The f/2 AI was the precursor of the f/1.8 AI series of lenses where Nikon needed to align itself to its competitor's normals after they started to produce their 1/3 faster equivalents. Nikon's Thousand Night's tale details the history of this lens, with its optical formula deriving from the 1964 Nikkor-H Auto 50mm f/2, and its short 2 year production run from 1977 before being superceded by the Nikkor 50mm f/1.8 AI a year into its run.
(C) Nikon - 50mm f/2 AI optical cross section
Performance
Nikon themselves highlight some of the perceived weaknesses of this lens:the spherical aberration is somewhat large [...] and therefore the contrast is somewhat reduced at full aperture. But this is a characteristic of the lens, and cannot be dismissed as an out-and-out defect.
In fact, this lens has a delicate balance of spherical aberration and residual peripheral coma flare [Ed: veiling and "glow"], which together with the outstanding image flatness gives excellently uniform imaging over the whole image area from full aperture.
[...] This lens exhibits a large change in imaging characteristics, from a distinctive fine and delicate image when fully open, to a crisp, high contrast when stopped down to f/4 or f/5.6.
Reviews tend to note the (perceived?) lack of sharpness wide open can be attributed to the veiling flare that minimises contrast and in some regard its similar comments to the Nikkor 50mm f/1.4 AI-S where its wide open performance is questioned although I've always found this to dependant on subject distance and light sources in frame. It's exactly these characteristics that intridged me to take a look at this lens which tend to appear at good prices on online auction sites.
There is a lot of contradicting information about the lens: "great sharpness wide open/best in the 50mm range", "smooth/jittery bokeh". Bjorn Rorslett 5/5 and comments suggest some of the online information is not all fanciful but we'll see:
[The 50mm f/2 AI and non-AI] delivers sharp and contrasty images already at wide apertures. Even at f/2, there is just a trace of flare and corner softness, and beyond f/2.8 it renders excellent image quality
Sharpess is more than reasonaby good wide open and as noted above, its quite sharp at f/4. It seems better when not close to MFD - take a look at the church window. Colour saturation is also very nice.
f/2 - ~50m away, centre crop
f/2
f/4
I find the lens flares and ghosts quite easily but surprisingly chromatic aberations seemed quite well controlled even wide open. I would not shoot this with a white background given its flaring tendencies.
f/2 - sun raking across from left and in frame - note limited CA except around the branches near the sun
Out of focus rendering is generally fine but its noted that with "high frequency" shapes/objects, it can get harsh/busy and potentially (although its subjective) slightly unappealing (see the leaves in the post box below).
f/2 - near MFD, sharpness seems to take a little hit
f/2 - the numbers are faded by the sun and not a constrast issue for the lens
f/2 - centre crop, some blooming from minor over exposure but little/no CA. Text is sun-bleached
f/2, f/2.8, f/4
From the examples I'm not really seeing the same levels of veiling/spherical aberrations wide open as say the 50mm f/1.4 AI-S but it is there and contrast can suffer.
f/2 - lettering was darker in reallife
Top: f/2 | Bottom: f/2.8 - significant increased contrast
f/2 - sharp eye but veiling noticable from the backlight onto fur
The delicate image is noticable with flash lit portraints as seen below - there is more apparent veiling at f/2 which gives it a very old look but its still more than acceptably sharp.
f/2
Compared
As noted, the Nikkor 50mm f/1.8 AI and subsequently the AI-S replaced this lens and its natural to compare them. The optical cross sections show that they are different.
50mm f/2 AI vs 50mm f/1.8 AI-S optical cross sections
The size and weight are almost identical, with the older AI 10g heavier at 220g but it has 230 degrees focus throw compared to 130 degrees which becomes very noticiable when you use side by side. Both lenses show a blue-ish tint on the elements.
f/1.8 AI-S (left) vs f/2 AI (right)
The images below are crops and staacked with f/2 on top - click through for large
Bokeh
It's very close but the f/1.8 has slighty smoother transitions, and we have elipitcal-rugby-ball point light sources from the f/2's 6x aperture blades which I prefer over the hexagonal shape of f/1.8
wide open at f/2 vs f/1.8
f/2
f/2.8Sharpness
Flash lit at 1/250th and live view focused - was (meant to be) on the camera's "Nikon" letteringAgain its very close - wide open we see the veiling flare of the f/2 but its still sharp and its better than the f/1.8, which carries through when the f/1.8 is stopped down 1/3 to match apertures. However, it looks like at f/2.8 the newer f/1.8 is a touch better.
wide open at f/2 vs f/1.8
f/2: for the f/1.8, I've missed focus and its on the lens serial number
f/2.8
f/4Servicing
These lens are close to 50 years old and it means that servicing is a must. My 50mm f/2 AI was a little dirty and dry and sometimes gritty focusing. Looking through Kenneth Olsen's YouTube videos or Richard Haw's guide for the 50mm f/2 K we can see they are near identical to the f/2 AI and shows that the barrel construction is very similar across the Nikkor 50mms at the time, especially the 50mm f/1.4 pre-AI and even older.Optics
As common at the time of AI lenses, the optics are housed in a single removable objective and is secured via a ring above the focus ring and not just the vanity name plate. Removing the securing ring, focus to MFD and loosening the grub screw will allow you to remove the vanity name plate. You can access the 6/4 optics now: front group (can be accessed with lens objective in place) behind the baffle that needs to be removed with a rubber cone tool, removing the entire assembling that needs a lens spanner for front element and twisting off the rear cemented couplet and the rear groups by twisting off the rear element housing.
both front/rear of this block are sealed, IPA is required in the access holes
Helicoids
I've skipped over the disassembly of one key item above but its critical for helicoid access and there are some oddities.- Remove focus ring retaining collar
This collar is really difficult to extract and you need a lot of patience, acetone and a rubber sheet. The grey rubber lens spanners are too wide to grip the collar so you need create something that will provide grip: I took a rubber sheet that I cut out for the front retaining ring - following this you slip the rubber over the front and then use the rubber lens spanner to rock back and forth until you break lose the collar and then spin it off: mkae sure you've applied acetone to the seam before hand to allow it to work behind and onto the thread.
- Remove optical block
- Remove rear mount, apeture ring and aperture coupling pin
Apply acetone to the bayonet mount screws before attempting to unscrew. When the rear mount and apeture ring is removed, note how the apeture coupling pin moves the ring mounted apeture fork
- Landmarks ahead of separating/removing helicoid key
Mark infinity on the barrel and one side of the detent spring.
Remove the chrome grab ring/distance scale which may be glued. You may get away with leaving the detent spring in place if you squash it down to allow the chrome grab ring/distance scale to slide over and out. On re-assembly you can reform the detent spring by inserted a small flat driver and lifting up the raised hump.
Take note of the infinity and MFD stops on the inner helicoid are built into a ring and hit but the aluminium limiting block - mark one side of the limiting block to ensure you know orientation for reassembly. The helicoid key is sunken - take note of orientation (the flat lip faces to rear of the lens) and you can remove this - you can't separate the inner and central helicoids because of the MFD/infinity stop BUT you can separate the central and outer helicoids.
With the helicoid key removed, collapse the central-outer helicoid and mark this position (my copy had a factory scribed mark already as well as a fully collapsed central-outer helicoids mark). This is the reference point I use later and also gives guidance for correct assembly - collapsing may seem difficult due to the solidified undisturbed grease. - Separate outer-central helicoid
From collapsed position, it's about 1 1/4 turns until separation - mark the central helicoid aligned to the infinity reference on the outer helicoid - Separate inner-central helicoid
Expand the helicoid so you are at MFD and exposing the limiting block again, remove the screws and remove. I found that you could NOT separate by collapsing further (past the MFD side) and you MUST separate through the rear of the central helicoid; ie the lens objective notch is collapsed through the central helicoid to separate
Mark the inner (aluminium) helicoid aligned to the reference on the central helicoid - Clean, relube and remate helicoids
I prefered a lighter grease, JHT #10, for lens due to the longer focus throw.
Remate in reverse order - the central and inner helicoid should feel very smooth and light once regreased
Make sure you are happy with the feel of the helicoids before progressing to reassembling. Remember that the optical block will add to the torque required to turn - Reinserting opitcal block and finishing
Once you have reassmebled the lens barrel, sans rear mount, set the aperate ring to f/16 and on the optical block manually closed down the apeture using the apeture pin. Its easiest to reinsert if you sit the optical block on a table and look down from the rear of the barrel to align the apeture fork with the optical block's apeture pin - when inserting this will also catch the notch on the inner/aluminium helicoid. Once inserted, secure the front retaining ring so the optical block will not move.
Re-attaching the bayonet mount requires you to match the apeture stop down fork to the corresponding tab - look down the back of the lens to see it and ensure to couple properly, line up the screw holes
At this point, infinity focus and when happy re-attach the focus ring collar and you're done.
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