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

Andreas Thaler · Mar 20, 2024

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 after 16 seconds, the same process repeats itself irregularly as soon as the metering System of the F4S is on.

My Nikkor AF 20-35/2.8 D behaves inconspicuously, so I looked for the fault in the lens after cleaning all the contacts on the zoom and camera.

That didn't improve things

so I looked inside the lens to see if I could clean the wipers and resistance tracks of the focusing and focal length adjustment.

I managed that after dismantling the rear lens part, but that didn't solve the problem either.

Maybe there is a contact problem somewhere on the flexible circuit board in the lens or its IC is no longer fully working. That's where my options end.

Anyway the fault is not tragic.

Here are some views of the partially disassembled lens:

Golden wipers and resistance tracks for focusing and …

… focal length.

Cleaning and lubricating with electronics cleaner and silicone oil of the aperture pawl and the corresponding stops on the aperture ring.

Assembly

+++

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

forest bagger · Mar 20, 2024

As far as I can see there runs the focal length brush not exactly in the middle of the contact tracks, and where ist the focusing brush?

Andreas Thaler · Mar 20, 2024

forest bagger said:
As far as I can see there runs the focal length brush not exactly in the middle of the contact tracks, and where ist the focusing brush?

The wipers for focusing are hidden:

forest bagger · Mar 20, 2024

When the focal length brush runs not exactly in the middle of the contact tracks the camera gets wrong informations about the focal length.
This causes erroneous movements of the focussing motor.
To check if this is the case or not you have to mount the lens to an DSLR like D3, D4, D700, D800... an read the EXIF informations thoroughly.

Andreas Thaler · Mar 20, 2024

forest bagger said:
When the focal length brush runs not exactly in the middle of the contact tracks the camera gets wrong informations about the focal length.
This causes erroneous movements of the focussing motor.
To check if this is the case or not you have to mount the lens to an DSLR like D3, D4, D700, D800... an read the EXIF informations thoroughly.

If the wiper is not running correctly on the track, would all focal lengths be affected, not just the 24?

Eric Rose · Mar 20, 2024

I have one of those lenses and it's the biggest piece of crap I have ever bought.

Andreas Thaler · Mar 21, 2024

Eric Rose said:
I have one of those lenses and it's the biggest piece of crap I have ever bought.

A zoom with a focal length range that is very useful for the city.

Once a hit with photo reporters because it combined super wide angle with light telephoto.

The distortion is noticeable, PTLens has a correction profile for this that works even without EXIF (unfortunately this software is no longer available).

I don't know why that's supposed to be crap. But that's not the topic of this thread.

Andreas Thaler · Mar 21, 2024

The design of the zoom was a particular technical challenge:

NIKKOR - The Thousand and One Nights No.58 | NIKKOR - The Thousand and One Nights | Nikon Consumer

Andreas Thaler · Mar 21, 2024

I opened the 24-120 a second time, checked the wipers on the resistance tracks, cleaned them again with electronic cleaner and worked on the contacts on the lens bayonet with the rubber polishing tip on the Dremel.

But unfortunately, no success.

Never mind, I'll just focus manually.

And there are quite nice replacements here which work faultlessly on the Nikon F4S

miha · Mar 21, 2024

Andreas Thaler said:
And there are quite nice replacements here which work faultlessly on the Nikon F4S

And way better optically I guess

forest bagger · Mar 21, 2024

Andreas Thaler said:
would all focal lengths be affected, not just the 24?

That depends on the grade of misalignment between the brush pins and the contact tracks.
In most cases the flex plate with the contact tracks has moved on its glueing to the tubus.

Andreas Thaler · Mar 21, 2024

forest bagger said:
That depends on the grade of misalignment between the brush pins and the contact tracks.
In most cases the flex plate with the contact tracks has moved on its glueing to the tubus.

Could the IC be faulty?

Andreas Thaler · Mar 21, 2024

miha said:
And way better optically I guess

At least the one on the F4S

In my copy, the well-known lens separation is only mildly pronounced.

Andreas Thaler · Mar 21, 2024

forest bagger said:
That depends on the grade of misalignment between the brush pins and the contact tracks.
In most cases the flex plate with the contact tracks has moved on its glueing to the tubus.

To get to the area with the wipers and resistance tracks, the zoom would have to be dismantled considerably.

A job that is not much fun and can also go wrong.

ic-racer · Mar 21, 2024

Just a thought:

Maybe a camera problem. The camera would sense the focus and turn the screw one way or the other to achieve focus. At 24mm the image on the camera's sensor would be harder to analyze and harder to tell if it is in focus. So the physical or electrical paramerters could be out of adjustment.

All you need is a Lark V to test and adjust the autofocus

Andreas Thaler · Mar 21, 2024

ic-racer said:
Just a thought:

I'd suspect a camera problem. Not shure what that type of lens electronics need for the foucs, as there is no focus motor in the lens. The camera would sense the focus and turn the screw one way or the other to achieve focus. At 24mm the image on the camera's sensor would be harder to analyze and harder to tell if it is in focus (just like the human might have difficulty with a 24mm manual focus camera).

I tested various other lenses on the F4S, including the ones you can see in the picture. All of them behave normally, only the 24-120 is an exception.

ic-racer · Mar 21, 2024

Andreas Thaler said:
I tested various other lenses on the F4S, including the ones you can see in the picture. All of them behave normally, only the 24-120 is an exception.

Ok, just wondering. But were any wide as 24?

By the way, that is great that you opened up an AF lens. I have yet to do that.

Andreas Thaler · Mar 21, 2024

ic-racer said:
But were any wide as 24?

Yes, I tested with the 20-35, 24-50 and 24 lenses.

ic-racer said:
By the way, that is great that you opened up an AF lens. I have yet to do that.

I hate doing this.

Zooms are complicated as it is and then there is the fragile electronics.

Michael @forest bagger is a specialist in those.

You can see his work at

Service Ecke

Der Informations- und Reparaturbereich im Nikon Fotografie-Forum

www.nikon-fotografie.de

„Beuteltier“

Andreas Thaler · Mar 21, 2024

ic-racer said:
All you need is a Lark V to test and adjust the autofocus

That would be the end of my photo gear and mine too. My wife has already been through enough of that

koraks · Mar 21, 2024

Andreas Thaler said:
Could the IC be faulty?

Exceedingly unlikely. You'd expect a hard & consistent failure, not something as vague and circumstantial as this. Sorry, that doesn't help much...

Andreas Thaler · Mar 22, 2024

koraks said:
Exceedingly unlikely. You'd expect a hard & consistent failure, not something as vague and circumstantial as this. Sorry, that doesn't help much...

Yes, it helps, because I can rule it out

Andreas Thaler · Mar 22, 2024

Confidence in DIY is important, but there are a few things you should know before you reach for the screwdriver.

A zoom like this is not for hobbyists, it is comparable in complexity to contemporary cameras (Nikon F4 and F5).

I have no assignment here

Nikon AF-Nikkor 24-120mm f3.5-5.6D Lens Service Manual

Factory service manual and parts list

learncamerarepair.com

Andreas Thaler · Mar 23, 2024

The zooms I have disassembled and reassembled so far have all been for manual focusing. And each of them is a nightmare if you don't have instructions.

Removing lenses for cleaning or removing the bayonet ring and aperture mechanism to clean oily aperture blades is usually doable.

But when it comes to taking the zoom mechanism apart to replace defective plain bearings (if you have spares) or cleaning and re-lubricating the mechanism, it becomes critical.

It's all nested and if you're unlucky, everything has to be readjusted - if you manage to put it back together.

Compared to the 24-120, these zooms were relatively simple in design.

I have great respect for the engineering of this lens.

forest bagger · Mar 23, 2024

Andreas Thaler said:
Could the IC be faulty?

No - if the IC is faulty the AF wouldn't work at all and the camera would show manual focussing.

forest bagger · Mar 23, 2024

Andreas Thaler said:
To get to the area with the wipers and resistance tracks, the zoom would have to be dismantled considerably.

When you can see the brush on its way there is no dismantling needed.
Just turn the zoom slowly from 24 to 120 mm and watch carefully the path of the brush.
In most cases the brush looses it connections at one end of the path - in your case on the short end.

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

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