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Exakta -> Pentax K adapter
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PostPosted: Fri Jul 25, 2008 6:32 am    Post subject: Exakta -> Pentax K adapter Reply with quote

After trying several approaches to the nagging problem of getting Exakta lenses to work on my Pentax DSLR, I came up with what I think is a reasonably successful contraption.

This adapter had to fulfill some challenging requirements -

1. It had to be strong enough to mount the Meyer Telemegor 400mm

2. It had to be very thin and add no more than 1-2mm to the flange-focal distance.

3. It had to be non-destructive to the Exakta-mount lens.

4. It had to be reasonably cheap and possible to assemble from readily available parts, with hand tools and at best an electric drill or Dremel tool. Most people haven't got a lathe.

5. It had to be removable and transferrable between lenses.

I think I've managed #'s 1-4

This is the adapter, mounted on the Telemegor -

Its not pretty, but it works.

On the Telemegor it gives me infinity also (with some focus adjustment). I took the Telemegor to the usual piers today for test shots, and it was perfectly stable.

It consists of -

- An M42-PK adapter, the cheap kind with a 1mm flange.

- An M39-M42 adapter, the simple thread ring used to adapt Russian M39 SLR lenses to M42.

- Three brass flat-head screws, US 4-40 1/4" in this case, but thinner ones would have been better I think - #3 maybe. For you Metric people 2mm or so may be best.

Hex-head set-screws may also work better and may be neater, if some way can be found to adjust their length. They are usually steel and very hard to file.

- Some electrical tape.

It also requires a tap for the screw size.

To build -

- Remove the locking lug from the Exakta mount. These usually just unscrew. Save this to restore the Exakta mount.

- Wrap electrical tape, one layer, around the base of the Exakta mount.

- Drill and tap three equidistant holes in the M39-M42 ring and the M42-PK adapter. Best to screw the two adapters together before drilling to make sure the holes line up.

- Open the drilled holes in the M42-PK adapter to accomodate the screw heads (bigger drill, file, whatever), so that they are flush or below the curve of the inside of the PK mount.

- Screw the M39-M42 adapter on the Exakta mount (it fits almost exactly, as the mount is 38mm in diameter) - it will screw in tightly on the tape, if done right.

- Screw the M42-PK adapter on the M39 adapter, and line up the holes.

- Adjust the size of the screws (I used brass ones so it would be easy to file them), both ends, to make sure they can be screwed in tightly against the tape and still fit flush or below the curve of the PK adapter.

An even more secure mount, and maybe without need of tape, could be achieved by drilling three dimples or even holes in the Exakta mount in the appropriate places, for a positive grip by the screws. This would be a disfigurement of the lens but it would still be usable as an Exakta-mount.

My adapter has one big problem - the M39 ring is screwed in so tight that it can't be unscrewed without destroying the M39 ring - its already been weakened with those big holes. So that is in there to stay.

As for infinity focus - this adapter adds about 1mm to the flange focal distance, and there is already a 0.8mm difference between Exakta and PK (45.5-44.7mm), so about 2mm needs to be made up somewhere. Many lenses can have their helical travel adjusted at least that much. On the Telemegor it is very easy - just unscrew the three setscrews on the focusing ring, turn it 1/3 backwards, and screw it in again - it is now possible to turn it 1/3 turns more, more than 2mm. On other lenses it is harder, and probably impossible on normal and wide angles.

A PK adapter without a flange could also be used, and there would only be that 0.8 mm to fix, but I wanted the flange for strength in this case.

PostPosted: Fri Jul 25, 2008 7:03 am    Post subject: Re: Exakta -> Pentax K adapter Reply with quote

That's a great idea Luis, it opens up a whole new range of lenses for your K mount cameras!

luisalegria wrote:
My adapter has one big problem - the M39 ring is screwed in so tight that it can't be unscrewed without destroying the M39 ring - its already been weakened with those big holes. So that is in there to stay.

If you ever make another one, I'm thinking maybe you could drill the three screw holes through both adapters together, before the M39 one is screwed in too tightly. Once the screws are in they will be locked together anyway. But there's no real need to separate them again, is there?

PostPosted: Fri Jul 25, 2008 7:58 am    Post subject: Reply with quote

Bright handicraft, Luis!
Congrats for the idea and the courage to execute it Smile
I'd love to see samples,
Great work!.


PostPosted: Fri Jul 25, 2008 8:13 am    Post subject: Reply with quote

Would it not be possible to just "plug" the adapter and drill a new 39mm hole and make a thread in it? Or just not make threads. Smile

PostPosted: Fri Jul 25, 2008 11:04 am    Post subject: Reply with quote

Many thanks for this very important guide!

PostPosted: Fri Jul 25, 2008 11:34 pm    Post subject: Reply with quote

The problem with the M39 ring in this case is that the setscrews bear on the electrical tape that the M39 ring is screwed on, and deform it, so it makes the tight fit even tighter. And it needs to be tight, as that tape/M39 connection is taking almost all the strain, rotational and directional.

My next version (already getting parts) will use an aluminum rear lens cap that will be press-fit on the tape, but because it will engage the Exakta bayonet (and take all the strain there) should be even stronger. All that needs to be accomplished with that one is to connect the PK adapter and the cap, plus keep the cap from rotating. It should be easily removable.

PostPosted: Mon Aug 18, 2008 1:38 am    Post subject: Exakta->PK adapter Mark III - All metal Reply with quote

This is yet another version of the Exakta->Pentax K adapter.

This one is based on a chromed brass flanged M42->PK adapter plus an aluminum rear lens cap for Exakta. I cut the end of the cap off, leaving a ring with the bayonet lugs.

In this one there are three holes drilled and countersunk for the screws in the M42-PK adapter, and corresponding holes tapped in the rear cap.

I used 00-90 screws on this, as I found my favorite hobby shop carried screws and taps in very small sizes. For you metric people, that would be about 1.2mm. This may be too small, but so far its holding up. The only things holding this together are the screws.

I had to put a rotation-stop on the rear cap, made of brass sheet held on with a screw -

There is no doubt a more elegant way to do this.

And to stop rotation in the other direction, I made a brass spring to fit in the space between the lugs, held in place with another screw. To remove the adapter, unscrew and pull out the spring.

So yet again this is not a terribly convenient adapter, though it is secure and it effectively turns the Exakta lens into PK.

PostPosted: Mon Aug 18, 2008 1:44 am    Post subject: Reply with quote

Nice one(s). You certainly know how to work around. Smile

PostPosted: Thu Aug 28, 2008 11:08 pm    Post subject: Reply with quote

I have a 1930's 75/1.5 that has the Exakta mount inside of a crude M42 thread adapter. I tested it on my Sigma SD14, and it was sharper than any other biotar 75 I have. Then I hacked out the jtat.com Sigma to M42 adapter so it would screw down all the way. Apparently your PK adapter does not have the aperture pin flange, which needs to be ground away on the jtat.

I have a 1950's biotar 75 with Exakta mount, and you have shown me how to adapt that to M42 or Sigma SA.

May I suggest aluminum hvac duct tape instead of electrical tape? One might even clean the glue off of that to make a really solid shim. Normally I like thick, but there is a new el cheapo corporate profit maximizing THIN aluminum duct tape that would even be better for this. Mineral spirits or paint thinner would probably take the glue offof the tape, and certainly acetone or nail polish remover would.

Exakta has the same flange focal distance as Sigma SA, which is the same mount as PK except for different flange focal distance.

PostPosted: Fri Aug 29, 2008 4:50 am    Post subject: Reply with quote

Hi Bob,

Thats a good idea on the aluminum duct tape, I'll have a look for that. I put in brass sheet shims on the last one afterwards, but maybe the tape was better, as it moves a bit now.

You obviously don't have a problem with the infinity adjustment on the Sigma, good thing too. The Biotar is easy, some of the others are a pain or plain impossible.

PostPosted: Sat Nov 29, 2008 4:23 am    Post subject: Reply with quote

I am just using aircraft-grade epoxy to put an M39-to-M42 ring on Exakta, after cutting the Exakta part away.

The fat biotar Exaktas all adjust to infinity as M42. Skinnies(Exakta to M42) are really hard to get to infinity(as M42), but I think that removing .75mm from the rear can edge on the helical end of the can would fix any that won't adjust to infinity. Be careful I haven't done anything but adjust them.


PostPosted: Sat Nov 29, 2008 4:27 am    Post subject: Reply with quote

I would put 3 screws through the M39-to-M42 and Exakta stub tube for that 400mm, but 75mm is OK with just epoxy I think.

PostPosted: Sat Nov 29, 2008 5:22 am    Post subject: Reply with quote

Hi Bob,

I'm using epoxy+screws for all of them now - I have three, and I'm making another for a member.

PostPosted: Sat Dec 05, 2009 3:52 am    Post subject: Reply with quote

Thanks for the instruction, might need to convert one into M42 or SA mount soon. It's very useful Smile

PostPosted: Wed Sep 15, 2010 1:33 pm    Post subject: BUMP! Reply with quote

Reviving an old thread:

I've tried (prototyped) this approach with my few Exakta bayonet lenses: Meyer Primagon 35/4.5 and Helioplan 40/4.5 pancake, CZJ Tessar 50/2.8, Schacht Albinar 135/4.5. (Prototype: Just tape the lens and add the M39-M42 and M42-PK adapters to test. Note: Electrical tape sucks.) In each case, infinity focus is lost, whether I use a flanged or non-flanged M42-PK adapter.

I'd probably be scolded again about chopping lenses so I won't suggest milling 1mm or so from the can base. Instead, I'd try this: screw a cheap M39-M42 ring into a cheap clone non-flanged (Bower etc) adapter, bonding the threads with good glue. Keep flush the back (camera-side) edges of the adapters so the M39 ring doesn't protrude. Then grind or mill away the PK adapter's front (lens-side) lip. The product is a M39-PK ring that's 4mm thick rather than 5mm. THAT should maintain infinity focus!

Now I need merely find somebody to do the milling. ;(

EDIT: Oops, I just thought about it (and tried another prototype) and I see that a thinner adapter would only work with small-can lenses whose base diameter is under 45mm or so, the width of a PK-mount's throat. Is that Primagon headed for the chopping block, or the auction block?

PostPosted: Wed Sep 15, 2010 3:35 pm    Post subject: Reply with quote

The closest to infinity yo will get, using standard parts, is with a somewhat destructive mod, but reversible. There will be a few small holes though.

M39-M42 ring inside a no-flange M42-PK adapter.

Remove the locking spring from the M42-PK adapter
Remove the locking lug from the lens

Screw the M39-M42 ring into the M42-PK adapter (Checking alignment with camera and lens)

Drill and tap a screw hole through both assembled adapters and put in a screw to hold them together.

Place the assembly over the 38mm Exakta mount barrel. It fits a little loosely. Epoxy it in place.

Drill and tap three holes through the assembly and PARTLY into the sides of the Exakta mount. Put in setscrews tightly. This holds the PK mount assembly securely.

Drill a hole in the Exakta lens mount for the PK locking pin.

You now have a PK mount lens that is about 1mm off infinity. Good enough for most tele lenses to be useful.

PostPosted: Thu Sep 16, 2010 12:34 am    Post subject: Reply with quote

Yes, I read your previous excellent instructions. For a long tele, that 1mm doesn't make a lot of difference. If I use a 200mm lens on a 1mm-thick flanged adapter, 'infinity' drops to maybe 50m-70m, which is certainly tolerable, and even less significant on longer glass. But with 35-40-50 and even 135mm lenses, 'infinity' becomes much closer and the lens is only good for close-ups. So I'll have to try thinning the adapter. It's either that, or amputate a dection of the lens can. Bother...

PostPosted: Thu Sep 16, 2010 12:42 am    Post subject: Reply with quote

Don't use a flanged adapter.

Use that M39-42 ring + a flush adapter (similar to the original Pentax M42-K adapter), plus the screws, etc.

That will get you 1mm closer to infinity.

All you do to the lens mount is drill four small holes.

This system will not work on all Exakta lenses, only those that are large enough to overlap the PK mount locking lug.