Nikkor 24-120/3.5~5.6 AF-D (IF): Fungus removed, assembling the rear lens block, infinity focus stop

Andreas Thaler · Dec 20, 2024

I actually bought this zoom lens for immediate use.

When I checked it, I found mold on several lenses.

That would be a reason to send the lens back to the seller straight away.

But for me it is a welcome repair project

I really appreciate this lens for my walks through the city because it covers a large focal length range up from f = 24 mm.

The obvious distortion can be easily corrected in Photoshop or with PTLens (which is unfortunately no longer available).

More about this impressive technical construction:

NIKKOR - The Thousand and One Nights No.58

This is not a photograph of the Earth taken on a NASA Apollo mission, but a view of the rear lenses of the Nikkor zoom in blue LED light.

The fungus sits on at least two lenses.

Fungus is also present on the front lenses

I had already dealt with another 24-120 AF-D once. It was about repairing an electronic fault:

Nikkor AF 24-120/3.5-5.6 D: Slow autofocus at focal length 24 mm (Nikon F4S)

Today my reporter zoom surprised me by focusing slowly and jerkily on the Nikon F4S at a focal length of 24 mm in autofocus mode. If I turn the focal length ring slightly towards a longer focal length, focusing works again at all distances even at 24 mm. If the metering system switches off...

www.photrio.com

I have no idea if I can remove the lenses for cleaning, so the next step is to look through the service manual to see how best to do this.

Stay tuned for more soon!

Andreas Thaler · Dec 20, 2024

From the service manual.

The good news:

The bad news:

Andreas Thaler · Dec 20, 2024

Removing the front lens should not be a problem.

The assembly of the rear lens unit requires centering, which I cannot do.

The only option is to mark the position of the lenses and then reassemble them exactly as they were inserted.

I don't feel comfortable doing this, but I don't see any other option if I want to get rid of the fungus.

But first I have to determine if the rear lens block can be disassembled at all.

Andreas Thaler · Dec 20, 2024

I'm wondering how I should understand Nikon's advice.

Does that mean that recentering is required after removing the entire rear unit or after disassembling the lens block?

I'm curious.

Andreas Thaler · Dec 20, 2024

From working on the other 24-120 AF-D before this shot.

Here you can see that the rear optical unit is attached with sealed screws.

This means that removing the entire unit requires centering afterwards.

So I understood the note in the service manual.

Perhaps I can disassemble the lens block without removing it ...

Andreas Thaler · Dec 20, 2024

This video also deals with cleaning fungus from the rear lenses.

Here the author removes the rear lens unit, which - according to Nikon - requires recentering.

forest bagger · Dec 20, 2024

Andreas Thaler said:
This means that removing the entire unit requires centering afterwards.

No, Andreas, only the rear lens block is the one that needs recentering after having removed it.
Additionally this rear lens block is the weak point of this AF 24-120/3.5-5.6 D because this rear lens block decenters sometimes by itself during heavy use...
So it is a good advice to leave the rear lens block unremoved, and clean only the one lens which is infected.

Andreas Thaler · Dec 20, 2024

forest bagger said:
No, Andreas, only the rear lens block is the one that needs recentering after having removed it.
Additionally this rear lens block is the weak point of this AF 24-120/3.5-5.6 D because this rear lens block decenters sometimes by itself during heavy use...
So it is a good advice to leave the rear lens block unremoved, and clean only the one lens which is infected.

Thanks, Michael!

I had to remove the rear lens block because the fungus was on the outside of the bottom lens.

Report follows.

benveniste · Dec 20, 2024

Good luck, and I hope you enjoy such projects!

Andreas Thaler · Dec 20, 2024

Removing and cleaning the rear lenses

Removal of bayonet and aperture ring.

The rear lens block is secured with three screws that are coated with varnish.

After unscrewing the retaining ring, two lenses lying on top of each other can be removed with the lens sucker.

There is some fungus on the top of the upper lens.

The lowest lens cannot be removed. The fungus is located on its lower side.

To clean the lens, I have to remove the lens block.

I use the diamond cutter on the Dremel Stylo to mark the screws and use the pointed tungsten carbide pen to make markings on the plate underneath.

This will allow the original position to be returned after assembly.

I do this for all three screws.

I remove the varnish best as possible.

Further markings.

The holder for the lens block with pressed-in lens has been removed. The fungus says hello.

Remove the mold with hydrogen peroxide. No residue remains and the mold has not yet etched the glass.

Andreas Thaler · Dec 20, 2024

The lenses are clean again.

Assembly

The shim under the rear lens block.

The lens block is reattached. The screws are aligned with the markings.

The movable part on the left must rest against the control curve on the right so that the aperture mechanism engages when the bayonet ring is put on.

The zoom is complete again.

I took some test shots with the Nikon F4 for later evaluation. Everything was sharp in the viewfinder, but that is not a reference.

Andreas Thaler · Dec 20, 2024

When focusing at the maximum zoom position (120 mm), the same problem occurred that I had already investigated and solved with my other 24-120: The AF focusing cannot find the focus point.

The cause is a slightly offset electrical sliding contact in the tube, which is probably caused by the zoom ring frequently hitting the end position.

See

Nikkor AF 24-120/3.5-5.6 D: Slow autofocus at focal length 24 mm (Nikon F4S)

Today my reporter zoom surprised me by focusing slowly and jerkily on the Nikon F4S at a focal length of 24 mm in autofocus mode. If I turn the focal length ring slightly towards a longer focal length, focusing works again at all distances even at 24 mm. If the metering system switches off...

www.photrio.com

I'll take a look at that.

But first I'll remove and clean the front lens.

Stay tuned!

+++

All information provided without guarantee and use at your own risk.

Andreas Thaler · Dec 20, 2024

benveniste said:
Good luck, and I hope you enjoy such projects!

Thank you!

Sometimes it's fun indeed

Andreas Thaler · Dec 20, 2024

Removing, cleaning and assembling the front lenses

The retaining ring for the front lens can only be removed with force and the assistance of WD-40.

Lifting off the massive front lens with the lens succer.

Ready for cleaning. On this lens there is no mold but some dust.

Done

The lens underneath in its plastic casing can be removed from its plastic mount by loosening three screws.

It doesn't look as high quality as older metal AF Nikkors. But it also saves weight.

The last seconds in the life of the fungus

Cleaned with hydrogen peroxide.

There is a greasy area here, probably from the lubrication of the helicoids. I clean it with isopropyl alcohol.

The lenses and the retaining ring are reassembled.

The latter could not be moved any more after screwing it in, but it is correctly positioned. I compared its position with my second 24-120 AF-D.

I hope that there will be nothing to clean here in the next few years

Some of the paint came off when unscrewing the retaining ring with the spanner wrench. This can be corrected with matt paint.

Andreas Thaler · Dec 20, 2024

Test on the Nikon F4E.

The viewfinder image is sharp, focusing now works even in the zoom end position at f = 120 mm.

Tomorrow I'll test the AF on objects that are further away than is possible in our apartment.

Repaired twice.

Test with film is still pending.

Conclusion

The front and rear lenses are easy to remove and clean, apart from the stubborn retaining ring and the tricky centering of the rear lens block. I hope my markings have preserved the previous condition.
If the focusing does not work in the zoom end position at f = 120 mm, I correct the position of the corresponding wiper contact, see links in the thread for the project.
The manufacturing quality of this zoom is not Nikon's peak performance. Misalignment of the wiper contact and rear lens block through normal use should not occur (see see Michael's @forest bagger comment). Nevertheless, it is a very versatile and technically impressive lens, see the link to Nikon in the thread.

+++

All information provided without guarantee and use at your own risk.

forest bagger · Dec 20, 2024

Hello Andreas,
the centering of the rear lens block is not done only by aligning the three screws with the marks you made before.
The holes for the 3 screws in the mounting flange of the lens block have a slightly larger diameter than the shaft of the screws, so that the lens block can actually be centered.

Andreas Thaler · Dec 21, 2024

forest bagger said:
Hello Andreas,
the centering of the rear lens block is not done only by aligning the three screws with the marks you made before.
The holes for the 3 screws in the mounting flange of the lens block have a slightly larger diameter than the shaft of the screws, so that the lens block can actually be centered.

I wouldn't have had a chance anyway, because markings don't help in this case either. I can't center anything myself, so I'll look at the test shots to see if the lens can be used.

Since I only use the zoom with film, there's no point in trying to adjust it myself, because I wouldn't be able to keep up with the processing and evaluation.

Since everything there is made of plastic, I assumed that by tightening the three fastening screws to different degrees the vertical position of the rear lens unit could be influenced.

How do you deal as a professional with centering problems like this?

Andreas Thaler · Dec 21, 2024

For DIY, we should try to make a statement about this lens in terms of repairability.

If the rear lens block is a weak point, the problem of blurring due to decentering will occur frequently.

And I actually had one years ago that I returned to the dealer because of this.

Similar to the Nikon F4, on which this lens was also used, there are limits to DIY. With the F4, the AF can therefore only be adjusted by Nikon.

If this project makes sense here, then it is also so that other DIYers know about it.

But maybe an autocollimator like the one shown here by @ic-racer could help?

In this case, I am very curious to see how the collimator project by @Reveni-matt progresses. I would be the first customer to buy it

Andreas Thaler · Dec 21, 2024

I could have marked the contours of the rear lens unit mount on the base plate, but I doubt that would be accurate enough.

Presumably the lens is adjusted by sight using a collimator and tiny adjustments are involved.

I am interested in how such an adjustment is carried out and what exactly is needed for it.

Or does this lens require a special device that only Nikon had, see below from the Nikon service manual?

forest bagger · Dec 21, 2024

Andreas Thaler said:
How do you deal as a professional with centering problems like this?

By pressing the lens block firmly onto its mounting plate while unscrewing it, and by noting the position of the holes in the lens block edge to the screw holes in the mounting plate before removing the lens block.

Andreas Thaler · Dec 21, 2024

forest bagger said:
By pressing the lens block firmly onto its mounting plate while unscrewing it, and by noting the position of the holes in the lens block edge to the screw holes in the mounting plate before removing the lens block.

Thanks!

What do you do if the rear lens unit of this zoom is already decentered?

Andreas Thaler · Dec 21, 2024

To what extent is the viewfinder image useful in assessing the centering of a lens?

In the viewfinder of my Nikon F4E I only see a very slight blur in the top right corner at the maximum zoom position.

Andreas Thaler · Dec 21, 2024

Andreas Thaler said:
If the focusing does not work in the zoom end position at f = 120 mm, I correct the position of the corresponding wiper contact, see links in the thread for the project.

When focusing on objects further away with the zoom set to maximum, the AF stutters. I will fix this by adjusting the sliding contact as mentioned.

I also think I have discovered another fungus star, so I will continue.

forest bagger · Dec 21, 2024

Andreas Thaler said:
What do you do if the rear lens unit of this zoom is already decentered?

Taking a picture of a test chart with a digital camera body that has a high resolution (at least 24 MPx, better 36 MPx), viewing the picture at 100% on a big monitor.
If there is not every corner of the picture ok, movig the rear lens unit some 1/10 mm, taking a picture.... and so on.
That takes more than one hour until some improvement is reached, and sometimes it won't get better...

Andreas Thaler · Dec 21, 2024

forest bagger said:
Taking a picture of a test chart with a digital camera body that has a high resolution (at least 24 MPx, better 36 MPx), viewing the picture at 100% on a big monitor.
If there is not every corner of the picture ok, movig the rear lens unit some 1/10 mm, taking a picture.... and so on.
That takes more than one hour until some improvement is reached, and sometimes it won't get better...

But does this technique work with all lenses?

I wouldn't know where I could move the rear lens on mine, they are all fixed in their mount and screwed in.

Nikkor 24-120/3.5~5.6 AF-D (IF): Fungus removed, assembling the rear lens block, infinity focus stop

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