The Nikon F-mount. Manual focus NAI, AI, AI-S, AI-P, Series E and auto-focus AF-D, AF-I, AF-S, D- and G-Type lenses. A compatibility inventory

I’m writing this post nearly 5 years after Can I use Nikkor AI and AI-S manual focus lenses on my Nikon DSLR?, because the interest of Nikonians for legacy lenses is undeniable, while my research into the topic has added some – hopefully worthwhile – facts.
At the same time, the value of manual focus NAI and AI(S) lenses is clearly on the rise, which is good for everyone who has a few of grannies F-lenses lying around, either for proper use or sale.

The Nikon F-Mount was launched with the Nikon F camera in 1959 and has remained virtually unchanged since, making it the only camera mount still in production almost 6 decades after its introduction.
The F-mount can be found in five versions: F, AI, AI-S, AF and Non-MF (Fig. 1).
The latter made its debut in 1987 on the F-401 (N4004), is used on the D40 to D100, D3000 and D5000 series and essentially a downgrade back to the original F-Mount, because it lacks the meter coupling lever to interface with non-cpu lenses. This is why none of these cameras exposure meter with MF lenses.

The F-mount has only seen two major updates.
The first from the original Auto-Nikkor to Auto-Indexing (AI), with the addition of the meter coupling lever – introduced on the Nikon F2 Photomic A in March 1977 – and the second from AI-S to AF, with the launch of Nikon’s first flagship auto-focus camera, the F3-AF, in April 1983, followed by the popular Nikon F-501 (N2020), two years later.

Where the manual focus system update from F-Auto to AI was basically oriented at making existing technology (aperture indexing) more user friendly, the update to AF may be considered break-through.
With it, Nikon replaces mechanical camera-lens coupling with a cpu-driven, electronic camera-lens interface, allowing for many posterior innovations such as aperture control from the camera body, electronic range finding, distance metering (with D and G-type lenses), EXIF data recording, AF-S, VR, iTTL flash, etc.

Nikkor lens categories
Nikkor lenses could be simply divided into two categories: compatible and not compatible without modification.
F-series lenses made before 1977 are called pre-AI or non-AI (from here on: NAI), while all lenses made afterwards – independent of whether they have a cpu or not – could be called AI (even if Nikon does not) because any post 1977 MF, AF Nikkor, Nikon Series E and third party lens conforms to the AI/AI-S spec.
While 99% of AI and AI-S compatible lenses can be mounted and used on virtually all post 1977 cameras, NAI lenses must have at least their aperture ring modified (AI-ed) before they can be mounted without the severe risk of rendering any modern camera service-prone or even useless.

The AI spec implied a substantial change to the NAI aperture ring, lowering the best part of its to top edge to below the lens bayonet flange, while leaving two protrusions – the so-called meter coupling ridge and the minimum aperture post – which sit more or less opposite on the AI/AI-S aperture ring (Fig. 2).

The modification is furthermore significant because it is the ONLY lens-mount upgrade that is not 100% backward compatible in the long existence of the F-Mount system, even though Nikon maintained NAI compatibility in certain post '77 cameras up to the 1988 Nikon F4.
For comparison, in the same sixty plus years Canon has modified its lens mount no less than five times, rendering many older lenses incompatible with last generation cameras.

The update from AI to AI-S, on the other hand, may be considered transitory, even if it enabled the new shutter priority AE mode on compatible cameras.
It consists in a modification that allowed for standardized aperture control across (then current) camera models, while also including a rudimentary focal length indexing ridge on the rear lens tube.
Under this spec the F-bayonet on the lens undergoes its only significant pre-AF modification, consisting in the inclusion of a small semicircular indentation – the AI-S lens identification gouge – on the flange (Fig. 3, right).

The next and final modification of the lens bayonet adds the AF drive screw, which couples the camera’s built-in AF motor with motor-less AF-D lenses.
On posterior G-Type lenses the aperture ring disappears entirely, while the F-bayonet rolls back to its original version.
Moreover, on some modern entry level AF-S G-Type lenses – such as the 18-55mm f/3.5-4.5G and 55-200mm f/4-5.6G – the F-bayonet is no longer made of metal, but rather of a high resistance polymer, which most of us would likely call plastic.

AI-S, Nikon Series E and Non-CPU lenses
The more economical Nikon Series E, launched in 1979, were the first AI-S compatible lenses, but an AI-S compatible camera – the Nikon FA – did not appear until 1983, the same year that Nikon launched its first auto-focus capable flagship camera, the Nikon F3-AF.
Nikon made only four cameras that can distinguish between AI and AI-S lenses: the FA, F-301 (N2000), F-501 (N2020) and F4.
They have a small, secondary feeler pin on the camera mount, just above the lens-lock pin (Fig. 4), that matches the position of the lens gouge. While AI lenses will compress the pin, on AI-S lenses it is left untouched.
The posterior introduction and later massification of AF and the consequent electronic camera-lens coupling made this modification superfluous.
Even though the AI-S lens identification gouge is still present on many modern lenses, the secondary pin on the camera mount is long gone.

What Nikon has come to call non-cpu lenses (rather than AI or MF) refers to all manual focus lenses that do not include a chip to allow electronic interfacing with the camera, including NAI, AI, AI-S and Nikon (not Nikkor) Series E.
Series E lenses are fully AI-S compatible and can be mounted and used without problems on virtually all Nikon cameras since 1977.
Factory and DIY post-chipped MF Nikkors and third party lenses have their own denomination – AI-P – and interact with almost all modern electronic cameras in a manner that is identical to factory chipped AF lenses.

Why the N in NAI means Nooooo!
NAI Nikkors come with the following nomenclature and must not be mounted on modern Nikon F and D cameras, unless modified: A, C, F, H & HC, K, N & NC, O, PD, Q & QD, S & SC, T and UD, often but not always in combination with AUTO.
The lens denomination letter(s) may come before the brand name on the frontal outer or inner lens rim, but generally come after: NIKKOR-QD AUTO 1:2

There are at least two essential parts of a modern camera that can (and will) be damaged when mounting an unmodified NAI lens.
The first is the meter-coupling lever, which, although it can be unlocked to allow mounting NAI lenses on cameras such as the Nikomat FT3, Nikon EL2, FE (Fig. 5), FM, F3, F4 and now also on the Nikon Df, on all other modern Nikons it is fixed.
It is interesting to observe that not even Nikon digital flagships (D1 to D5 series) include this feature while their film brethren do, in spite of the fact that the top D-models have been MF compatible since day one.

While most users are probably aware of the risk of damaging or breaking this part of their camera when mounting an unmodified NAI lens, the large majority is evidently unaware of the fact that Nikon DSLR cameras that do not meter-couple with non-cpu lenses and thus lack the meter-coupling lever – from the Baby Nikons to the D90 and D100 – have another (not well documented, if at all) dowel or lever located at the lower part of the lens mount.
The latter interact with the minimum aperture post on AI spec lenses, with the purpose of establishing whether or not an AF-D lens (with aperture ring) is locked at its smallest aperture.
They are responsible for generating the fEE error on the camera’s screen(s) when an AF-D lens is not locked, a necessary pre-requirement to manage its aperture from the control dial(s).
On the motor-less D40, D60, D3K and D5K series a plastic dowel is suppressed by the minimum aperture post on the lens, which measures less than half of the standard NAI aperture ring’s over-height (Fig. 6).

However, on the motorized models D50, D70, D80, D90 and D100 this function is carried out by a small lever that is not suppressed, but rather displaced by the minimum aperture post (Fig. 7, right).

While on entry-level Nikons the dowel will at least become severely compressed when mounting an unmodified NAI lens, on the motorized cameras the lever will likely bend and most probably break, if forced.
In any case, such damage means a trip to the repair shop, without even considering that select NAI lenses can possibly also break internal parts, such as the mirror, because their lens tube protrudes too far into the camera housing.

The following NAI lenses are incompatible with the Nikon Df: Nikkor H Auto 2,8cm f/3.5, S Auto 3,5cm f/2.8, S Auto 5cm f/2, Q Auto 13,5cm f/3.5, Micro 5,5cm f/3.5, Medical Auto 200mm f/5.6, Auto 85-250mm f/4-4.5 and Auto 200-600mm f/9.5-10.5.
If you really want to risk mounting these – even if modified – on any other Nikon DSLR, be my guest.
Be particularly cautious about the S Auto 5cm f/2, because I have seen this lens offered on numerous occasions.
In some cases the incompatibility only applies to certain serial numbers. If you have any of these lenses on hand, I suggest you check the Nikon Df user manual, page 320.

Post chipped lenses: AI-P
Nikkor and third party manual lenses can, apart of being AI-ed, also be modified to communicate with an electronic body through the inclusion of a so-called dandelion chip.
Although Nikon offered AI to AI-P conversion sets for a fair number of years, today dandelion chips and lens modification sets are only available from third parties.
The main advantage of chipping a non-cpu lens is that, with the obvious exception of auto-focus, it enables features similar to the ones available with any other cpu lens, such as range finding, minimum, maximum aperture and focal length read-out to EXIF, focus-confirmation, color matrix metering and iTTL flash, among others, including on cameras that cannot meter at all with MF lenses.

Keep in mind that Nikon cameras from the D7K series - with exception of the D7500 - and up (D2, D200 and later) are already fully MF compatible.
This means that they do not need to communicate with the lens, as long as the user includes lens data (focal length and maximum aperture) in the camera’s data-base (Set-up menu > Non CPU lens data), with the only limitation that aperture cannot be controlled from the command dials, and must be set manually.
Even so, chipping and AI-converting old lenses may be a worthwhile exercise for use with most cameras, especially if one has some of granddads F-series collecting dust.
Moreover, F-series NAI as well as AI/AI-S lenses – especially the moderately fast ones – can often still be had on auctions for considerable less money than their modern descendants, offer generally exceptional build, mechanical quality and have only one disadvantage: they don’t AF...

While manual focusing on the matte can be pretty darn difficult with modern cameras, especially crop-models, correctly configured MF compatible Nikons and chipped lenses will both confirm correct focus acquisition through the focus confirmation >•< dot at the left hand bottom of the viewfinder (Fig. 9: 4), which, in my experience, works flawlessly.
Although I consider AF particularly useful in combination with long(er) lenses, with the classical focal lengths such as 28, 35, 50 and 85mm I have found that I can perfectly live without.

Nikon AF cameras and lenses
Nikon AF capable cameras come either with or without a built-in focusing motor, which determines which types of AF lenses are AF capable on a particular camera.
With exception of the D50, Nikon entry-level cameras (D40(X), D60, D3K-series, D5K series) will only auto-focus with motorized AF-I, AF-S lenses and not with motor-less AF-D models.
However, they can still inter-communicate with the latter because they are chipped. On these cameras, AF-D lenses offer the same functions as AF-S models, but must be focused manually.
This is not an easy task, though. On most AF-D lenses the focusing ring is less than 10 mm wide, while also extremely loose in comparison with MF lenses.
Furthermore, the mentioned cameras and even not-so-entry-level motorized Nikons, such as the D70(S), D80, D90, D100 and now the D7500, cannot exposure meter with non-cpu lenses, which means that the user must guess exposure when such a lens is mounted.
Fortunately, with modern DSLR cameras it is fairly easy to make educated guesses, especially with the help of the histogram and the blown-highlights overlay in image review.

The Nikon D50 and all cameras from the D70 to D100 and up (D7K series, Dx00 series and Dx series) are motorized and can AF with any AF-D, AF-I and AF-S Nikkor, while Nikons from the D7000 and up also meter with manual focus lenses.
The metering modes and other camera functions available depend on the camera model and the selected exposure mode, but typically include at least range finding, focus confirmation and color matrix metering (A and S modes), among others. Consult your manual and/or this article for more information.

Nikkor AF lenses come in four flavors: camera driven (AF-D), with a built-in focusing motor (AF-I, AF-S), with an aperture ring (Type-D) and without one (Type-G).
Although the large majority of Type-G lenses include a focusing motor, the 10.5mm f/2.8G and 70-300mm f/4-5.6G Nikkors as well as some third party models, such as the Tokina AT-X 12-24mm f/4 DX (picture above) do not, which means that they will not autofocus on entry-level cameras.
Similarly, while AF-D models generally lack a built-in motor, there are a few that do, such as the AF-S 17-35mm f/2.8D and the AF-S 300mm f/4D.
Independent of whether an AF-D lens has a built-in focusing motor or not, it must always be set and locked at its smallest aperture or else the camera will refuse to fire while showing a fEE error on its control panel(s).

Nikon also specifies the little known AF-I lens series as compatible with both motorized and motor-less cameras, which has people wondering what AF-I is.
AF-I was the initial version of Nikon’s motorized lens technology, and features internal focusing, hence the “I”. It was launched in 1992 and abandoned for AF-S four years later.
The technology was exclusively used in fast aperture super tele-lenses, namely: AF-I 300mm f/2.8D, AF-I 400mm f/2.8D, AF-I 500mm f/4D and AF-I 600mm f/4D.
Most of these lenses have kept their value very well. An AF-I 600mm f/4D IF-ED in mint condition can be found on eBay for around USD 6.500, in comparison with an AI-S 600 f/4 IF-ED in the same condition, which, although cheaper, would still set you back a respectable USD 2.500.
The AF-S G VR version of this lens has a suggested retail price of USD 10.300 (Nikon USA), while the latest model, the AF-S 600mm f/4E FL ED VR costs an additional two grand ($ 12.300).

From AF-D to AFS-G
Over the last half decade Nikon has been upgrading the best part of its lens line-up from AF-D D-Type to AF-S G-Type, which now covers both the classical focal lengths 20, 24, 28, 35, 50, 58, 85, 105, 200, 300, 400, 500 and 600mm, as well as focal lengths specifically geared to crop sensor cameras (APS-C or DX), such as 10-24, 12-24, 16-35, 16-85, 18-55, 18-200 and 55-200mm, among others.
While all D-Type lenses are compatible with cameras back to 1977, G-Type lenses lack an aperture ring and can only be operated from more recent camera bodies that feature at least one control dial.
I expect that the large majority of AF-D Nikkors will ultimately disappear from the line-up, with the exception of a few “classics”, much the same way a few manual focus Nikkors are still in production today, such as the AI-S 28mm f/2.8, 35mm f/1.4, 50mm f/1.4 and 105mm f/2.8 Micro, for example.

Conclusions
Comprising well over 400 different DSLR camera compatible lenses, the Nikon F-mount is, by far, the largest camera lens ecosystem available, today.
Until the early nineties Nikon built – rather than manufactured – lenses, a dedication that shines through in a solid, full metal construction and outstanding mechanical quality.
Especially the more expensive lenses also feature exceptional optical quality and personality, which is the reason why many decade-old models still sell second hand for considerable amounts of money.
I’m not quite sure about the original retail price of my AI 28mm f/2, for example, but today an EXC++ copy goes for around USD 700 on Grays of Westminster.

On the down side for buyers and upside for sellers, in a day and age where computer aided design has improved optical formulas to near perfection and material design has bettered polymers to the point of almost unbreakable, let alone cheap, many photographers want something more imperfect, analogue and human.
This is why NAI and AI lens pricing has risen like foam in a beer glass, to the point where a second hand lens in mint condition may be only 25% cheaper than a brand new G-Type plastic equivalent, while certain exceptional samples may sell at several or dozens of times their original retail price.
I cannot demonstrate this, because 20.000 dollar plus Nikon collector items, without exception, disappear from eBay in the blink of an eye...

When we take a look at three popular and fast classical focal lengths, f/1.4 35mm, 50mm and 85mm, both the age-old NAI and AI (-S) versions currently sell for only about 50% less than their modern equivalents recommended retail price.
A mint AI-S 35mm f/1.4 sells on eBay for approximately USD 600 (USD 1.200 new), a mint AI-S 50mm f/1.4 for USD 220 (USD 469, new), while a mint AI-S 85mm f/1.4 goes for about USD 730, which is roughly half of what a brand new f/1.4 G-version costs and almost 50% more than a new 85mm f/1.8G (USD 500)
Fortunately, slightly slower versions – between f/1.8 and f/3.5 – are still much more affordable, especially if one is patient.

Considering that modifying and chipping NAI lenses is fairly easy, finding those might be worth you while, since they still sell in mint or excellent plus condition for less than their AI equivalents, let alone modern G-versions.
All in all, even if it has become increasingly hard to find bargains, the Nikon F-mount ecosystem still offers its users the unequalled quality, compatibility and versatility worth investing in.

You might also want to read:
Is my camera motorized or motor-less, compatible with NAI, AI, AI-S, AI-P, AF-D, AF-I, AF-S lenses and GPS or not? Answers.
Can I use Nikkor AI and AI-S manual focus lenses on my Nikon DSLR? Yes, you can!

Sources:
Wikipedia: Nikon F-Mount
Thomas Pindelski: Adding a CPU to MF Nikkor lenses (Pt. II)
Mir: A brief history of Nikkor Lenses
Nikon: From external light meter to TTL full-aperture exposure metering
Nikon USA: Nikkor camera lenses
eBay search: “Nikkor AI”