Nikon F Focusing Problem

basicn · Jul 22, 2019

You should check the mirror position. There should be a screw for adjusting the position of the mirror somewhere inside the body, near the mirror.

Main_Cogg · Jul 22, 2019

ronnies said:
Is the mirror out of alignment ?

Ronnie

That has crossed my mind. Can the mirror be far enough out of alignment to cause the focusing problem and yet still go through it’s travel? I wouldn’t think so, but I really don’t know.

Dali · Jul 22, 2019

Main_Cogg said:
That has crossed my mind. Can the mirror be far enough out of alignment to cause the focusing problem and yet still go through it’s travel? I wouldn’t think so, but I really don’t know.

Yes it could! I had once a mirrror issue with an OM1 and it was clrealy impossible to focus correctly The mecanism was worn out and the mirror position was not correct anymore.

KN4SMF · Jul 22, 2019

I never thought of mirror. THAT'S IT!

Main_Cogg · Jul 22, 2019

I’ll have a different screen by the end of the week, so I’ll set this project aside for now. I’m not going to mess with mirror until I’ve ruled out the focusing screen. I have plenty of other projects to deal with in the meantime.

GRHazelton · Jul 22, 2019

Bronica S2a cameras can have similar problems, caused by foam spacers in the focus screen assembly and by foam mirror bumpers, one or the other or both. Ecch! Given the NOISE the S2a makes on firing, why is foam needed for the mirror bumpers? No chance of sneaky shooting with the Bronny!
Blasted foam is the Achilles heel of so many cameras. Back in the day "good" cameras - like the Nikon F - used good engineering to keep the back light tight, or string or yarn which didn't deteriorate. Now moths might be another story....

reddesert · Jul 22, 2019

Both of the focus offsets: lens at infinity has groundglass focus at 7 feet, and lens at 24 inches has groundglass focus at 19 inches - are consistent with the groundglass being about 1.2mm too far away from the lens. So that is the size of offset you are looking for.

The Nikon F has an easily removable focusing screen, but I think it is possible to put the focusing screen back in without really getting it all the way seated. You have to push the finder release button very firmly to get the screen to drop all the way into place.

Other possibilities could include that there is some junk, bent metal, etc, either keeping the focusing screen from dropping all the way onto the frame.,or keeping the retaining mechanism from holding the focusing screen properly. Or that the focusing screen and its frame has been damaged in some way so it doesn't sit properly.

If the mirror were 0.6mm too far back/down it would create a 1.2mm offset at the groundglass, but I think it's far more likely to be the groundglass position, as that could have been played with any number of times by a user over the 50+ year lifetime of the camera, while the mirror depth would be hard to get at.

The prism isn't an issue because the prism is just a device for looking at the groundglass; the GG sets the focus position. (I think it was verified upthread that the problem exists with the prism removed.)

As the pictures Brian posted show, an F focusing screen is a thick item, much thicker than a screen for an FE or similar. If one tried to put the F screen in upside down, the camera wouldn't focus at all.

abruzzi · Jul 23, 2019

not to derail, but I have a question about mirror position: if the mirror is out of position, would the focus error manifest more at the upper part of the image than at the lower? Assuming the pivot point of the mirror is fixed of course.

Willyekerslike · Jul 23, 2019

reddesert said:
If one tried to put the F screen in upside down, the camera wouldn't focus at all.

Yes it will focus with the screen upside down. The screen as most call it is in two parts the lower part, if fixed in the camera correctly, is the ground plastic part that you focus on. The second part appears to be glass and is I think to magnify the image. When you put the screens in upside down the point of focus is just a couple of feet away at the infinity mark on the lens.

To strip the screen down you undo the screws and the screens lift out fastened together with clear stick tape. Remove this tape and you have the two parts seperated. This makes it easier for cleaning. When you put the two pieces back tpogether after cleaning you need to get both parts the same as they were before you took them apart. If you get the botton part, the focus screen upside down it will not focus at infinity when thre lens is set to infinity. It will be further away from the camera than the two feet you get when you put a correctly asembled unit in upsidown. That is my experience using a 50mm lens yours may be different. A perevious poster KN2SMF gave this information previously in this thread #20.

swchris · Jul 24, 2019

abruzzi said:
not to derail, but I have a question about mirror position: if the mirror is out of position, would the focus error manifest more at the upper part of the image than at the lower? Assuming the pivot point of the mirror is fixed of course.

I would think the same.

basicn · Jul 24, 2019

abruzzi said:
not to derail, but I have a question about mirror position: if the mirror is out of position, would the focus error manifest more at the upper part of the image than at the lower? Assuming the pivot point of the mirror is fixed of course.

No

abruzzi · Jul 24, 2019

basicn said:
No

Why not?

Theo Sulphate · Jul 24, 2019

abruzzi said:
Why not?

Draw the lines - you'll see distance is the same regardless of upper or lower part of mirror.

reddesert · Jul 24, 2019

Willyekerslike, you are correct, I didn't think of the possibility that the groundglass plus magnifying lens had itself been disassembled and put back together wrong (and on more careful reading, I see that is what poster KN2SMF was suggesting). I haven't ever disassembled the focusing screen assembly (or gotten it so dirty that I needed to).

The question of mirror position: if the mirror was at the wrong angle (meaning the pivot point at one end is fixed), then what happens is the image moves relative to the focusing screen. The center of the image is displaced and the image rotates, so the top and bottom of the image focus at different places. If the entire mirror rather than pivoting moved forward/back or up/down, the image would be displaced away from the focusing screen, causing a focus shift. But it's hard for me to imagine that this is the root cause of the problem unless someone was really monkeying with the insides of the camera. The Nikon F is a very robust camera if not horribly abused. I've had one for over 30 years and have never even dreamed of adjusting the mirror. If I have good fortune, hopefully I'll have it for another 20-30 without touching the mirror, and after that, I probably won't be able to lift it.

Main_Cogg · Aug 23, 2019

Finally got back to this project. I received the 2 focusing screens that I found on the bay, both are in very good condition. Tried them both in the camera, no change at all. This camera may become a shelf queen.

__Brian · Aug 23, 2019

This problem is a puzzling one- I had a similar problem with an SLR with a broken mirror, I found a replacement- but it was too thick. The mirror is silvered on the front, so the overall distance to the finder was less than the film. The reverse would happen if the mirror were too thin.

COuld you post some pictures of the mirror box?

Main_Cogg · Aug 24, 2019

A few shots of the mirror, maybe somebody will see the problem.
Thanks again

__Brian · Aug 25, 2019

I just looked at my 641xxxx Nikon F: your mirror looks just like it does, small cut-outs at the lower edges..

With the lens at infinity, the actual focus through the viewfinder is too close, but the actual focus on film is good: that means the path to the focus screen is too great and needs to be reduced. There are shims under the focus screen- removing these might be enough. Otherwise, polishing down the viewfinder frame might be enough.

This is an early Nikon F. If you want a working one, you could put this one up for trade for another F body.

Main_Cogg · Aug 25, 2019

I tried removing the shim, no noticeable change. I took out the ~2mm thick spacer that sits on top of the shim. That moved the focus just a little bit too far the other way. I could make a custom thickness spacer, but that’s not really fixing the problem.

jerrybro · Aug 26, 2019

What is that horizontal line across the bottom of the mirror? It looks like a scribe mark.

__Brian · Aug 26, 2019

I see the problem: your picture #3 shows the cause of the error. The frame that holds the screen is at an angle to the grooves of the light baffles of the mirror box. The frame should run evenly across the tops of the lines.

Uploaded shot of my Nikon F. The metal of this frame is very thin and easy to bend - I know first hand, after screwing it up being too rough cleaning the old foam of the mirror bumper out. I had to take it out and bend back into the correct shape, then insert into the mirror box. Whoops.

__Brian · Aug 26, 2019

This is a shot of the mirror box from my Black Nikon F that was professionally CLA'd some 20 years ago... I did not screw this one up!
I also checked the focus on the camera that I screwed-up then repaired: the focus through the viewfinder agrees with the distance scale.

Main_Cogg · Aug 26, 2019

Thanks Brian, that would make sense. I'm off on a vacation for the next week , I'll get my big hammer out when I get home and straighten things out.

Main_Cogg · Oct 4, 2019

I finally got back to this project and I’ve found the problem, it’s the mirror. As the shutter is wound the mirror moves moves up about 2 mm and suddenly all is right in the world. I still can’t believe that in all the time I was messing with this that the shutter was always released. I guess just from years of habit, cock shutter, shoot. Don’t leave the shutter cocked.
I shot a roll with it, on a tripod checking the distance scale to a tape measure, close enough for most work. The negs came out good, except for a light leak. The shutter curtain has a tear that’s about 3 mm long. I don’t think that I’ll put any more effort into this one, it can have a rest on the shelf.
Thanks again to all who have helped on this one.

Nikon F Focusing Problem

Privacy & Transparency

Privacy & Transparency