Nikkor Tales: The Nikon 105/2.5 AI Nikkor

Early AI Nikkor 105/2.5 w/ custom rabbit ears fashioned from a non-AI lens version

  Updated Sept. 3, 2024

   If the Minolta MD Rokkor-X 100/2.5 we looked at previously is the definition of a sleeper, the subject of this post is anything but. I feel quite safe in postulating that there has been more virtual ink spilled over the Nikkor 105/2.5 (in all its guises) than any other medium (75-105mm) telephoto of the vintage 35mm era :-). Well, let's spill a little more, but hopefully we will cover some fresh territory in the process and see why Nikon kept this lens design all the way to the end of the film era in the early 21st century and why it remains one of the best performance-to-price ratio vintage lenses you can lay your hands on today :-).

  Origins

    The lineage of the 105/2.5 Nikkor can be traced to Nikon's (Nippon Kogaku at the time) earliest post-WWII efforts to establish themselves as a player in the 35mm rangefinder camera market, over 75 years ago. Design work on the 105/2.5 itself (labelled 10.5cm, as was industry practice into the 1960s, in many cases) was started in 1949 by Zenji Wakimoto, but the basic optical layout was derived from the Sonnar-type 85/2 Nikkor-P (also labelled as 8.5cm; with the P = Pente = 5 elements) that Saburo Murakami had started development of in 1946 and was released for production in 1948 with Wakimoto continuing to improve the design over the following year or so before embarking on the 105/2.5 project. The similarities can be clearly seen below. 

Nikkor-P 8.5 cm f/2

Nikkor-P 10.5cm f/2.5

    It was the 8.5/2 Nikkor-P that put Nikon on the map with photojournalists, when LIFE photographer David Douglas Duncan was introduced to it by Japanese photographers Ryuichi Murai and Jun Miki in June 1950. Upon seeing a portrait of himself snapped by Miki-san, Duncan was duly impressed and the very next day they travelled to the Ohi plant where Nikon was producing its lenses at the time. Duncan purchased the 5cm/1.5 and 13.5cm/4 Nikkors in M39 Leica Thread Mount (LTM) directly from the factory and had both lenses with him when the Korean War broke out later that month. The resulting images in LIFE magazine made both Duncan's and Nikon's reputation. The 10.5/2.5 Nikkor-P was released for production in the spring of 1953 (coincidentally, about six months after the end of the Korean War) and went on sale in mid-'54 in both M39 LTM and the Nikon S rangefinder mount. Almost 22,000 rangefinder 10.5/2.5s were produced (predominantly in S-mount) by Nikon, edging out its illustrious progenitor, and a sign of even greater popularity to come in SLR (single-lens-reflex) form.  

    With Nikon's decision in 1957 to prioritize development of the F-mount SLR, there came a fresh challenge for their optical designers. Because the addition of the reflex mirror mechanism necessarily pushed the rear of the lens further away from the film plane, designers could not simply change the mount of the S-mount rangefinder lenses to F-mount and be done with it. Compensation had to be made for the increased distance (termed back-focus). The 8.5/2's formula simply could not be compensated enough, so Nikon abandoned any thought of adapting it to F-mount and began development of a clean-sheet 85mm lens designed for the SLR from the outset. However, by making a 1.0mm reduction in the thickness of the rear-most fifth element of the 10.5/2.5, Wakimoto was able to achieve sufficient back-focus for the F-mount with the imaging characteristics of the lens remaining virtually unchanged, permitting the lens to debut among the original six F-mount lenses that accompanied the Nikon F in June 1959. The 10.5/2.5 in F-mount was a massive hit, with sales of over 131,000 units from 1959 to 1971, remaining Nikon's most popular short telephoto (the 6-element 85/1.8 Nikkor-H in F-mount, finally released in 1964, sold just under 75,000 copies by 1971). Nevertheless, Nikon would not rest on its Sonnar-based laurels; they felt that they could still improve the mighty 105. And they would be right :-).

Schematic of the Sonnar-type F-mount Nikkor-P 105/2.5 circa 1965

    In similar fashion to Wakimoto's standing on the shoulders of Murakami to reach even greater heights of Nikkor performance with the 85/2, in 1966 Yoshiyuki Shimizu, Wakimoto-san's prized pupil, developed an entirely new version of the 105/2.5, based on the Xenotar optical layout. That Nikon waited another four-and-a-half years to introduce it to the buying public was a testament to just how well the Sonnar-type 105/2.5 was doing. A 16-year run for any lens design has to be considered a success :-). But the new 105 would make that achievement look merely ordinary, and would go on to sell for a staggering 34 years with the only optical changes consisting of improvements in coatings, making it the second longest-lived F-mount Nikkor optical design, ever (the rights of seniority belonging to the 35/2 manual focus Nikkor produced for a full four decades :-0). The new and improved 105/2.5 would sell 138,000 units in just under four years, eclipsing the 12-year total of the original F-mount lens. By the end of a decade in existence it had tacked on another 214,000 sales for a total of over 352,000 in 10 years, with the AI version being the most popular, averaging sales of 3,275 per month from Sept. 1977 to Sept. 1981. The final AI-s iteration would be the most numerous of any generation with almost 164,000 sold in the remaining 24 years of production, a remarkable achievement for a manual focus (MF) lens that lived all but four of those years in the auto focus (AF) era, when MF sales had dropped by 90% by the end of the 1980s.

Xenotar 105/2.5 optical layout from 1971 - 2005 w/ AI-s version shown circa 1988

   All versions of the 105/2.5 were designed with portraiture as a main priority and aberration correction was weighted towards closer focusing distances, particularly with the Sonnar version. Spherical aberration was slightly undercorrected to provide improved background bokeh and a gentler falloff from focused to out-of-focus areas at the expense of a bit of overall sharpness, which suited portraits just fine. Astigmatism at infinity was also undercorrected in favor of full correction at closer distances. Wakimoto chose to undercorrect for coma, too, so as to keep contrast a bit more muted. There is no big jump in contrast as you stop down with the Sonnar as compared to many other vintage lenses of the time, and as we will see next, the new Xenotar version of the 105. 

    How many times is a sequel to a great book or movie even better than the original? It happens just often enough to be noteworthy but is definitely on the rare side. When Nikon set Shimizu-san to the task of redesigning the already-legendary 105/2.5, he had the unenviable task of trying to improve the lens technically, while not losing any of the "je ne sais quois" of its ancestor. To say he succeeded would be an understatement, although, of course, there are dyed-in-the-wool Sonnar fans that still look upon the Xenotar with disdain ;-). With any basic optical formula, there are inherent strengths and weaknesses, and optical designers have to prioritize where they will spend their limited optical correction budget. The adoption of the more symmetrical Xenotar layout for the new 105/2.5 versus the strongly asymmetric Sonnar gave Shimizu more freedom in some areas:

• There is inherently more distortion in an asymmetric design than a symmetric one, so Shimizu had less to do in correcting for distortion than Wakimoto did with the Sonnar (which was still well above reproach with only 0.5%). 
• There is less field curvature with the Xenotar layout.
• Astigmatism is also very small, so it can be more easily corrected at both closer and further distances, rather than having to make the trade-off of better correction at one end of the focusing range or the other.
• Shimizu kept close-range spherical aberration undercorrected to retain smooth background blur at wider apertures. As the user stops down, the effects of SA are lessened.
• Coma is also corrected better than with the Sonnar, but still slightly undercorrected to maintain lower contrast at full aperture. Stopping down virtually eliminates coma.
• Particularly with the multicoated versions, the Xenotar exhibits far more flare- and ghosting-resistance, something that will matter not a whit to the Sonnar-lovers out there ;-).

    Whereas the Sonnar maintains a very consistent look and makes smaller gains in contrast upon stopping down, the Xenotar is very portrait-friendly from f/2.5 - f/4, but flips a switch at f/4 - f/5.6 with a distinct boost in contrast and sharpness. That makes it a bit more versatile, being very effective for landscapes out to f/11 or so before diffraction starts creeping in. On the iterations with f/32, you would be best served to avoid that aperture setting, as diffraction is going to negatively impact image quality far more than any gain in depth of field will yield benefits, and that is likely why Nikon chose to go back to an f/22 minimum aperture with the final AI and AI-s generations.

  105/2.5 Nikkor Versions 

    The fact that the 105/2.5 was produced in quantity in both optical versions means that prices remain reasonable and availability is not an issue. In then-classic Nikon form, all versions conveniently took 52mm filters and other lens accessories (not so much nowadays ;-)). The pre-AI Sonnars remain the least expensive (starting at around $100 USD in Very Good to Excellent condition) with values incrementally rising with each generation and the latest AI-s copies on top at $250 - $300 USD in Excellent condition. Here is a brief identification rundown of each major iteration:

• 1959-71 Nikkor-P (Sonnar) - Silver nose and lens barrel with black, knurled-metal, short-grip focus ring. The rear element is considerably smaller than on a Xenotar version. Minimum focus distance of 1.2m/4'. 370 grams/13 oz. Single-coated. f/22 minimum aperture painted yellow. Straight aperture blades. 180-degree focus throw. AI conversion kit #40. Serial # 120xxx - 286xxx.
• 1971-73 Nikkor-P (Xenotar) - Black nose and lens barrel with long-grip, knurled-metal focus ring. Minimum focus distance of 1m/3.3'. 435 grams/15.3 oz. Single-coated. f/32 minimum aperture painted blue. Curved aperture blades. 170-degree focus throw. Ai conversion kit #39. Serial # 407xxx - 473xxx.
• 1973-75 Nikkor-P-C (Xenotar) - Same physical appearance and specifications as preceding version except for multicoatings (thus Nikkor-P-C). Curved aperture blades. AI conversion kit #39. Serial # 500xxx - 571xxx.
• 1974-77 Nikkor K (Xenotar) - Rubber focus ring. Improved, colour-balanced Nikon Integrated Coating (NIC) multicoating. Otherwise identical to previous iteration. AI conversion kit #39. Serial # 673xxx - 729xxx.
• 1977-81 AI Nikkor (Xenotar) - Automatic Aperture Indexing. Rubber focus ring with slightly coarser texture than K version. Back to f/22 minimum aperture painted yellow. Otherwise identical to K. Serial # 740xxx - 897xxx.
• 1981-2005 AI-s Nikkor (Xenotar) -  AI-s has linear aperture actuation for proper exposure in Shutter-priority and Program modes for so-equipped Nikon SLRs. Straight aperture blades. Built-in sliding lens hood. f/22 minimum aperture painted orange. 140-degree focus throw. Super Integrated Coating (SIC) from Serial # 1043xxx up. Serial # 890xxx - 1053xxx.

  A Tale of Two Nikkors - Which Will You Choose?

    Your personal vision of what you want out a lens will be the most important factor in deciding whether to go Sonnar or Xenotar. If you prefer lower contrast, more propensity for flare, and are not a resolution or sharpness junkie, the Sonnar could hold more appeal. If you want more versatility, with the ability to crank up contrast and sharpness with a twist of the aperture ring, the Xenotar might be more your speed. You cannot go wrong with any version of the 105/2.5 Nikkor for portraiture. Personally, the AI-generation (or an AI-converted K-generation) hits a nice balance for me, for the following reasons:

• I enjoy the optical versatility of the Xenotar design.
• I prefer rubber focus rings in colder weather.
• It will meter with the widest range of Nikon film and digital SLRs, both pre-AI and AI-equipped, as long as it retains the rabbit-ears for use on pre-AI bodies.
• It was the last version with curved aperture blades to abet the lovely bokeh characteristics of the optics.
• The longer 170-degree focus throw allows for a hair more precision in focusing than the AI-s version. (Some people prefer the slightly quicker focusing of the AI-s; horses for courses :-))
• It still costs less than the AI-s ;-)

    A plus of having 46 years-worth of lenses in six major iterations to choose from is the ability to drill down and find one with the most appealing combination of features for you personally:

• If you are a mirrorless user, you can likely get by with any of the four pre-AI versions without any of the clearance issues of pre-AI lenses on AI SLRs, thus saving a bit of cashola. Just check your adapter to make it will be compatible before buying a lens or vice versa if you procure a lens first.
• Do you favour metal focus rings? You can get a single-coated Sonnar or Xenotar, or a multicoated Xenotar, so-configured.
• If you prefer single coatings or multicoatings, you can get the Xenotar in either Vanilla Nikkor-P or  Neapolitan Nikkor P-C flavours.
• If you like a shorter focus throw or the convenience of a built-in hood, the AI-s is there for the taking, at a bit of a premium, of course :-). A final note on the AI-s version: the last 10,000 lenses (from Serial # 1043xxx +) were given Nikon's Super Integrated Coatings (SIC), developed in the late-'90s, for that last tiny bit of improvement over the excellent Nikon Integrated Coatings (NIC) that were used from the K version onward. Be prepared to fork over a few more bills for such a copy.

    As with most vintage Nikkors, you may very well encounter a copy with a dry-feeling or stiff focusing ring due to decomposition of the grease in the helicoids. Any version of the 105/2.5 is definitely worth a re-grease/CLA and that will make the lens an absolute joy to use. 

  Wrap-Up

    While there are plenty of excellent manual focus Nikkors to be had these days, the 105/2.5 remains the best combination of capability, availability, and accessibility for the focal length, in my books. My early-production AI version remains my favorite Nikkor, regardless of whether it's mounted on an F2, N2000, F90X, or a D300 (I just back up a little farther due to the APS-C crop  factor ;-)). For portraits...set it at f/2.5 and forget it; for landscapes...f/8 and be there. It cares not about whether the camera is an all-mechanical classic, a noisy 1980s film-burner, an icky-sticky, polycarbonate-encrusted '90s AF model, or a newfangled, yet already "obsolete" DSLR ;-). In other words, a timeless lens :-).   

An amazing AI aggregate. Nikon F2A with the accompanying 105/2.5 circa 2022

  References:
 
    The Thousand and One Nights No. 5 @ https://imaging.nikon.com/history/story
    The Thousand and One Nights No. 36 @ https://imaging.nikon.com/history/story
    The Thousand and One Nights No. 45 @ https://imaging.nikon.com/history/story
    One Minute Story - 1950 @ https://www.nikon.com/about/corporate/history
    Nikkor Rangefinder Lenses Sales Brochure @ https://www.pacificrimcamera.com
    Nikon Dealer Notebook 1965-66 @ https://www.pacificrimcamera.com
    Nikon Dealer Notebook - SLR Lenses (1988) @ https://www.pacificrimcamera.com
​    Roland's Nikon Pages @ http://www.photosynthesis.co.nz/nikon/lenses.html