The Kadapter !

© , 2005

Or how to mount Pentax K/Praktica B lens on a EOS body.

For more info on these lens, be sure to visit my Manual Focus Forum !

I had 2 bayonet lens collecting dust because I couldn't mount them on my 350D, for lack of adapters. I knew the depth of the this mount was greater than the EF mount, so it was theoricaly possible to mount them, but various web pages were stating that due to the aperture lever sticking out too much, it was impossible without modifying the lens.

But, well, I never believe what I read on the internet anyway, so I took my Vernier and measured the whole lot, and to me, in the case of my two Zeiss lens, there was plenty of room to spare for having the lever in the 350D body without changing anything.

Problem is, since "one can't mount the lens" current belief, no adapter exists. I knew from other readings that the M42 and K mount share the same distance-to-film (but, it turns out it's not true either!), so I assumed that if it was possible to mod a M42->EOS adapter the focus plane would be correct.

So that's just what I did. I measured a lot, drew some, and decided to sacrifice one of my M42 EOS adapter in the name of science.

Note that there are plenty of generations/variations of K mounts. This one works, but I can't guarantee all K or Praktica B will. I'm getting a couple more lens to see what is different.

Left: The Offending Bayonet, on an otherwise lovely Carl Zeiss Jena (Prakticar) 135mm f3.5. Note the aperture control lever, and some electrical contacts that I totally ignored. They are in fact a potentiometer that tells the body (by varying resistance) the currently selected aperture on the ring. Simple, efficient!
The red ink is some "debug" marks I did to check which part of the kadapter needed extra work

The result! Compared to an untouched M42 adapter.

I had to machine out a whole lot of metal to first make the base bayonet diameter passage, then the "ears" of the bayonet passages, then the extra indentations so the ears can "turn" and lock.

I had to take extra precautions because the 3 "ears" are of different size, to prevent mismounting the lens.

So I had to figure out the ideal place to dig in so that the lens, mounted on the adapter, still have the "text" and the aperture mark on top when mounted on the Canon camera.

Right: The adapter on the right has some black paint so (well, I figured it couldn't hurt) it reduces internal light reflections in the body.

Here are the indentations I dug into the brass to allow the rotation of the bayonet ears to lock the lens in place. Requires a ton of patience and trial & error to get these to work with each others!

Now this was not easy. This mount has LESS depth than the M42 lens.

So even tho with this original M42 adapter I could focus *past* infinity (the adapter was technically a little too thin) when I first mounted the bayonet lens on it, I couldn't focus at infinity!

I had to shave off a reasonable amount of metal from the adapter flat plane to make the lens sit closer to the body and reach infinity.

I know it looks a bit rough finish here, but well, it's a prototype! I'll do like Nike and hire underage slaves workers to polish these off once the production reaches the first million !

Details on the adapter, mounted on the lens.

You can notice the "ears" snugly fitted in my indentations.

The other important thing to note is the aperture lever, that doesn't stuck out that much from the whole lot ! In fact, I placed it so that it's position in the EOS body will *not* allow me to mount it if it is too long and would potentially damage the mirror.

As it turns out, there is plenty of room in there, as my vernier had predicted.

Note that I am speaking of my 350D body only. I'm sure Canon made other bodies that could cause problems.

And there we are, all done! it mounts perfectly on the body, allows infinity, doesn't shatter the mirror, is securely held, and didn't require any surgery of the lens. Amazing !

if you want to contact me about this adapter. I'd be willing to ship the prototype to anyone willing to produce these.


Chinon 50mm f1.7
Nice & sharp little 50! Bought it for nothing, to test the validity of the adapter. This mount is a plain K mount, and it works just fine.