20mm f/2.8 AI-s - obscure elderly kit question of the day

Discussion in 'Nikon' started by Andrew Garrard, Oct 11, 2017.

  1. Hi all. I notice a news story about Metabones having produced a focal reducer for F-mount lenses to the Pentax Q.

    This is of precisely no interest to me (or almost anyone else), but there's a comment that it's "compatible with all lenses except the 2.1cm f/4 and 20mm f/2.8 AI-s", which confuses me.

    Okay, the 2.1cm f/2 has an invasive rear element (and I'd put money on some old fish-eyes not mounting either), but I've not heard of anything odd about the 20mm f/2.8 AI-s which would make it unusually incompatible, unless I'm missing something. This kind of thing normally talks about F3AF lenses (not that there's AF) and the medical Nikkor, some old shift lenses and ones using the focus unit, but I don't normally see the 20mm in the list.

    Is it a typo, or is there something obscure I can learn?

    In mildly related news, I'd only recently noticed the D3400 is missing the switch that engages the aperture ring driver, which I assume is why it can't work with pre-G lenses. I'm a little surprised you can't override that (can you?), given that a non-G lens should work just fine if set to minimum aperture; Nikon seem to have put idiot proofing ahead of compatibility, admittedly on their most idiot-friendly DSLR. Another year, another "so much for compatibility with everything from the 1960s" F mount change.
  2. I think it's to do with the clearance between the rear of the lens and the Metabones.
    Some (all?) versions of 20mm f/2.8 Ai-S have a rear element protector that protrudes a long way out from the mount.
    I noticed this the other day when I was looking at a used 20mm Ai-s. It struck me as odd because I own a copy of this lens with a damaged front element, and hadn't remarked on the protruding piece of metal before. - Must dig mine out of storage to check if it's the same. (I've been waiting for a sample with another fault to turn up cheap.... for about 15 years now!)
  3. > Some (all?) versions of 20mm f/2.8 Ai-S have a rear element protector that protrudes a long way out from the mount.

    Thanks, Joe. Do they? I don't think I've ever seen one in person, but going off this (am I allowed to link to Hypnoken these days?) there doesn't seem to be anything protruding other than the AI focal length ridge. Is it worse than, say, the 50mm f/1.2 AI-S? I get the 2.1cm intrusive element, but is this different? I'm surprised there's anything to clash that wouldn't already hit either the mirror or some of the other feelers in, say, the FA.

    I struggle more than I feel I should to find a good reference for all the variants of the F mount; I can find several partial ones. I really think we should have some sticky threads somewhere...
  4. Sandy Vongries

    Sandy Vongries Moderator Staff Member

    Did you Google - nikkor 20mm 2.8 ais specifications? Seems like a lot of info from various sources. My 20 3.5 has absolutely no projection on the back, I don't recall one on the 2.8. The ancient 21mm did have a long projection.
  5. Incidentally, through the Nikon F-Mount - articles is the most useful resource I stumbled across for this. (The Photography in Malaysia site has lots of information, but I couldn't find equivalent detail on it.)
  6. > Did you Google - nikkor 20mm 2.8 ais specifications?

    No, but now I have. I don't see anything obvious that would explain an oddity, though?
  7. Vincent Peri

    Vincent Peri Metairie, LA

    I just checked my 20mm f/2.8 AIS. It has a lens protector tab that extends up 1/4".
  8. Vincent Peri

    Vincent Peri Metairie, LA

    Here is a rear view of the 20 2.8 AIS:

    Nikon 20mm f/2.8 AI-s

    scroll down to the rear view and you'll clearly see the rear projection.
  9. Doubtful as the glass element in the speed booster appears far enough recessed. I thought that the CRC of the 20/2.8 Ai-S might cause interference of the rear element but believe that the rear element actually moves forward when the lens is focused closer.
    Those are not protector tabs but indexing posts or ridges: the top one is the focal length indexing ridge and the bottom one the lens speed indexing post. Both are used in the Nikon FA (and F4, F-501, not sure if there are others). Possible that the lower post interferes with the inside of the metabones adapter. It appears to be actually two posts, and I believe only the lower portion is the actual indexing post, the other appears to have no useful purpose.

    FWIW, the incompatibility warning is the same for every metabones speedbooster adapter that features a Nikon lens mount.

    EDIT: my above assumption appears correct: here is a post with a response from metabones: Nikon 20mm f/2.8 AI-S for BMCC Speedbooster
  10. Yes, that's the image I linked to above. There's a projection, but I believe it's the speed post (not the focal length ridge as I claimed earlier; sorry).

    Oops, crossed over with Dieter, so I'll stop typing.
  11. > here is a post with a response from metabones

    Ah, thanks Dieter.

    (Other resource I'd meant to find earlier, on the Photography in Malaysia site.)

    As far as I'm aware (from images - I don't have a suitable body, which is not the first time in my life I've heard that) the lens speed indexing post pushes the lever sideways. It seems a little odd that it should stick out a non-standard amount, especially on a relatively slow (f/2.8) lens where it looks like Nikon had some design leniency. I'm curious whether there's a reason for it, or whether Nikon just didn't have a standard depth in mind. But only mildly. :)

    I wonder whether the "upper ridge" actually is just for rear element protection - it doesn't look like it's quite in the right place for the focal length ridge (which might be irrelevant anyway for a 20mm lens - IIRC it's only a 135mm switch so it wouldn't be pressed anyway).

    Anyway, odd little lens. I'll stick to my 14-24. Thanks for clearing things up!
  12. It does. It's not unique to Ai-S lenses though. For example, my 105/2.5 Ai has it too. No need to be as massive as on the 20/2.8 Ai-S (which actually does look like two separate posts/ridges; the Rockwell image is from an odd angle and doesn't show it clearly). One would think that in order to protect the rear element, the height of this ridge and the focal length indexing ridge opposite would be identical (so that the lens could stand without a rear cap). The only other thing needing protection is the aperture lever - and the extra ridge appears to be just the right height.
  13. Here's an F4 to show both the maximum aperture feeler at the bottom of the mirror box and focal length feeler on the right. I'm pretty sure the focal length feeler is just an on/off switch switch that-as mentioned-is used in cameras like the FA to shift to "high speed program" mode. I'm not sure whether or not it does anything in the F4 since high speed program mode is also manually accessible.


    Here are two lenses and one loose lens mount. On the left is an 18mm 3.5 AI-s, and on the right a 85mm f/2 AI-s. The center mount is from the 135mm f/2 AI-s that I'm still trying to get back together(the focusing helical was gummed solid when I got it, and I was an idiot and didn't mark it when I took it apart). As can be seen, the 18mm and 85mm ridges are roughly the same height, while the 135mm has an extra "step" in it.

    Although all three of these are AI-s lenses, the same arrangement is also present on AI lenses-I thought I was throwing an AI lens into the mix, but got a pleasant surprise when I realized my 85mm was AI-s(and I'm too lazy to dig out an AI lens and re-take it).

    Andrew Garrard likes this.
  14. Since both my FA and F4 are non-working paperweights/doorstops; to me the rear protrusions are only useful as element protectors, regardless of their intended purpose.:p

    So, to boil all this down. It is the chunk of metal hung on the back of the lens that prevents it being used with a Metabones. Useful to know it can easily be unbolted though - for next time I'm chosen for a space mission and have to save every gram in weight.
  15. Yes - I think the only mechanical interface difference for AI-s is the cut-out (and the coloured minimum aperture in the finder view). Plus, obviously, that the aperture lever is linearised. (I'll say it yet again: I don't understand why a body which has been told the lens is AI-S rather than plain AI - bearing in mind you already have the option to enter the max aperture and focal length - can't control the aperture from the body like it does with an AF or AI-P lens.)

    That's a good point. Maybe my Canon history is showing, but roll on E-aperture lenses!
  16. "I'll say it yet again: I don't understand why a body which has been told the lens is AI-S rather than plain AI - bearing in mind you already have the option to enter the max aperture and focal length - can't control the aperture from the body..."

    You could always add a Dandelion chip.

    "...but roll on E-aperture lenses!"

  17. > You could always add a Dandelion chip.

    Which would also allow trap focus to work. Another pointless restriction based on the Nikon firmware.

    > Noooooooooo!

    Electronics in cameras. Whatever next? :) (Don't worry, I'm sure there's a market for a battery with a dynamo in it so you can wind up the dSLR before use. Actually, that's not a terrible idea...)
  18. As far as I know, the only camera that even makes use of that information is the FA.

    If you have an AI-s lens mounted and are in P or S mode on the FA, the camera just uses the pre-determined exposure. If you have a non-AI lens mounted, it stops the lens down then takes another meter reading and corrects the shutter speed based on that.

    Of course, the unspoken advantage is that you actually get the aperture that the camera says.

    In any case, as you can see from my above photo, the lens mount(albeit out of focus) on the F4 has the AI-s "feeler." Still, the only AE mode available is Aperture Priority-I'm not sure the pin that tells whether or not an AI-S lens is mounted does anything.
  19. Allegedly the F-301 and F-501 use it like the FA does. I'm also unclear what the F4 does with its AI-S feeler. (I actually thought it did something, but I now can't find what. Maybe it was supposed to, but didn't work?) Not that I have any of these.

    To be clear, while I'd love to see a Df2 with a "fully compatible with absolutely everything" lens mount (just as an engineering example), all I'm talking about for a "normal" DSLR is that the user check a box to say the lens is AI-S rather than just AI. If they lie, the exposures are wrong; how sad.
  20. No S mode. I owned an F-301 but honestly can't recall what did and did not work with the various lenses. The FG also has P mode - which works both with Ai and Ai-S lenses: through the Nikon F-Mount - the difference between an AI lens and an AI-S lens
    Quite clear: nothing at all.

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