If you are like me and love the classic manual focus Nikkors (ignoring for now the non-AI lenses which may require modification), then you know there are two subtly different mounts, the automatic indexing AI mount and the slightly newer, slightly more intricate AI-S mount.
When shopping for lenses, you will often see (what appears to be) the same lens in both AI and AI-S mount, and it will usually be cheaper in the AI mount. Why are the AI-S lenses more expensive? They are slightly newer, but the Nikkor lenses are built to last, so if you can find your desired lens in good condition in both AI and AI-S mounts, which should you choose?
From afar, it is usually impossible to tell the difference between the AI and AI-S versions of a lens. Generally the optics and overall cosmetics are the same, (although the focusing scales tend to be on the black barrel on AI lenses and on the silver grab ring on AI-S lenses - I find the AI-S lenses generally a bit better looking) but the mechanics differ slightly. The quickest way to identify an AI-S lens from an AI lens is that AI-S lenses always have the smallest aperture on the aperture direct read-out scale written in orange and a small semi-circular (in cross-section) notch cut out of the back of the mount.
AI lenses: note that many of these have the smallest aperture in orange on the large scale, but none do on the smaller aperture direct read-out scale.
AI-S lenses: have the smallest aperture in orange on both aperture scales.
AI lenses: no semi-circular cut out in the mount.
AI-S lenses: have semi-circular cut outs in the mount.
There are very specific instances when AI-S lenses will work better with specific bodies in specific modes. AI-S lenses will work slightly better if shot in shutter priority and program mode on the few bodies which support them, and long (135mm + ) lenses can tell certain bodies to use "P-Hi" mode which prioritizes faster shutter speeds to help you hand-hold the longer lens. Personally I know I will never be in a situation where having an AI-S mount instead of an AI mount will matter to me.
I have long understood that there is usually very little reason to pick an AI-S lens over an AI lens, all else being equal. But recently I realized there is often a very good reason to pick the AI lens: they almost always have a longer focus throw.
There may be shooting situations where you might prefer having a shorter focus throw for faster focusing, but in those instances you'd probably be better off with an automatic focusing lens anyway. I posit for the longer and faster lenses, and for the more meticulous shooters the AI lenses with longer throws are the better purchase.
Below I have listed the differences between AI and AI-S versions of lenses. I hope these help you make an informed decision. If a specification is not listed then the AI and AI-S versions are otherwise identical or the differences are negligible. All the information was culled from the Photosynthesis site which has a fantastic amount of obsessively gathered information on Nikon lenses.
20mm f/3.5
AI: 100° throw
AI-S: 70° throw
This lens is wide enough and slow enough that an extra 30° throw probably is not worth worrying about, but the AI version is still cheaper.
24mm f/2.8
AI: 160° throw
AI-S: 60° throw
I have the AI-S version and I would very much like the longer focus throw of the AI version. It may be surprising with a wide and not-very-fast lens that precise focusing is a problem, but I do have trouble nailing focus with the AI-S version.
24mm f/2
AI: 160° throw
AI-S: 80° throw
After learning my lesson with the 24mm f/2.8, I intentionally sought the AI version for the longer throw. I am glad I did.
28mm f/3.5
AI: 200° throw
AI-S: 90° throw
28mm f/2.8 - not the same lens!
AI: 190° throw, (7/7), 0.3 meter close focus, does not have CRC
AI-S: 170° throw, (8/8), 0.2 meter close focus, has CRC
The AI-S version is generally (universally?) viewed as the better lens, and I love the close focus ability. However, I haven't tried the AI version.
28mm f/2
AI: 120° throw, 0.3m close focus, 7 curved blades
AI-S: 120° throw, 0.25m close focus, 7 straight blades
I'd get the AI-S version for the slightly closer focus.
35mm f/2.8 - careful!
AI "old": (6/6), 195° throw
AI "new": (5/5), 100° throw
AI-S: (5/5), 120° throw
The older design (on the right) has the depth of field scale at a lower level than the focusing ring, on the newer version (on the left) they are on the same level. I am not certain which lens is held in higher regard optically. This is the one case where the AI-S version of a lens has a longer focus throw.
Nikon's Thousand and One Nights goes through the designs of the various 35mm f/2.8 SLR lenses, starting with the original (7/5) design (3.5cm f/2.8 S non-AI), to the classic (7/6) design (35mm f/2.8 S non-AI) to the (6/6) design of the "old" AI-design, but omits the latest (5/5) design in the "new" AI lens, which is shared by the AI-S version.
35mm f/2
AI: 190° throw
AI-S: 120° throw
It seems many people think the f/2.8 or f/1.4 lenses are better.
35mm f/1.4
AI: 180° throw, 7 blades
AI-S: 105° throw, 9 blades
I'd get the AI version and live with "only" a 7-bladed aperture. At f/1.4 I think the extra focusing precision would be worthwhile. The cosmetics differ slightly, the front of the AI is conical where as the AI-S is (to me) more aesthetically appealing.
50mm f/1.8
AI (older style): 210° throw
AI-S (older style): 130° throw
50mm f/1.4
AI: 210° throw
AI-S: 140° throw
50mm f/1.2
AI: 180° throw, 7 blades
AI-S: 110° throw, 9 blades
I'd get the AI version and live with "only" a 7-bladed aperture. At the wider apertures I think the extra focusing precision would be worthwhile.
58mm f/1.2 Noct
AI: 230(?)° throw, 7 curved blades
AI-S: 140° throw, 9 straight blades
I'll never be able to afford one (or find one) but hypothetically I'd get the AI version and live with "only" a 7-bladed aperture. At the wider apertures I think the extra focusing precision would be worthwhile.
85mm f/2
AI: 255° throw
AI-S: 170° throw
105mm f/2.5
AI: 170° throw, no built-in hood, 7 curved blades
AI-S: 140° throw, built-in hood, 7 straight blades
These two look different cosmetically. I may get the AI-S version for the completely shallow reason of looks (like the 35mm f/1.4 AI the 105mm f/2.5 AI has a sort of conical front) and built-in hood, but the slightly longer focus throw of the AI might make it worth considering.
135mm f/3.5
AI: 220° throw
AI-S: 180° throw
135mm f/2.8
AI: 270° throw
AI-S: 180° throw
135mm f/2
AI: 270° throw
AI-S: 200° throw
I'd get the AI version.
180mm f/2.8 - not equivalent lenses!
AI - no ED glass, (5/4)
AI-S - ED glass, (5/5)
I'd certainly get the AI-S version with ED glass.
200mm f/4
AI and AI-S have same specs. Might as well get the cheaper one.
200mm f/4 micro
AI and AI-S have same specs but the AI-S one is supposed to have a better tripod foot.
300mm f/4.5 (non-ED)
AI: 4m close focus, 6 blades, f/22 min aperture
AI-S: 3.5m close focus, 7 blades, f/32 min aperture
It is really better to save up more for the ED-IF version (skipping the ED non-IF AI version in between (unless you are a collector, these are rare!)).
300mm f/4.5 ED-IF
AI: 7 blades, f/22 min aperture
AI-S: 9 blades, f/32 min aperture
I would get the AI-S version.
If you want to know more about the variations on the Nikon F mount, check out
By Thom
Camera Quest
Photosynthesis
KEH blog
Nikon USA
Ken Rockwell
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