Cleaning, Repairing and Servicing a 50mm f/1.4 AI

The 50mm f/1.4 AI and its optical formula was inheritted by its successor and beyond which meant it was in production from the late 70s to 2025 so this lens as an option comes with a well known optical performance and can be found a slightly lower cost than its succesor. The f/1.4 AI ran from 1977-1981 so servicing is a must, with my copy having sticky and unusable focus as well as being very dirty.



When casually examining the 50mm f/1.4 AI it's >very similar in appearance to its succssor and a departure from many AI lenses that have distance scale printed on a sleave (like the 50mm f/2 AI) but the 50mm f/1.4 AI lens barrel is very similar to other AI-S. However, with many AI lenses, the internal optical elements are housed in a single removal optical unit rather than separate optical groups secured into the lens barrel.

The construction is different from its successor, the Nikkor 50mm f/1.4 AI-S (even though that also houses the elements in a single optical unit), with this lens actually a little easier to service. Unusually, Richard Haw's site does not include an article for this specific lens - many New-Nikkor (sometimes called Nikkor K) are similar to their AI successors, but this is NOT the case with the 50mm f/1.4 AI and the 50mm f/1.4 K. An alterntaive resource exists here.

Access to the optical block

• Remove the front retaining ring by extending to MFD and releasing the grub screw. Be careful as the retaining ring secures optical block. Pull the optical block out.
  
  
  
  The optics are arrange in as 7 elements in 6 groups, split into the front (3 elements, in 3 groups) and rear (4 elements, in 3 groups). If you need to put down the removed optical block, use the front retaining ring (face down) to sit the lens to avoid damage to the front element.
  
  
  
• Front optical group
  • spin the nameplate off which acts as a retainer for the front element. Spinning this can remove either the nameplate or the entire front optical group and its 3 elements.
  • the rear of front optical block houses the 3rd element that you can spin off and this will allow access to free the 2nd housed element from the rear of the block
• Rear optical group
  • spin off the rear opitcal group - similar to the front, it may bring with it the cemented 4th-5th element (in front of the aperture blades) but this is usually sealed. If the cemented 4th-5th element remains in place you will need IPA to soften the seal and then removal via a slotted lens spanner
  • the 6th element is secured by a retaining ring that will require lens spanner to remove from housing
  • the rear (7th) element is not removable from its housing

Access to the Helicoids

• Remove the optical block as above
• Remove the rubber focus ring to access the sealed access hole and add IPA to soften. Spin off the ring which will expose the 3x focus ring screws.
  
  
  
• Remove the rear bayonet mount - I find that many of these 50mms have at least one of the bayonet screws glued - to free on this lens, I splash acetone onto the mount around the screw head and surrounding areas and leave to soak before removing
• Remove the aperture ring - Locate the 2x screws on the aperture ring and remove then remove the aperture fork and lift the aperture ring
  
  
  
• Take note of the helicoid keys and mark infinity - note that at infinity and MFD the inner helicoid position
  
  
  
  If you do not need to fully clean, you can leave the chrome grab ring in place,
• Remove the focus ring, via 3x screws and slide off the focus ring and the distance scale
  
  
  
  At this point you can unscrew and seperate the inner helicoid so be careful.
  
  Note that the focus ring has 4x screw holes - at the top under inifinity, there is a group of 3x screws with the central hole aligned to the optical block notch which in turn is at the infinity position; mark this hole to reference
  
  I would normally mark infinity position on the central helicoid but its not that necessary here.
• Remove the helicoid key, collapse the central helicoid past infinity and mark the central helicoid aligned to infinity - this will let us verify assembly. Take note of the ~3/4 revolution required to collapose when mating and aligning. If the helicoids get stuck at the fully collapsed position, reattach the focus ring to get more leverage to free yourself
  
  

  Inner helicoid

  Locate the mark you previously made on the central helicoid and slowly rotate the inner helicoid til it separates - mark separation point as it aligns to your central helicoid mark
  
  
  
  Be as accurate as you can be for this separation mark, which can be difficult with old sticky grease. There is an helicoid mating point very close to the correct mating position - this means its a little painful to mate and if you get this wrong, you will notice on reassembly that the optical block sits too deep in the barrel and you'll have to go again.
  
  

  Outer helicoid

  Hold onto the central helicoid and rotate to separate. Once separate, mark alignment of central helicoid to the outer helicoid's infinity position.
  
  
  Clean thoroughly with lighter fluid and IPA. I used a light and greasy NGLI #00 grease (NPC BC-13A) for this lens but very very thin on the inner helicoid and slightly more on central. When remated, the inner helicoid should be very smooth and light otherwise it'll be tiring to turn with the required torque.

This lens is quite simple to service and the only gotcha is the inner helicoid mating point and ensuring correct choice of grease but its probably one of the easier lenses to clean.