Nikkor-H.C 50/2 front focus

[brbo]

My new Nikkor-H.C 50/2 LTM is front focusing pretty badly. I've tested it side by side on M6 with CZ Sonnar 1,5/50 that is optimised for f2.8-4 and J-8 (optimised for f2). Event though at f2 my CZ Sonnar still front focuses my new Nikkor has much more front focus.

Here are crops at moderate distance:

Nikkor-H.C. @f2:

Sonnar 1,5/50 @f2:

Full size shots here - Nikkor-H.C., Sonnar 1,5/50.

Same story at close distance, front focus about 5cm at 1m distance (Nikkor-H.C., Jupiter-8).

I don't think it's a problem in my M6 rangefinder as all other lenses I have focus correctly. I know that Sonnars shift focus, but this Nikkor only matches my CZ Sonnar at f11 or f16 where the CZ Sonnar already has so much back focus that it looses sharpnes if I don't correct for it (focus closer). I've tried 3 different LTM-M adapters, all performed exactly the same in close focus test (and with a standard 50mm lens this, I guess, is expected, only difference that different adapter thickness would be visible at infinity).

The lens otherwise seems to be in good condition. Retention ring is secured, no wobble in lens.

Can front focus be fixed in Nikkor-H.C.? I've opened and cleaned a few lenses so far but never needed to adjust focusing. Can it be fixed as a DIY? If I unscrew the retention ring and remove the optical module what should I check? I hope to find some shim that somebody put there and hopefully removing this will be all that is needed. What if there is no shim? How do you move the focus further back then?

Any help will be appreciated.

 

[Dez]

Having the lens front focus is rather unusual. How much is it off? I generally use something like this as a test bed:



The measuring tape is arranged at about 45 degrees to the camera, here testing a J3 at 1m distance. It back focuses by about an inch as expected. I may get around to changing the shims one day.

With the Nikkor, if you are absolutely sure that the optical block is assembled correctly, and there is no other factor causing you grief, you can correct front focusing, but it's not easy. I have never seen a shim in one of these lenses, or any obvious adjustment mechanism. I believe your only option is to put the focusing mount in a lathe, and ever so carefully skin off a bit of brass from the surface against which the optical block abuts. Needless to say, you need to be very certain that you have the part dead square in the chuck.

But I wouldn't undertake to do something like that unless the focus error were severe, and if the focus error were severe, I would consider an incorrect focus mount to be one of the least likely causes.

The effect of a different adapter would be apparent at any distance. While cheapo adapters sometimes have problems with the bayonet edges, the flange itself is just a 1.0mm flat bit of metal, and it is very difficult to get that wrong.

Cheers,
Dez

 

[brbo]

Thanks Dez!

As I said it front focuses by about 4-5cm at 1m. Several meters at 10m or so. If you download the full size pictures that I've linked you'll get the idea. When I shot it side by side

I've taken out the optical block and there doesn't appear to be any additional shims in the lens. I don't know how to check if the optical block is assembled correctly.

 

[Dez]

Please ignore the stuff about lathe work I said above- it's all wrong: I was thinking of a different lens. I don't have a Nikkor LTM 50 f2 at the moment, but I just took my 1.4 apart to check, and the arrangement is the same as shown in your picture.

The easiest way to adjust this lens is by the thickness of the little circular ring at the bottom left of your picture. Again, it is unlikely for this to be wrong, but I guess it could happen. This ring goes into the mount, past the key, and defines the position of the back of the optical block. Make sure the optical block is screwed together tightly, and that there is not crud under the shim ring. You can try removing the ring altogether, but the focus mount probably won't go all the way to infinity.

Also, I think the focusing mount itself is adjustable. I have never done this, but it appears that if you remove the two tiny setscrews, you can rotate the large brass ring with an appropriate spanner, shifting the helical slightly. When you get it right, put back the setscrews. If the setscrews are missing in your lens, a shift in this brass ring may be the culprit. My concern here is that this may shift the position of the rangefinder cam, so some experimentation is in order. It may be that this is an adjustment for the RF cam only and shouldn't be messed with. Please let me know how you get this resolved.

Cheers,
Dez

 

[brbo]

I tried without the ring and it doesn't work. Some sort of ring is needed or else the lens will have a "wobble".

The two tiny setscrews seem to be in place and I tried if the large ring can be rotated freely (it can't be), so I guess this is not something that changed position during transport.

Otherwise, rangefinder is in perfect alignment at infinity so anything that messes with that is out of the question. I'll have another look when I get home from work and try to figure out if adjusting the position of the big ring will allow for moving the optical block further back without changing the rangefinder cam position.

Edit: Thinking now, every adjustment to the thickness of the small ring or the position of the big ring will have effect on rangefinder infinity position and since this is spot on now, it will be off after such adjustment. Right? So, I think calibration of this lens is not something I can do myself...

 

[Dez]

I would be leery of moving the large ring if I were you, given that it is still apparently in its original position. It is useful (Crucial?) to completely understand how something goes together and why it is designed the way it is before taking it apart. The times when I ignore that advice tend to be the times I learn a painful lesson.

The small ring buts against the rearmost ring of the optical block, and defines its position relative to the focus mount. In my 1.4, when I removed it I could still reassemble the lens, but I found that the optical block had shifted far enough rearwards that the aperture ring would hit the focusing ring just before the lens was focused to infinity. In your case, since it's loose I suppose the ring holding the optical block into the focus mount runs out of thread before the block can be drawn all the way in when the shim ring is removed. You don't have much to play with in making changes to the thickness of the shim.

Look at the design of the focus mount. A change in the thickness of the shim ring will move the optical block relative to the focus mount, but won't change the focus mount itself, and therefore the rangefinder cam at all, so it won't affect the rangefinder of the camera.

The thing that concerns me though is that Nikon didn't do a poor job of assembling lenses, so having a problem like this seems very unlikely at the time the lens left the factory; something changed. Maybe this is not the original focusing mount for this optical block? I'd want to understand what changed to cause the problem before you start making irreversible changes to the lens.


Cheers,
Dez

 

[brbo]

Look at the design of the focus mount. A change in the thickness of the shim ring will move the optical block relative to the focus mount, but won't change the focus mount itself, and therefore the rangefinder cam at all, so it won't affect the rangefinder of the camera.

You are, of course, right about that. So, I guess this is doable, but I don't think I can file down the ring to absolutely correct thickness (evenly). And I don't have a digital M camera to check. Anyone can come up with math roughly how much thinner the original ring needs to be made if the lens front focuses by 5cm at 1m distance? I'm afraid that making the ring thinner will lead to a wobble when the optical block is put back in the focusing mount.

I'm thinking of sending this lens away for adjustment. Anyone know of a repairman in EU (Austria, Germany...) that is proven to do a good job and is not too expensive?