This is what the smallest aperture on a Nikkor AF 35/2.0 D should look like. If the Nikon F4 has the common aperture control problem, the aperture closes with a delay or not completely. This can result in exposure issues.
The Nikon F4, Nikon's fourth generation of top-of-the-line SLRs, introduced in 1988, often has a problem with the aperture control. In this case, the aperture on the lens does not close completely and/or with a delay. This can result in exposure issues.
Lack of lubrication, no defect, no replacement parts required
The cause is a lack of lubrication on a ratchet gear in the aperture control, which is located inside the mirror box on the wind side. This prevents the ratchet gear from turning freely and causes the aperture control electronics to receive incorrect values.
This ratchet gear in the aperture control of the Nikon F4 is missing a drop of oil.
The connected aperture lever in the mirror box, which couples with the lens, then moves only slowly or not completely down when the shutter is released.
Aperture lever in the mirror box.
As a result, the aperture on the lens closes with a delay or not completely.
This is not a defect, and no replacement parts are needed to fix it.
Check a Nikon F4 to see if the problem is present
To check whether an F4 is affected by this, I recommend the following:
- attach a lens (AF or MF),
- for an AF lens disable AF on the F4,
- select manual exposure mode (M),
- set the shutter speed to 1 second (or another value),
- select the smallest aperture on the lens,
- release the F4 and look into the lens from the front. The aperture should close completely to the set value without delay (compare the aperture opening with the lens removed from the camera).
- If it does not, the problem is likely to be present.
An additional indication of the problem is a screeching noise when the depth button is pressed. This causes the ratchet gear to turn quickly and the lack of lubrication causes the noise.
When you press the depth button (green arrow), the aperture lever (red arrow) moves down. This causes a connected lens to stop down to the set aperture value.
Sustainable solution: removal of the front plate and lubrication
To fix the problem, the front plate of the F4 with the mirror box must be removed and the ratchet gear lubricated.
Front plate with mirror box removed (below in the picture, interior view). Blue arrow: position of the ratchet gear, wind side.
I have written a comprehensive article on this topic on PHOTRIO, which deals with the problem in detail and gives some background information:
Nikon F4: Mission Aperture Lever - two ways to fix the aperture problem
Among other things, a shortcut was also investigated that solves the problem from the outside without removing the front plate, but it is problematic (oil is introduced into the camera with insufficient control. This can lead to further problems).
Here, I will show you the sustainable and correct solution that we have already discussed here on PHOTRIO.
Article in SPT Journal as a basis
Complete instructions for removing the front panel of the Nikon F4 can be found in the SPT Journal, which describes all the necessary steps in an understandable way and which we are working with here.
The problem and its solution are also described there. However, there is only a mention that the ratchet gear can be lubricated from the outside without disassembling the F4. Unfortunately, there are no instructions on how to do this. Therefore, the only option is the more laborious method of removing the front panel and mirror box to access the area and apply lubricant precisely.
Comprehensive technical information on the Nikon F4 in SPT Journal (39 pages)
These instructions are a prerequisite. I am supplementing them with photos and comments and providing tips from my own experience.
The prerequisites for successfully fixing the problem DIY are:
- SPT article.
- Basic experience with servicing/repairing electromechanical SLRs and the necessary basic understanding; soldering skills.
- Tools for precision mechanical and electronic work, in particular: JIS crosshead screwdrivers, sizes 0, 00. Soldering iron or soldering station. Light oil for lubrication of fast moving parts, I use Nyoil.
I strongly recommend reading the mentioned article on PHOTRIO and the article in the SPT Journal first and then deciding whether to do the work yourself or take your F4 to a qualified repair shop.Fixing the aperture control problem involves the following steps:
- Removing release-side top cover
- Removing rewind-side top cover
- Removing bottom cover
- Removing front plate/mirror box assembly
- Lubricating the ratchet gear
- Reassembly
- Test
Practical work on a Nikon F4
After this introduction, we will move on to the practical work and look at the individual steps on a Nikon F4.
An article on this topic will be published on PHOTRIO in 2026.
Additional information: function of the ratchet gear in the aperture control
I looked in the Nikon Repair Manual for the F4 to see if I could find any information about the ratchet gear, which plays a central role in the aperture control.
The ratchet gear (blue arrow) rotates quickly when the aperture mechanism is triggered (second picture).
It is coupled to a disk (white arrow) that runs through a U-shaped photointerruptor below.
There are notches on this disk that interrupt a light beam (infrared supposed) between sender and receiver of the photointerruptor. This generates electronic pulses that work as a counter.
For example, after X pulses per unit of time an action is completed or an action is started. This could be closing and opening the aperture to a certain extent.
It is therefore clear that this ratchet gear with the coupled disk has an important control function within the surrounding aperture mechanism.
What is a Photointerrupter?
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A word of cautionPlease keep in mind that you’ll be using highly flammable solvents for service work, and their fumes are harmful to your health. Soldering also produces fumes that should not be inhaled. Dangerous high voltages can be present when handling electronics, especially in conjunction with electronic flash units and mains. Therefore, familiarize yourself with the safety regulations beforehand and ensure your safety. Depending on the activity, protective gear may be advisable.
All information provided without guarantee and use at your own risk.
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