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"Wild" mod on 50mm Praktica B mount with EOS adapt
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PostPosted: Sun May 08, 2011 4:37 pm    Post subject: "Wild" mod on 50mm Praktica B mount with EOS adapt Reply with quote


Yesterday I disassembled my Pentacon 50mm f/1.8 in order to clean the aperture blades which were full of grease.
Cleaning was successful but, when the lens reassembling was mostly completed, I had an insane idea.

I guess you already know, Praktica B mount lenses on Canon EOS DSLR with adapter can't achieve infinity focus. The problem is the register distance which would require an adapter thinner than 0,5mm.

But... If it would be possible to move back all the lenses by about 1mm, in order to have them nearer to the camera sensor, the problem would be solved.

While I was reassembling my Pentacon, I tried to figure out how to achieve this, and I clearly didn't find any easy way. But I found it's very easy to get the back lens group nearer to the sensor, just leaving it a little unscrewed.
The downside is that this "wild" mod increases the distance (maybe also the focal?) between the front and back lenses groups.

I guessed that this would just screw up all the thing, but I wanted to do some test anyway and... surprise: I got infinity! Shocked

The global optical quality doesn't seem to be affected but with f/1.8 aperture: the picture gets a strange "dreamy" soft tone, as shot with a toy camera. But with any other aperture everything seems ok.

I post here some tests I made. I would really like to share this with you and see what you think about it.

Thanks! Smile

This is the first test, from about 2 meters, in order to be sure that the lens settings were not screwed up. All pics taken with 350D+adapter.

At f/1.8:
(click to enlarge)

100% crop at center:

Very soft, quite unusable....


At f/2.8
(click to enlarge)

100% crop at center:

It's better....

And finally:

At f/5.6
(click to enlarge)

100% crop at center:

So the lens seems to work fine, except for the f/1.8 aperture. A little cost to pay if I can achieve infinity focus.

Time to go for infinity focus test....

At f/1.8:
(click to enlarge)

100% crop at center:

Here again, the picture is soft. Maybe ok for some special effect, but not interesting in everyday use.

At f/2.8:
(click to enlarge)

100% crop at center:

Not so bad....

At f/5.6:
(click to enlarge)

100% crop at center:


At f/11:
(click to enlarge)

100% crop at center:

Still ok! Very Happy

Maybe my tests are not 100% accurate, but it seems to me that they show that this "wild mod" achieves acceptable infinity focus starting from f/2.8.
Thus, this mod is easy to undo: just unmount the lens, screw the back lens down until the end, mount it back, go for portraits with few light at f/1.8 Wink

PostPosted: Mon May 09, 2011 1:32 pm    Post subject: Reply with quote

I think the "dream" look is due to spherical aberration, which is obviously misscorrected. The result is not exactly soft, but the superposition of a sharp image, formed by the central part of the glass, and a kind of "halo" made of a softer image from the outer part of the lens.

If you change the distance between groups, this is the typical consequence (as in the "macro" position of some lenses or the focussing method is some old folders). I kind of like it for some subjects. As you close down, this will be less evident, but the image degradation will be always present, but this is not an issue in your case: it looks sharp enough at 2.8.


PostPosted: Fri May 13, 2011 8:32 am    Post subject: PB lens Reply with quote

Well done!

I believe the aberrations are still present stopped down, but are acceptable, particularly if you intend to use for portraiture, having an antique look!

Especially useful that it corrects so quickly, leaving the 1.8 as a soft portrait special!

Thanks to the expert who confirmed that we might do this with other lenses, just for the effect!

Great posts!

PostPosted: Mon Jun 06, 2011 2:54 pm    Post subject: Reply with quote

When I get home in a few weeks, I'll deal with my Prakticar MC 50/2.4 PB, try to fit it to a PK mount. In earlier attempts, I couldn't see how to disassemble it. How did you take apart your Pentacon 50/1/8? (I have one of those too, and an Oreston 50/1.8, both in M42, and I haven't yet found how to unravel them.) Is there any commonality with my 50/2.4?

Oh wait, I just remembered that someone suggested using a round rubber block to unscrew the front element housing. Is that the trick?

PostPosted: Tue Jun 07, 2011 12:02 am    Post subject: Reply with quote

The plastic bodied Pentacons are very easy to take apart and convert to other mounts, I have converted both a 2.8/28 and 1.8/50 from PB mount to EOS.

All you do is pop out the black plastic colalr at the back to reveal four screws, remove these and the PB mount comes off.

All you then need to do is remove the three electrical pins then take a cheap M42-EOS adapter or the mount from a broken EOS lens and trim it down in diameter a couple of mm so it is the same diamater as the PB mount you just removed, then drill four holes in it that match up to the holes in the lens, screw it in place et voila, the lens mounts direct on an EOS and hits infinity perfectly.

PostPosted: Tue Jun 07, 2011 2:45 am    Post subject: Reply with quote

Thanks so much! I've bookmarked your post, and I'll do this when I get back home. In my case, it will be with some M42-PK adapter, and I may have to adjust for register difference. But that shouldn't be too hard a trick. I hope...

PostPosted: Tue Jun 07, 2011 5:04 am    Post subject: Reply with quote

RioRico wrote:
Thanks so much! I've bookmarked your post, and I'll do this when I get back home. In my case, it will be with some M42-PK adapter, and I may have to adjust for register difference. But that shouldn't be too hard a trick. I hope...

It was dead easy to do. In the case of the 2.8/28 I had to use epoxy to hold the EOS mount to the lens, but it works fine and takes great pics.
I have two more 1.8/50s in PB mount (one German, one I think is Japanese) another 2.8/28 in PB, a Sigma made 2.8/135 in PB and a pair of Sigma PB zooms. I might convert them all to EOS eventually although I might buy a PB body and shoot film with them.

PB mount lenses go for very little on ebay, hence I have most of them.

PB mount 2.8/28 or 1.8/50 can be had for under 5ukp inch shipping and an M42-EOS adapter from Hong Kong is just under 2.50ukp so it is very worthwhile converting these PB mount lenses as they are very good lenses and only useable with PB mount bodies unless converted.

I am not so sure you need to do this conversion with PB mount lenses as the PB mount is virtually identical to the PK mount, the only difference is the thickness of the flange on the lugs, I think it is 1mm on the PK and 1.6m on the PB, this means you can mount a PB lens on a PK body but might not be able to get it off.

You can take a small file or a dremel with adrasive tool fitted and remove 0.6mm from the flanges on the PB lens and you just turned it into a PK mount lens, of course the aperture won't link to the body, the levers being different.

If you google PB to PK conversion you will find all the info, people have done this conversion before in order to use Pentacon lenses on Pentax bodies as Praktica bodies often broke, especially the later PB ones and th Pentacon lenses definitely stand up well against Pentax ones in IQ. Pentax PK bodies are undeniably better than Prakitca PB ones imho.

PostPosted: Tue Jun 07, 2011 5:16 am    Post subject: Reply with quote

I've just compared the mounts on a Pentax-M 2/50 PK lens and a Pentacon 1.8/50 PB lens and they are the same, the only difference is as I said - the flanges on the lugs on the PB are a wee bit thicker and you could just file them down and it would fit straight on a PK body.

Hope I haven't just gone and increased the price of PB mount lenses before I've collected them all myself! hehe

PostPosted: Tue Dec 30, 2014 2:52 pm    Post subject: Reply with quote

Seems to be possible reach infinity with this adaptor? but I can see this thread is some years old now.