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Instructions for how you rebuild a Prakticar 2,8/135 MC Pentacon B mount to Pentax K mount.

Warning: This walkthrough involves disassembling a lens. Proceed at your own risk.

What you need:

First you have to have a K-bayonet. I took my K-bayonet from an old extension tube.
Three M2 screws with conical head apox. 6mm long. You can try to reuse the tre screws that holds the the ring below the bayonet.
One 4mm end mill.
Table drilling machine.
2 mm drill.
Small countersink.
95% alcohol for milling and drilling (do not drink it).
A fine metal file.
Flat screwdrivers of small dimensions.
A Prakticar 2,8/135 MC lens with Pentacon B bayonet.

Remove the Pentagon B bayonet and the upper part (picture 2a, 2b and 2c) from the lens. You need to remove all that has to do with the electrical contacts and wires. Are not needed.
Be careful that you don't lose the small ball inside the aperture ring.

In picture 1 you can see the two bayonets. K bayonet to the right. The best is a bayonet that is not black anodised. The contacts on new Pentax cameras must be shorted to work properly with M lenses in manual mode (M).

Picture 1: B bayonet and K bayonet.

Picture 1: B bayonet and K bayonet.

Picture 2a: Aperture automechanism
Picture 2a: Aperture automechanism.

Picture 2b: Back side and the three holes to use for mounting the K bayonet.
Picture 2b: Back side and the three holes to use for mounting the K bayonet.

Picture 2c shows the tre M2 holes (red arrows) that you will use to mount the new bayonet. Picture 2d the diameter of the bayonet.

Start to measure the diameter of your K bayonet. It must be the same as the B bayonet. If not, you have to file it down to the same diameter as the B bayonet (58,5 mm).
You have to file the back side of the K bayonet, if there are edges.

Picture 3: back side of my bayonet.
Picture 3: back side of my bayonet.

Next you have to mill the inner edge on the upper ring. Picture 5a and 5b shows the final result.
The easiest way is to remove all the parts on the ring (picture 2a and b) and mount the ring on a wooden plate so you can hold it tight and move the plate when you mill. Use alcohol (95%) as ”grease” when you mill.

picture 4
Picture 4

Picture 5aPicture 5b
Picture 5a and 5b: final result of milling. Not a masterpiece but it do no matter when the K bayonet are placed on the ring. Paint the metal black.

Next step is to drill the holes for the three M2 screws. A little tricky to place it right. Place the K-bayonet on the milled upper ring and use the measurements in picture 7a and 7b to place it right.
Try not to move the two parts when you lift them away from the lens. From the back side you can mark where the holes should be. Drill carefully and countersink the holes.

Picture 6: Drilled holes
picture 6: Drilled holes

Picture 7a and 7b: The left picture f 2.8 (5mm) and the right f 22 (2mm).

Picture 8
Picture 8: The pin should be 6mm high. I used a polyester filler (in Sweden Plasic Padding kemisk metall) to make the pin little longer. I will paint it black.

Picture 9
Picture 9: When it is done it should look like this (or better).

Now you have to adjust the lens so you can focus to infinity. Flange distance for Pentax K is 45,46 mm and for Praktica B 44 mm. So the optics must come 1,46 mm closer to the sensor.
There are three small screws on the focusing ring (picture 10).
1) Set the focus ring to infinity and loosen the three screws.
2) Move the focus ring to 20 m without changing the position of the optics.

3) Tighten the screws.
4) Now focus to a point far away.
5) Gently losen the screws again and turn the focus ring to infinity mark.

6) Tighten the three screws.
Now you have adjusted the lens to infinity. If you do not get the infinity then start over from step 1.

Picture 10: Focus ring.

Picture 10: Focus ring.

Lons mounted on Pentax K-1
The lens mounted on Pentax K-1

A fast test with the lens on Pentax K-1.

huvudbild av test
The small letters on the paper are 2 mm high. Pentax K-1, FF, ISO 400, f 5.6.
Pictures below is the center cropped marked in the image above.

f 2,8f 4
f 5,6f 8
f 11f 16
f 22

The lens vignettes vigorously.  Below two pictures showing vignetting. Vignetting will be less with APS-C sensor.

Vinjettering 2.8Vinjettering 8
Vinjetting, full frame on Pentax K-1: left f:2.8 and right f:8

The question is: Is it worth the work?
My answer would be: Hardly worth the trouble, but fun to try. It can give some nice softnes at f 2.8 and f 4. Would like to say that this lens fits best as a portrait lens.

Below are some pictures taken with the lens monunted on Pentax K-1.

f 8, +4D
f 4
f 4par. klippa
f 4Sandra
f 4