Nikkor AI problem

[Jonas Kvist Jensen]

So I have a set of old Nikon primes, all of them working great except the 24mm, which apparantly is a AI (not AIs) lens. The tap on the rear end of the lens is longer than the AIs lenses, and somehow that doesn't work with the DSMC Nikon mount.

I was actually considering simply sawing the thing off. Is this a completely retarded idea? Has anyone had any luck with such an operation?

 

[Robert Hart]

Unnfortunately, I cannot find my old post over on dvinfo.net. I had addressed this problem when making AGUS35 adaptors and ran foul of it again on the SI2K. I posted some measurements relating to a very early version of the P+S Technik IMS-Nikon "Professional" F-Mount. This mount ( and I think also the RED iteration ) eliminates the normal twist and lock operation of offering up and moving the entire lens body to eliminate flange wear.

Hopefully, the Optitek guy who conrtributes here will post a response as I think he will be across the issue with more competence than myself.

From memory, the older Nikon F-Mount ( four screws ) on older cameras like the FM2, had the standard three lugged bayonet profile, the same flange to focal plane distance, the same internal shoulder diameter and a similar stainless spring ring which engaged under the lugs as the lens was rotated to ensure snug fitment of the flange faces. This stainless steel spring accommodated any differences in the lug front face to flange distance and compensated for wear.

The newer F-Mounts, with a four screw and a five screw fixture to the body appear identical however there are two subtle differences.

The older AI lenses as you have discovered, have an overhang of the iris ( aperture ) actuation ring which comprises the entire circumference of the ring. The older mounts have a shoulder relief cut out of the outer diameter of the mount to allow this overhang to clear. The newer mounts appear to have this shoulder relief, however it is of a slightly wider diameter and from memory, 1mm less deep from front to back. Therefore, on the newer mounts, this ring overhang rides on the wider diameter of the shoulder relief and its rear face.

On some lenses, the inner diameter of the ring will just clear the shoulder or force over it but the rear face of the overhang bottoms out on the rear face of the shoulder relief. Depending on wear and tear, the lens may partially fit or even install to the newer Nikon F-mount but the aperture ring more often than not becomes locked through becoming jammed.

The third slight difference between the lenses now comes into play. Some lenses have a deeper distance between the front face of the lugs and the flange contact face of the lens. This added clearance means that these lenses, if modified by trimming the overhang away will not mount firm in some positive lock style Nikon mounts or third party Nikon mounts with tapered rear lug faces instead of Nikon's cam spring to maintain flange face contact. As mentioned above, the spring in the newer Nikon mounts makes this issue go away in most instances although the lens may be moveable against the spring's pressure.

These positive lock style mounts employ a rotating collar on a precision thread. With some of these mounts, like the very early P+S Professional, the ring will not rotate across the lugs bcause the lens is not in true flange contact but riding forward on the iris ring overhang. The Tarzan factor then comes into play. The strongarm person brute-forces the ring over the lugs and gets lucky if they are worn, with just enough bite to hold the lens on. Sometimes, infinity focus can be achieved, sometimes not, depending on whether the lens focus movement turns past inifinity. The witness marks are wrong though.

Over time, the Tarzan doctrine breaks the screwed mechanical relay pillars between the outer ring and the threaded cylinder inside. Tarzan then ends up with a lens stuck in the mount, a jammed or free-spinning clamp ring and a stalled production.

If the iris ring overhang is removed and the lens offers up to true flange face contact, with some of the positive lock style mounts, the rotation of the clamping ring is limited and hits a stop before it comes into pressure contact with the lug faces. the lens then rattles in the mount, won't hold focus and sends the focus puller insane.

I understand that the P+S IMS-Nikon F Professional mount was soon modified to allow a greater arc of movement of the clamp ring or employed a coarser thread pitch to compensate for clearance difference between the two lens models. I never did find out what their solution was.

Later models of the AI lens or S-modified ones retained the overhang of the iris ring but cut much of it away to form a mechanical interface between the iris ring and the camera for metering. These lenses will force more easily onto the mount and in some instances the operator will not observe there is anything wrong except perhaps an initially baulky fit and maybe a tight iris actuator ring.

Im my instance, rather than attack the lens itself and devalue it, I took the newer mount ring from my otherwise standard Nikon F-Mount and later, the IMS. These are readily replaceable if one stuffs the job up. In a lathe, I skimmed the iris ring clearance shoulder diameter down a very, very small amount and machined the rear face furthur rearwards by 1mm from 0.7mm to 1.7mm to allow clearance for the overhang. When using the older AI style lenses on the very early P+S mount, there was an internal adjustment available to accommodate the added clearance between the lug front faces and the flange. I could not mix and match AI and AIS together but could adjust for either. The later IMS-Nikon Professional mount fixed this issue.

Cutting off the iris actuator ring overhang from your lens will not solve your problem entirely. There may not be sufficient arc of movement of the clamping ring on the RED Nikon F-Mount to allow the added clearance between the front faces of the bayonet lugs to be taken up and your lens may then rattle in the mount. If the thread pitch of the clamping ring is too fine, then the ring, if the arc of movement permits, may then pass the bayonet lug clearances around to the next clearances and the lens will fall out before lockdown can be achieved.

I do not know the RED Nikon F-Mount so this is all speculation on my part. Take heed of the wise minds you encounter here.

 

[Jacek Zakowicz]

So I have a set of old Nikon primes, all of them working great except the 24mm, which apparantly is a AI (not AIs) lens. The tap on the rear end of the lens is longer than the AIs lenses, and somehow that doesn't work with the DSMC Nikon mount.

I was actually considering simply sawing the thing off. Is this a completely retarded idea? Has anyone had any luck with such an operation?

I believe you're right and the tab indeed interferes with the internal mechanism on the Red Nikon mount. There was a thread about some guys just cutting it off like you were planning. Nothing wrong with that . The fixed tab is there to protect the rear element that;s quite big on the 50 f1.2 and sticks back a bit. I don't know about the old 24mm but post a picture and I'll try to help...
I personally would not cut anything off a pristine Nikkor 50 f1.2 but that's me....
Just make sure you are not cutting off the spring loaded moving AI tab as this is the link to the iris control through the mount. if you cut it off you'll lose the iris control through the camera.
hope it helps....

 

[Matthew Scott]

I used pliers, rip it off!

 

[Jacek Zakowicz]

BTW our Nikon ProLock has no issues with ANY Nikon F mount lenses. No hacking or ripping necessary.
But, to be fair- we do not have any motors or actuators inside the mount...

 

[Jonas Kvist Jensen]

Thanks guys. I'll post some pictures later today.

 

[Jonas Kvist Jensen]

Ok, so here's a quick snap of the problem. That's my 24mm on the left, and my 28mm on the right. As you can see the 24 (AI) has a slightly longer tap than the 28(AIs).

 

[Jacek Zakowicz]

Yes that seems to be the perpetrator. Maybe try to file a little off first so the tab is the same as on the 28mm?

 

[Jonas Kvist Jensen]

Yes that seems to be the perpetrator. Maybe try to file a little off first so the tab is the same as on the 28mm?

Yeah, that's sort of what I had in mind. Do you know if it is possible to detach the black piece of metal (I'm sure it has a real name), so I can file it down without risking to scratch the glass?

 

[Jacek Zakowicz]

yes it could be just a shroud held by screws around the mount

 

[Vance Colvig]

I removed the Nikon DSMC mount and put a cresent shaped sliver of one-sided tape on the bottom of the DSMC mount so I could detect a rub or interference. I then set the lens on a towel on a hard surface and rotated the lens so the tab was on the bottom. Took a flat jewlers file and removed some of the material on the tab by filing bit by bit until the lens would lock in the Red DSMC mount without moving or touching the tape in the mount. When desired clearance was acheives, a little matte black paint from a spraycan on a tiny brush to touch up the exposed filed area. A black magic marker or sharpie would probably work also, but may easily wipe off. I have a complete set of vintage Nikon lenses that I am unwilling to modify, but I do have Optitek's Nikon Pro-Lock for just those lenses. Love that mount! Hope this helps.

 

[Ulf Krentz]

+1 for the Optitek for the older Nikon lenses, we did not like to modify our old glass and had trouble with:

20 2.8
24 2.0
35 1.4
50 1.2
55 1.2

That said, for new Nikon glass the original RED Nikon mount is probably the most solid solution.

Cheers, Ulf

 

[Jonas Kvist Jensen]

Thanks all. I think I'll try the filling option (once I've gathered the courage). It doesn't seem too big an operation. I'll post the results - good or bad

 

[Ulf Krentz]

Jonas,

Your choice, a lot of people did so, I never heared of any problems so far. Would be a good idea to take the part off the lens though...

Cheers, Ulf

 

[ALLSTAR]

A-mount system dosen`t have Nikkor AI problem.

 

[Erick von Schulz]

Converting AI to AIS solves alot of issues.

John White does it for about $25

http://www.aiconversions.com/

I had the issue in the past and used him to convert all my old Nikon glass to AI-S. Now they work on every camera and mount with no issues.

I hope that helps some people. Like an earlier post mentioned it was an issue on the old PS Technic adapters and Newer Nikon Cameras as well.

Also you can use the glass on all of your newer Nikon Bodies with no issues after the conversion. I have had at least 10 lenses converted over the years from John White.

 

[Jacek Zakowicz]

I don't think that this has anything to do with AI/AIS conversion

 

[Ulf Krentz]

Actually, having a greater focus rotation AI lenses are considered better for cinematography...

 

[Jonas Kvist Jensen]

Mission accomplished!

Took out the five screws and removed the back part of the lens. Then used a small hack saw to cut of a bit of the tap. Reassembled with no problems and the lens is attaching perfectly to my Scarlet now.

Thanks for the input everybody.

 

[Nick Morrison]

That "shark fin" comes up in Contax glass, too. People regularly just saw them off. They would hit people's 5D mirrors, etc.

As someone mentioned, they were designed to protect large back elements.