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ANNOUNCING: Easy cheap non-destructive Exakta-Pentax adapter
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PostPosted: Fri Dec 03, 2010 1:26 pm    Post subject: ANNOUNCING: Easy cheap non-destructive Exakta-Pentax adapter Reply with quote

Yes, I have done it! A simple adaptation to put Exakta bayonet-mount lenses onto Pentax PK-mounts, with infinity focus! No machine tools are required! All that are needed are:

1) a cheap Bower-type M42-PK adapter
2) a cheaper M39-M42 adapter ring
3) a sheet of 100- or 120-grit sandpaper
4) water-soluble glue such as Elmer's Glue-All
5) a snip of metal tape or electrical tape
6) a dry sponge or pad, comfortable to hold
7) jeweler's screwdrivers and a razor knife

We will not seriously modify a lens. We will modify an ADAPTER! To prepare the lens:

1) Remove and save the index screw from the lens base, for reversion.
2) Put the snip of tape around the bayonet, to shim-out the M39-M42 ring.
3) Squirt some glue around the bayonet -- just enough to secure the ring.
4) Place the M39-M42 ring around the bayonet, flush with the lens base.
5) Let dry for a few hours. Scrape off any excess glue with a razor knife.

Look at the Bower-type M42-PK adapter. You will see a screw and clip. REMOVE THEM or the adapter will be TERRIBLY DIFFICULT to remove from a camera body. Notice that the adapter has 3 bayonet flags on one side, and a 48mm diameter flange on the other side. The flange is just over 1mm thick. We are going to grind off about 0.6mm from that.

Now for the hard part. Tape the sandpaper (face-up) to a flat hard surface such as a large ceramic tile. Place the M42-PK adapter flange-down on the sandpaper. Use the sponge or pad to hold the adapter while you work, or your fingers will be sore. Swirl the adapter over the sandpaper to grind away the flange's thickness. Continue until the flange is a bit more than razor-blade thick.

Without a micrometer, how do you know when you have ground away enough of the flange? Test the adapter on the lens. I have split-focus VF screens on my SLRs, so I simply focus at a horizon or distant object to determine if infinity focus is achieved. Grind and test, grind and test, repeat as needed.

The register (flange focal distance) of an Exakta bayonet is 44.7mm. The register of a Pentax mount is 45.46mm. For perfect infinity focus on many lenses, you should remove about 0.75mm from the flange, but that might not leave enough material to seat securely on the PK mount. I removed 0.6mm from my adapter (first try). I can achieve infinity focus with a CZJ Tessar 50/2.8 (12 iris blades) and with a pancake Meyer Helioplan 40/4.5, but not quite with a Meyer Primagon 35/4.5 (infinity is about 5m). I guess I need to grind another adapter thinner.

This approach is not rock-solid, but I wanted to keep it non-destructive and reversible. To revert the lens for Exakta use, just remove the adapters and glue, and re-insert the index screw. To secure the adapters for a more solid fit, drill-out and tap the hole in the M42-PK adapter where the clip was screwed down, and run a longer screw through both adapters into the lens body. I have not gone that far yet.

And now I need to buy some more Exakta lenses for experimentation. This only enables and drives further LBA. I am doomed.

Last edited by RioRico on Sat Dec 04, 2010 12:12 am; edited 1 time in total

PostPosted: Fri Dec 03, 2010 1:37 pm    Post subject: Reply with quote

Good job! It is nice to see other people willing to experiment.


PostPosted: Fri Dec 03, 2010 5:10 pm    Post subject: Reply with quote

Thanks for sharing!

Could you please post some pictures on the process and/or the finihsed adapter?.

Thanks in advance!


PostPosted: Fri Dec 03, 2010 5:43 pm    Post subject: Reply with quote

super - see alreay the Exacta lens prices rising Smile

thanks for sharing

PostPosted: Sat Dec 04, 2010 2:40 am    Post subject: Reply with quote

der einrahmer wrote:
super - see alreay the Exacta lens prices rising Smile

Oh damn...

Jesito wrote:
Could you please post some pictures on the process and/or the finihsed adapter?.

I did not shoot the process -- the sandpaper grinding was my brother-in-law's idea, and his sandpaper, and his micrometer, done at his house, and was too boring to shoot. But I can show what is involved.

First, preparing a lens: On the left is a plain Exakta-mount lens. On the right is a prepared lens. These are different lenses. On the prepared lens:

A- the Exakta bayonet
B- a bit of metal tape under (D)
C- the index screw, removed
D- the M39-M42 adapter ring (black)

Next, modifying a M42-PK adapter: On the left is a plain adapter; on the right is a ground-down adapter. You can see that the grinding has removed some chrome and exposed the brass structure, and that the flange (on top) is thinner. On the far right you can see a small notch -- I will explain that below. On the plain adapter: (A) is the clip and (B) is the screw that must be removed.

Finally, the finished product: This is a CZJ Tessar 50/2.8 with 12 iris blades. On the lens:

A- the lens body
B- the modified M42-PK adapter
C- the thinned flange of (B)
D- the M39-M42 adapter ring (black)
E- a bit of metal tape under (D)
F- the Exakta bayonet

I cut the small notch in the modified M42-PK adapter in an earlier experiment. The notch was to allow clearance for the index screw to remain in the lens body. This did not work. I must say that shooting and processing these pictures above has taken much longer than the modification work did. Such a bother!