"Wild" mod on 50mm Praktica B mount with EOS adapt

[spacexion]

Hi!

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!

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!


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....

But...

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:



Interesting....

At f/11:
(click to enlarge)



100% crop at center:



Still ok!


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

[wupdigoj]

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.

Javier.

[pat donnelly]

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!

[RioRico]

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?

[iangreenhalgh1]

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.

[RioRico]

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...

[iangreenhalgh1]

[iangreenhalgh1]

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

[keithcanisius]

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




Thoughts?

https://www.jackthehat.co.uk/praktica-mount-canon-lens-adapter-p-804.html