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CZ 28mm Praktica B testing delayed by adaptor problem
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PostPosted: Wed Jul 13, 2011 6:51 am    Post subject: CZ 28mm Praktica B testing delayed by adaptor problem Reply with quote
PB to NEX (Emount) arrived from Hongkong yesterday.

Well made...fits lens But don't think they got the Flange focal distance right

Been lucky with all the others (m42, pk, fd) but now it's my turn for the dreaded adaptor.

The good news thought is that this looks to be easily fixed and I should be able to post some pics soon.

The problem is as usual related to infinity but unlike the poor eos guys it the reverse issue. Infinity is reached to early. The adaptor is too short.

Infinity on the 28mm is around the 6meter markings.

So unfortunately this effects the macro end of the scale and won't be truly representative.

To fix the problem looks to be as simple as unscrewing the PB mounting end(3 screws) and adding a shim/washer of the desired thickness.

Question: anyone done this before recommend what I should use as a shim?

I'm thinking layers of photo paper of known thickness, which can be cut with scissors.

Will post updates


PostPosted: Wed Jul 13, 2011 6:55 am    Post subject: Reply with quote
What a pain. Most of my adapters are too thin, for Nex and EOS. Only the really high quality (read: expensive!) ones seem to be spot on. I've never tried a shim before, but that seems like a sensible approach.


PostPosted: Wed Jul 13, 2011 9:00 am    Post subject: Re: CZ 28mm Praktica B testing delayed by adaptor problem Reply with quote
saffersteve wrote:
PB to NEX (Emount) arrived from Hongkong yesterday.

Well made...fits lens But don't think they got the Flange focal distance right

Been lucky with all the others (m42, pk, fd) but now it's my turn for the dreaded adaptor.

The good news thought is that this looks to be easily fixed and I should be able to post some pics soon.

The problem is as usual related to infinity but unlike the poor eos guys it the reverse issue. Infinity is reached to early. The adaptor is too short.

Infinity on the 28mm is around the 6meter markings.

So unfortunately this effects the macro end of the scale and won't be truly representative.

To fix the problem looks to be as simple as unscrewing the PB mounting end(3 screws) and adding a shim/washer of the desired thickness.

Question: anyone done this before recommend what I should use as a shim?

I'm thinking layers of photo paper of known thickness, which can be cut with scissors.

Will post updates


Normal A4 printing paper will work very well as a shim and easy to cut. Just don't tighten down on it too tightly. Surprisingly, it won't compress much.

Worth making sure it is the adapter that's at fault and not the lens. I have seen lenses before which have been assembled incorrectly and so hit infinity at the wrong point. Usually a giveaway is that the markings on the barrel get covered up when the lens is screwed all the way in.


PostPosted: Wed Jul 13, 2011 2:57 pm    Post subject: Reply with quote
The difference in helicoid position between infinity and 6 m for a 28mm lens is only 0.131mm, so a piece of thickish paper will probably the best option for a shim. I have some 160 g/m^2 Staples premium paper which seems about right according to my vernier, but I can't find my micrometer to check its thickness more accurately.

Realistically, though, such a small error will not have a big effect on the min focusing distance (for example about 2cm at 30cm focus distance).

Mark


PostPosted: Wed Jul 13, 2011 3:59 pm    Post subject: Re: CZ 28mm Praktica B testing delayed by adaptor problem Reply with quote
martyn_bannister wrote:


Worth making sure it is the adapter that's at fault and not the lens. I have seen lenses before which have been assembled incorrectly and so hit infinity at the wrong point. Usually a giveaway is that the markings on the barrel get covered up when the lens is screwed all the way in.


Ahh yes...very good point and yes this was the very first thing I tried.

I have 3 other lens and a old pb mount camera.

All lens fine though the view finder on the 35mm camera, but same issue on the adaptor.

Leaving work now so will try the paper and let you know.

Will photo graph the process incase this is a general issue with this mount.


PostPosted: Wed Jul 13, 2011 4:48 pm    Post subject: Re: CZ 28mm Praktica B testing delayed by adaptor problem Reply with quote
saffersteve wrote:
martyn_bannister wrote:


Worth making sure it is the adapter that's at fault and not the lens. I have seen lenses before which have been assembled incorrectly and so hit infinity at the wrong point. Usually a giveaway is that the markings on the barrel get covered up when the lens is screwed all the way in.


Ahh yes...very good point and yes this was the very first thing I tried.

I have 3 other lens and a old pb mount camera.

All lens fine though the view finder on the 35mm camera, but same issue on the adaptor.

Leaving work now so will try the paper and let you know.

Will photo graph the process incase this is a general issue with this mount.

You got there before me! Hope it works OK and interested to see the photos of the process.


PostPosted: Wed Jul 13, 2011 7:37 pm    Post subject: Reply with quote
WOW !!!!!!!

4 x shims required.

Please don't attemt to do this under the influence of alcohol or while operating any heavy machinery Very Happy

Steps
1) unscrew
2) play with a printer
3) play with scissors (don't run)
4) Put shims on top of each other....make holes with a sharp screwdriver
5) screw back together











Hope to do some testing tomorrow.


PostPosted: Thu Jul 14, 2011 7:04 am    Post subject: Reply with quote
Excellent engineering! Look forward to the results. (Don't run with the sharp screwdriver either)
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