Wide and Obscure: the Nikkor 18mm f/4 AI

The 18mm f/4 AI is another lens documented on Nikon Thousand and One Nights tale. The ultra-wide lens was produced from 1977-1982 as a premium lens coming from the New-Nikkor/K incarrnation, and was replaced by slightly faster 18mm f/3.5.

This 18mm f/4 is not very common on the used market in the UK - Roland Vink's data showing around 9000 New-Nikkor/K and AI units produced - and when it's available is reasonably pricey - it has a mixed reputation as per Richard Haw's usual great write up and further feedback from the Nikon Gear forum.

It's an interesting looking lens for sure.



Even in its short production run it had a number of makeovers. The lens featured a 86mm filter ring and slightly bulbuous focus ring that was slightly remminisent of the 20mm f/4 AI slip on HK-3 lens hood in the way the focus grip is altered.

The optical construction is a 13/9 design:


^{(C) Nikon - 18mm f/4 AI optical cross section}

Performance

The lens has some very vintage characteristics. Sharpness at the wider apertures isn't great and at infinity/extreme far distances can be quite mushy - it seems better closer at these apertures. Stopped down to around f/8 it gets much much better but any modern day lens at f/8 is going to be very good.





It's performance with bright light in the frame is ok: it's very flare resistant but can be suspetable to ghosts.






Decent sun-stars can be acheived:



One of the criticisms of this lens is the moustache distortion:



At near focus (~0.5m):




Colour is good and comparable to the contemporary 20mm wides - DX/crop images below are colour corrected to a consistent WB using the WhiBal reference card. The 18mm f/4 AI vignettes heavily but you see the minor effects on the corners (even in DX crop).

Servicing

This lens isn't hard to service but it does have an unusual set up for the optical block that typically houses the lens elements and the aperture mechanism.

• Remove the shallow hood and use lens spanner to remove the front optical block
  
  Set the lens at infinity and aperture at f/22
  
  
  
  
  
  The front element is bulbuous so be careful how you sit this - sit it in the removed hood and set aside
  
  
  
  
• Remove the rear optical block
  
  The rear block is secured by 3x secures and has a guide pin (see the pin at the 5 o'clock position below)
  
  
  
  
  
  Note the position of various parts - you may wish to mark the inner brass helicoid for the orientation after removing the focus ring for easy reassembly, as this is at infinity and at f/22 (note the position of the aperture fork)
  
  On the optial block you will see the hard focus stop - on the image below its the chrome/grey out of focus square blob.
  
  
  
  
• Remove the parts to get the the helicoids
  
  Nothing too complicated in at this point, remove the rear mount and pull off the aperture ring, remove the screws to free the focus ring and then the aperture coupler screw.
  
  
  
  
  
  Remove the 3x screws on the chrome grab ring and slide off towards the rear - remove the ident spring (after marking one side of it) as its easier to reassmeble.
  
  
• Mark the infinity position on the rear and central helicoid
  
• Separate the outer and central helicoids
  
  These steps will be very familiar for anyone who has serviced a 50mm f/2 AI and the like. The helicoid post is at the TOP, towards the front for the lens and the lip faces the rear. Unscrew the helicoid key screws and slowly seperate, marking the off separation point.
  
• Separate the central and inner helicoids
  
  The inner helicoid separates towards the front of the lens (unlike the 50mm f/2 AI). Mark the position of seperation
  
  
• Clean and relube the helicoids
  
  Clean with the usual tissues, lighter fluid, dish soap, kitchen grease remover (Cilit Bang kitchen grease remover works well once you've done with lighter fluid!) and final clean with more lighter fluid and IPA. When ready use a thin layer of soft grease, like NPC BC-13A, on the inner helicoid surface and very thin on the central helicoid mating surface. The front optical block with its 8 elements sit on this helicoid so we want the helicoid not to bind. Once remated turn them to ensure its not too heavy but not too dry.
  
  A thin layer on the central helicoid as you mate to the outer helicoid.
  
  The helicoids on this lens aren't very troublesome to remate.
  
  

Conclusions

I find the old wide angle Nikkors relatively fun to shoot with because of their perceived optical flaws, in particular ghosting. This lens doesn't compare to modern equivalents and dare I say even some of the modern kit lenses but for the time it was released the handling of flare is impressive although probably just below the likes of the 20mm f/3.5 AI and 20mm f/4 AI. Compared to the 20mms, the focos throw is shortest (at 70degrees) and then lens is the heaviest of the 3 at around 320g - the large 86mm filter ring makes it not as pocketable as the others.

But this is an 18mm and gives a 100 degree field of view - as with wider lens that extra 6degrees can be very noticiable and useful in tight spaces. Using the 18mm for things other than architecture where you need to be careful of your parallel lines and using it for street photography can be hit and miss: the wider you are the closer you need to be and again the wide angle will be pushing objects in the background further away which has its own challenges.

It's a handsome lens with a unique look and fun to use and has an appeal for vintage manual shooters but there are better modern optical options but most people finding and using this 18mm f/4 AI will already know that.