CV 75/2.5 Heliar RF adjustment?

[Braeside]

I have recently got an M8 and one of my lenses suffers from front focus. I now have 4 lenses (CV28/1.9 Ultron, CV35/2.5 Skopar classic, 50/2 Summicron and CV75/2.5 Heliar)

The odd one out is the lovely Heliar.

If the lens is set to the infinity stop then the RF patch has crossed over, but distant objects are in focus OK, however if I focus using the RF patch then the focus is out.

I did initially tweak the infinity setting on the M8 to suite the lens and that worked fine, except of course the other lenses now back focussed :bang:

So, I surmise that the CV75 RF adjustment is set wrongly. I have checked (with a rather poor analog vernier) and it looks like the RF ring is about 0.3mm too far out when at real infinity. (This is comparing against my other lenses).

Anyone familiar with the CV75 RF adjustment?

Is there a way to adjust the moving part (I don't want to move the lens, just the RF coupling).

Or do I have to lap down the end of the tube?

I took off the rear plate and there is no sliding adjustment plate that I have seen in other CV lenses mentioned in this forum.

 

[Braeside]

Digital caliper today showed error much less than I had thought, less than 0.15mm, so very little to polish down on end of RF ring.

I am still puzzled by the construction of the RF ring, cannot determine if it is one piece or if the top inner part is threaded into the lower part.


Some photos of the rear of the Heliar with lensmount plate removed.

 

[ARI_1969]

the little mechanic...

the little mechanic...

hmm, from the pictures, it is hard to determine, wheather there are two parts or not. (personally, I think it is one piece)
But I think, this would not be the major point, because the hard thing is to counter (hold) the thicker diameter and find an "appropriate spanner" to loosen the thinner diameter... if there are two parts of course).

If I get it right, you are looking for a solution to have 0.15mm more distance between the end of the ring and the RF mechanism (or the bayonett, which is just a question of offset)... so why not look for a 0,15mm piece of sheet metal and mount it below the M39 thread...

am I correctly thinking????

 

[Braeside]

Thanks for that - no, if I shimmed the mount then the lens focus would move with the RF ring which I don't want as the lens focuses correctly by scale as it stands.

Good news - today I went ahead with plan A - that was to lap down the end of the RF tube.

I took advice of a friend who recommended starting with 600 grit wet and dry paper glued onto a really flat surface, I used a piece of kitchen laminate. Then I wet the paper and carefully rubbed the end of the RF tube in a figure of eight motion a couple of times. I then cleaned the end of the tube and refitted to camera, tested and found I had reduced the RF error by about half already.

I repeated this with finer grade of wet and dry (1200 grit) again just a couple of gentle figure of eight turns. Checked again and very close to perfect.

Finally I used a bit of "Brasso" metal polish on the back of a piece of wet and dry stuck down to do the final bit. Cleaned all up and checked again, it seems almost perfect, perhaps a fraction short of where it should be, but I am stopping there as I do not want to go to far, as there is no way to easily lengthen the tube!

Of course all of this did not go entirely to plan, at one point I dropped a tiny screw in the kitchen and it disappeared under the washing machine. To recover this I had to move dishwasher and washing machine and crawl around with a torch and magnet for half an hour until I found it, not under the machine, but on the floor tiles in the middle of the room!

 

[payasam]

You are daring, I must say, in comparison to most others here. Well done.

 

[Livesteamer]

Lapping is a permanent solution to a problem that probably has an adjustment. Removing materiel is easy, putting it back on is much harder. Please don't be hasty until you have explored this problem completely. I have this lens, it's wonderful. I hope you have not damaged this one. Also, a digital caliper is not of sufficient accuracy for measuring this mechanism. Joe

 

[ARI_1969]

aaah ok, I understand

aaah ok, I understand

and thanks for the idea with the laminate... when I needed flat surfaces for "sanding" down, I always took a thick glass (approx. 16mm) which I got for free from a local craftsman as it was scrap material. For "weting" the paper, I use normally WD40 (or Ballistol) because this gives a little corrosion protection after finnishing.... and makes sure, that sanding residues got bond.
Moving like an 8 is perfect and is even better, when you turn the part by some 10 degrees after each 8... helps taking out the "manual process tolerances"...

was it on piece? (the tuebe)

best ragards

Achim

 

[Yves]

Hi,
I did RF adjustment some time ago with an Ultron 28/1.9.
I am pretty sure you can do it the same way with the Heliar 75/2.5 (I own one, but did not disassempled it yet). See at the end of the thread http://www.rangefinderforum.com/forums/showthread.php?t=75050
Yves

 

[Braeside]

Lapping is a permanent solution to a problem that probably has an adjustment. Removing materiel is easy, putting it back on is much harder. Please don't be hasty until you have explored this problem completely. I have this lens, it's wonderful. I hope you have not damaged this one. Also, a digital caliper is not of sufficient accuracy for measuring this mechanism. Joe

Thanks Joe, I appreciate there may be an internal adjustment someplace deep within the lens, but this solution worked for me, it was non intrusive and probably less risky than disassembly.

I used the camera not the micrometer to measure the final results. My M8 rangefinder is working well with a standard Leitz 50/2 lens and other CV lenses, so I wanted to get the CV75 to focus as well. The CV75 collimation was good - infinity on the lens was truly in focus, it was just the RF coupling that was slightly too long giving me front focus.

I am very pleased with the outcome, please note this is not something I would recommend anyone else to do, this was an inexpensive used lens, not a kilobuck item, but the results are impressive.

 

[Braeside]

Hi,
I did RF adjustment some time ago with an Ultron 28/1.9.
I am pretty sure you can do it the same way with the Heliar 75/2.5 (I own one, but did not disassempled it yet). See at the end of the thread http://www.rangefinderforum.com/forums/showthread.php?t=75050
Yves

Thanks Yves, I didn't want to have to disassemble if at all possible for fear of damage. I have made a note of your thread for future reference though.

 

[Braeside]

and thanks for the idea with the laminate... when I needed flat surfaces for "sanding" down, I always took a thick glass (approx. 16mm) which I got for free from a local craftsman as it was scrap material. For "weting" the paper, I use normally WD40 (or Ballistol) because this gives a little corrosion protection after finnishing.... and makes sure, that sanding residues got bond.
Moving like an 8 is perfect and is even better, when you turn the part by some 10 degrees after each 8... helps taking out the "manual process tolerances"...

was it on piece? (the tuebe)

best ragards

Achim

Yes Achim, the tube appears to be one part. Under the surface of the black it is brass, so is easy to sand down.

I was surprised how little I needed to remove to make this adjustment.