Prakticar Pentacon B PK PKA Pentax K
mount bayonet rebuild 2.8/135 2,8/135
lens DIY Pentax K-1 FF full frame lens
test optics ombyggnad av kameraoptik
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.
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
Be careful that you don't lose the small ball inside the
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 2a: Aperture
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.
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 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 7a and 7b: The left picture
f 2.8 (5mm) and the right f 22 (2mm).
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: 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
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.
The lens mounted on Pentax K-1
A fast test with the lens on Pentax
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.
The lens vignettes vigorously. Below two pictures
showing vignetting. Vignetting will be less with APS-C sensor.
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.