pin on one lens is NOT removable. I used the Dremel to cut a notch about 2mm wide and deep into the adapter, to fit over that pin. And I screwed an M39-M42 ring into the adapter and glued that over the lens bayonet. So now that is a permanent mod -- unless and until I use thinner to dissolve the glue and return the lens to its pristine state.

I hope this clarifies the procedure.
08-08-2011, 10:30 AM   #15
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Thanks Rio, if I see an Exacta or Praktica B lens cheap, I will definitely give this modification a try.
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