Another closer look: Micro-Nikkor 105mm f/4 AI

The Micro-Nikkor 105mm f/4 AI was released along side the 55mm f/3.5 AI in 1977 and offered a more comfortable working distance and narrower field of focus over the 55mm - the longer lens is double the weight heavier (500g) compared to 245g of the 55mm.


The 105mm micro is larger than the 55mm micro and the 105mm f/2.5 AI-S even if its feels lesss dense than the latter, with handling on modern bodies is comfortable. When the lens is fully extended its in the ballbark of a 24-70mm f/2.8G or 180mm f/2.8 ED AI-S.


^{Micro-Nikkors 55mm vs 105mm | 105mm f/2.5 AI-S vs Micro-Nikkor 105mm f/4 AI}



Whilst the 105mm micro focuses down to 0.47 - over the 2.41 of the 55mm micro - distances closer than ~1.3m (providing 1:10 reproduction ratio) will require exposure compensation as per Nikon's own user manual. Conversely, the non-macro 105mm f/2.5 AI-S has a 1m MFD which gives a 1:7:7 reproduction ratio (which is aligned to the macro lens) without any exposure compentation required! Similarly, a 180mm f/2.8 ED AI-S at 1.8m gives a 1:7.5 reproduction ratio.


^{(c) Nikon - from 105mm user manual, TTL Compensation values}

Using the micro 105mm in non macro applications is fine - when compared to the 105mm f/2.5 AI-S which at f/4 is incredibly sharp but the macro lens appears significantly better. At macro distances, even wide open, resolution and sharpness is already very good with perhaps some CA/fringing.


<sup>centre crop, near MFD, f/4


centre crop, near MFD, f/5.6


centre crop, near MFD, f/4


centre crop, f/4</sup>

Servicing

The optical design is relatively simple and common for AI lenses of the period, the lens objective is in a single unit that can be removed from the lens barrel before further disassembly. The decendant f/2.8 AI-S was a lot more complicated with internal CRC mechanism but designed to keep the shorter barrel size.


^{(c) Nikon - Micro-Nikkor 105mm f/4 AI optical cross section}

To access the lens objective, you will need to locate the grub screw hidden under the lens hood - note its note at the full extended hood position so you will need to patient. Once located, loosen the grub screw and unscrew the hood.





The rear block is accessed by removing the rear baffle but this is secured via lacqure so you will need to run a line of acetone into the hole and wait. The rear baffle also secures a mechanism to move the iris - a pin exists in the block so you must be careful to align when reassembling. The rear element can be freed with a lens spanner once the baffle is removed. The front block requires unscrewing the front via a rubber cone tool to have access - once the front block is removed you will have access to the apeture blades so need to be careful.

Re-installing the objective needs care and better with the bayonet removed so you can clearly see the alignment of various peices: the iris actuation "foot" and the stop down lever. With the optical block inserted, you can align the bayonet mount stop down lever/fork with the "pin" of the iris actuation foot.


<sup>L: long rod is the iris actuation lever | R: note the outward position of the "foot" that actuation lever sits



Top: pin required to align with the bayonet's stop down lever/fork - the pin MUST be positioned perpendicular to accept the stop down lever/fork</sup>

Helicoids

Accessing the helicoids requires you to fully dismantle the rest of the lens - as Richard Haw notes in his article the screws securing the helicoid key are increbidly difficult to remove: I was unable to free those screws even with soldering iron/IPA/acetone/lighter fluid.

• Lift name plate after removing the 3x screws - this will expose screws securing the focus ring
  
• Remove the 3x screws from focus ring and lift
  
  
• This silver key limits rotation (note the notch on the exposed helicoid. You will need to remove this key to seperate the helicoids.
  
• Remove the 3x screws on the chrome grab ring - mark the focus position to have a reference point before removing the sleave to reveal the helicoid screws.
  
• This is where I get stuck - these screws would not budge no matter what I tried. If you get here and you can remove the helicoid key you will be able to seperate the helicoids for cleaning.
  
  
  Given this copy was suffering from gritty/sticky focus - and the inability to remove the helicoid keys - I took a less than optimal approach; from the rear of the extended lens it is possible to somewhat get access to some parts of the helcoids and similarly when the focus ring is removed. With this limited access I was able to flood/clean the exposed parts of the helicoids with lighter fluid and then 99% IPA using a small headed toothbrush - with constant extend/collapse of the focusing helicoid I was able to flush a lot of old grease and more importantly gritty/dried lumps of grease. When it felt that there were very limited grease left - the extend/collapse feeling very light and dry - I applied a very thin layer of new (Japan Hobby Tool #10) grease using the same exposed helicoids and this seemed to help. Not perfect but effective (so far) with no apparent binding/clumping with the old grease.
  
  
  ^{view of helicoids from rear}
  

It must be said, the seals on this lens were an absolute pain. The helicoid keys didn't move and the bayonet mount screws were also very stubborn but at least those could be removed.