Before I get started, I need to be up front and honest about today’s post. Although I am a Nikon guy, I have been trying to keep my posts general enough, or at least cover multiple brands, so that it has some relevance to photographers who don’t shoot Nikon. With that being said, today’s post is specifically for photographers who use Nikon SLRs. Stay tuned though as to make up for a post I was not able to get up a couple of weeks ago, I will be having a special edition post on Thursday where I’ll be talking about reflex mirror lenses and whether or not they are worth it. Also, in coming weeks I am going to continue my series on focal lengths followed up a new series (beginners are going to pay attention to that one). As always, I may not do the series in consecutive weeks to keep it interesting for readers who may not be interested in a series that lasts for a month before getting on to a different topic. Now on to today’s post about what the heck AI is in terms of Nikon lenses and how to make use of your older lenses that may not have it.
As mentioned above, this week’s post is really only relevant to the users of Nikon SLR and DSLRs. The topic is automatic indexing, or AI. I know with a topic like that you are on the edge of your seat (sarcasm abounds), but please bear with me as this is very relevant, may prevent you from breaking your camera, and may save you some money (more interested now?). Back in my post on lens mounts, I stated that Nikon had not really changed their lens mounting system, known as the F-mount, since its inception in 1959. This is a fantastic feature of the Nikon system and for Nikon users as it means that lenses that came out from the point on can be used on the vast majority of Nikon camera bodies including today’s modern DSLRs. Well, that is mostly true, there is one big caveat to that feature. In 1977, Nikon added a feature to its camera system that would be the only change that really affected the ability to use a lens from 1959 on a camera body made in 1978, or 1994, or 2011….you get the point. This feature was known as automatic-indexing, or AI for short. For the purposes of this post and as you look it up on the web, lenses before 1977 will be referred to as non-AI or pre-AI, lenses after will be referred to as AI lenses, and those that have been converted with be known as AI’d lenses. Lenses after that each have their of official letter designations (such as AIs, AF-I, AF-D, AF-S, etc). All of these terms are pretty standard in the Nikon community.
So what is automatic indexing, and why do you need to care? Well, to answer the second part in brief, you don’t need to care if you don’t use any lenses that pre-date 1977, but still read on as you may find a more inexpensive way to get some good glass. If you are really knew to cameras and only used DSLR and CPU based lenses, you may not know that before the chip was placed in the lenses back in the late 80s and early 90s, the camera had no idea what that aperture was on the lens without a mechanical linkage. (If you are really new to it and have lenses without aperture rings, you may not even realize that up until the last decade or so, the aperture was completely controlled by a ring on the lens body.) This linkage between the lens and body had a couple of different executions in the history of Nikon SLRs.
The Nikon F camera introduced in 1959 (after which the F-mount was named) utilized a pin-and-yoke system to tell the camera what aperture was currently in use on the lens. There was a lever with a bin mountd just above the throat of the camera where the lens attached. There was a corresponding bracket on the lens, specifically on the aperture ring, called a yoke. When attached the lens you needed to make sure the pin was captured by the yoke on the lens. As you turned the aperture ring, the yoke would move the pin, and this would inform the metering system on the camera as to what aperture was in use. However, because there was no CPU or way of telling the camera what lens and how many f-stops it had for its aperture range, the photographer had “index” the lens. The yoke on the lens was mounted above f/5.6. When the photographer mounted the lens, he/she had to turn the aperture ring all the way to the widest setting and then back to the aperture he/she wanted to use. The distance the pin traveled would tell the camera what the actual f-stop was on the camera (essentially calibrating the metering system to the lens). If the photographer didn’t index the lens, the meter would be way off and so would the exposures. Well as you can guess, this could become a common problem and as you were shooting with film, there was no way to know until you developed a roll to find all of your photos were either under or over-exposed.
To help aid this process, in 1977, Nikon introduced the automatic-indexing system on its line of cameras and lenses. This feature would do exactly as it sounds: it would index the lens automatically so all the photographer had to do was attach the lens and shoot away. It accomplished this by inserting a tab on the outside of the flange where the lens attached to the camera body. The aperture ring had a indexing ridge on it (essentially ring on the AI lens was narrower than the non-AI ring except for a ridge that was left at the same length). This ridge would engage the indexing tab on the body when the lens was inserted and inform the camera meter what f/stop the lens was set at. No manual indexing was required. In addition to the addition of the metering tab and the ridge on the lens, the AI lenses also had a second aperture scale in smaller number on the edge of the ring that could be seen through the viewfinder of some models of cameras. The yoke on the lens was left on for a couple of years, but was removed in 1979 as Nikon wanted people to move on to their newer cameras with AI and not preserve the old pin-and-yoke mechanism. Nikon did run a service to convert non-AI to AI lenses for a period of time (up until 1991) for a small cost which allowed many photographers to continue using the non-AI lenses they loved on their newer camera bodies.
Ok, great, so I’ve explained and given the basic history of the changeover from non-AI to AI, but how the heck does this affect you. This was decades ago, right? Again, if you don’t ever use an older lens, then it doesn’t really affect you at all. But, read on for good measure. Ever since the 1980s and 1990s, the majority of Nikon’s lenses have contained a computer chip. This chip told the camera what lens was on the camera, what aperture was set and, in some iterations, the distance from the camera to the subject. This chip communicated to the camera through a set of electrical contacts that Nikon placed around the bayonet of the lens and inside the flange on the camera body. However, in an effort to maintain compatibility with older lenses (again, a benefit of the Nikon system that Nikon recognizes their customers appreciate), they kept the metering tab on cameras all the way through to today. This means that the D7000 made in 2011 has a metering tab that would allow it to work properly with AI lenses made in 1977. Almost all of Nikon’s mid-range and higher end cameras (D300 and up) today have a metering tab and a minimum aperture contact (allows use of AF-D style lenses). As such, if you try to mount a non-AI lens on one of these camera bodies, it does not have room on the aperture ring to allow for the tab, and as such can cause the tab to bend, or break-off, essentially breaking part of the functionality of the camera. The lower mid-range cameras (D90 ,D7000, etc) do not have a metering tab, but do have a contact to tell the camera the lens is set at its minimum aperture which also can be damaged by the non-AI aperture ring. The entry-level line of cameras (D40, D40x, D60, D3000, D5000, D3100, D5100, etc.) do not have either metering tab or the minimum aperture contact. The entry-level line will only work properly with AF-S lenses where the AF motor is in the lens body. However, because they do not have a tab or contact on the mount, these entry-level cameras could accept lenses dating back to 1959 allowing the photographer to use them in complete manual mode (but no metering).
If you have a lens that pre-dates the 1977 introduction of AI, then you should not mount it on your new DSLR until you find out if it will accept it with no damage (you should check the manual anyways before mounting any lens just in case). I had one of these lenses that I used with my Nikon FG film camera. It is a Nikkor 28mm f/2.8 and it was made before 1977 (see below to tell quickly if your lens is AI or not). I really liked this lens and wanted to continue to use it on my D90, but because of the minimum aperture contact tab, I could not. And since Nikon no longer converted the lenses for a fee, I turned to a 3rd party to help solve my problem. I don’t make it a habit to promote one place over another on this blog and certainly do not get paid or any benefit from promoting another business or product, but I will tell you which place I have used for a specific service, and whether or not I had a good or bad experience. For this AI-conversion, I sent my lens off to John White (www.aiconversions.com). For a reasonable fee, John takes and machines down a section on the back end of the aperture ring to accommodate the metering tab on the camera. He then inserts a label with the secondary aperture scale mentioned above (only useful on some cameras made in the 1980s, but nice to have nonetheless). All and all it was a great experience and I would definitely consider using him again as the service was fast, reasonably priced and professionally done. If you are so inclined you could do this service yourself, but given the low cost of getting it done by someone who has done it before, I decided it was no worth the risk of doing it wrong and breaking my camera anyways, but you could certainly do it if you have better skills with tools than I do. Now I can use the lens on any of my cameras including my Nikon FG or even my father’s Nikkormat (pin-and-yoke style).
I mentioned above that reading this post could save you from accidentally breaking your camera by attaching a non-AI lens to your camera body which I explained above. However, I also mentioned that this could potentially save you money. If you are a person who likes manual focus and are looking for some good inexpensive Nikon lenses, then I highly suggest checking out online auctions or other sources for used gear and pick up some non-AI lenses. Since they lack compatibility with many modern cameras in their unmodified form, they often go for real cheap, but with some still fantastic glass (in fact many of the AI-s lenses still available from Nikon today are based off these non-AI lens designs). Then just send out the lens for conversion or do it yourself, and amazingly you have a usable inexpensive manual focus lens from Nikon. This may not be the best approach for you and I would only recommend it if you prefer older manual focus glass. If you are in that boat then this a good way to save some money without risking your camera.
So before I wrap up this post, I mentioned I would describe how to tell if the lens you have is non-AI or AI (or AI’d). The biggest give away is that the yoke on the non-AI lens is solid (no holes except the slot to accept the pin). It is possible the yoke will still be solid if the lens was previously converted to AI. If the yoke has additional holes (designed to let in more light to see the additional aperture scale on some cameras) then it is AI compatible. Also, on AI lenses you should be able to see the metering ridge on the back edge of the aperture ring. If the ring has a slot cut away, it is possible that it was AI converted (AI’d). If the ring is solid all the way around check some of the features before trying to mount it. As mentioned above, AI lenses also have a smaller, secondary set of aperture numbers toward the back edge of the aperture ring. If you only have ne set of larger numbers it is likely an non-AI lens. In many AI conversions these numbers will be added by way of a thin label. If you are buying a lens and are unsure about the AI status, ask the seller if the lens was converted or if they know when it was made. If they are reputable used lens dealer they will know and tell you (many will often taught that it has been converted as it justifies them selling it at a higher price), but always check the other features listed above. There are a couple of more ways which use the color of numbering of the smallest aperture on the scale, but they can vary so I won’t go into them. There are some other great sources on the web for more information of the progression from non-AI to AI, so please go check some of them out as well (as much as people in forums like to harp on Ken Rockwell, he does have a lot of good information on his site, www.kenrockwell.com). You can also always take a photo of the lens and post it in a forum, or send it off to a conversion service to ask them.
So that wraps up this week’s post. I promise to not have too many of these Nikon only posts, but I thought this was one that was relevant and important information. It also ties in to my posting in conjunction with the photos I’m taking as part of my 365 photo challenge. As I mentioned there will be another one a little later this week which will not be specific to any one brand. In the meantime, get out there and capture the world around you.
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