AI indexing. I don't get it

[sanmich]

I am musing about a small come back of my F into action, and maybe I will give a try the old nikkors, some of which I would like to adapt to AI.

Now, If I get it right, all I need to do is to file a certain prtion of the aperture ring. That brings me to that question:

I see how the AI conection with the camera "tells" the camera what is the current aperture on the lens. What I don't understand is how the lens "tells" the camera what is its larger aperture.
I mean, the camera needs to "know" at what aperture the light is curently measured right?

Help me! cure my ignorance! 🙂

 

[Vilk]

after a quick look in my lens drawer--

the AI tab's position is fixed relative to the max aperture. so the light meter knows "how much light am i getting?" (that's the full opening) and "by what factor do i need to calibrate/adjust my circuit?" (that's from the position of the tab)

in other words, the meter doesn't know what the max aperture or the working f-stop may be. it knows the absolute amount of light and the correction factor for the working f-stop, enough to match the needle

that's why you have to enter the max aperture of the manual lens on cameras like D3 or D7000--so you can get the F display on camera (handled optically on older cameras)

seems to make sense, but i frequently type faster than i think 😎

ps. looks like i did make some sense, http://www.cloudoff.com:8080/showIte.../11142771.html 🙂

 

[sanmich]

Let me ask it this way:

I have only two lenses right now, a 35 2.8 and a 50 1.8.
I can get the small metal piece on the camera reach almost the same position, in front of the aperture window on an FM, on the 35 by setting the aperture to f/8, and on the 50 to f/11.

If the only communication is throught this ridge, how does the camera knows the difference between the two lenses??

 

[Spavinaw]

What tells the camera the maximum aperture of the lens is how much of the aperture ring is missing. For example, if you were modifying an f1.4 lens you would file away the aperture ring until you were exactly opposite the f8 mark on the ring. However, if you were converting an f2.8 lens you would continue filing until you got almost to the f16 mark.

 

[Vilk]

michael, like i said above, the camera couldn't care less what lens you mount. the meter simply compares the energy of the light coming in with a reference level--the aperture ring only calibrates the incoming value. from the camera's point of view there might just as well be no markings on the lens

sorry, i'm afraid my didactic powers end here 🙂 best of luck with your quest!

 

[Vics]

I remember watching PJs shooting Fs and when they changed lenses, they'd crank the aperture ring back and forth, lock to lock. I asked why they did that and they said it was to let the meter know what the max aperture of the lens was. I have lots of NAI lenses that have been AI'd but I don't really understand it myself. I do know that some NAI lenses can't be modified like that.

 

[sevo]

Pre AI lenses use a coupling prong at a fixed location on the aperture ring - the camera/finder has a latching mechanism that locks at the widest open aperture if you "index" the lens by ratcheting it back and forth.

AI replaced that with a cutout in the aperture ring which is at a constant angle to the wide open position, and does not need that manual indexing any more - but with many PJ's it stuck on as a habit for years after the upgrade to AI.

 

[sanmich]

michael, like i said above, the camera couldn't care less what lens you mount. the meter simply compares the energy of the light coming in with a reference level--the aperture ring only calibrates the incoming value. from the camera's point of view there might just as well be no markings on the lens

sorry, i'm afraid my didactic powers end here 🙂 best of luck with your quest!

It seems your didactic powers are not that bad...😉

Let me see if I got it right: from the camera point of view you need to close the aperture a certain number of stops from the current measure. the "meaning" of this number of stops is different from lens to lens, but doesn't matter to the camera.

Did I pass? :angel:

 

[Vilk]

...from the camera point of view you need to close the aperture a certain number of stops from the current measure.

if by "current measure" you mean max aperture (i.e. lens wide open) then yes. the camera will then adjust the current in the match needle circuit by a factor of 2 to the power of the "certain number of stops." by setting the aperture 3 stops from wide open you're effectively telling the camera, "hey, you may be seeing all that bright light coming in, but when we take the picture there will be 8 times less of it." (2^3 = 8)

...the "meaning" of this number of stops is different from lens to lens, but doesn't matter to the camera.

only if by "meaning" you understand the absolute f-stop number (4, 5.6, 8, etc.). from the camera's point of view, the "meaning of this number of stops" is exactly the same from lens to lens--it's the adjustment factor i explained above. the actual labels--4, 5.6, 8, etc.--yes, those would mean nothing to a match needle system

it's a bit like comparing the amount of liquid in two vessels. you see when it's the same, when one has twice as much liquid, etc.--and that's what the meter does. you can write a numerical scale on the side of the vessels, but that will not affect the sense of "equal" or "twice as much."

Did I pass? :angel:

the proof is in the pudding. are you having fun? that's all there is to it! 😎

 

[Gary E]

I don't get what this is all about. Sanmich said he was getting his F back into action. The F has no AI indexing around the body mount. The F meter requires prongs on the lens aperture ring to "index" the meter. Once you put on the lens on a metered F camera you have to turn the aperture to it's max value (1.4, 2.8, etc.). This will tell the meter your lens' max aperture.

In more modern Nikons such as F3 and on, the ridge on the aperture ring will move the aperture coupling ring on the body to a different resting position once the lens is mounted. This tells the body the max aperture of the lens.

This being said, I miss both my F2 and F3 though my newer AI lenses (without prongs) didn't meter on my old F2 🙁

 

[sanmich]

I don't get what this is all about. Sanmich said he was getting his F back into action. The F has no AI indexing around the body mount. The F meter requires prongs on the lens aperture ring to "index" the meter. Once you put on the lens on a metered F camera you have to turn the aperture to it's max value (1.4, 2.8, etc.). This will tell the meter your lens' max aperture.

In more modern Nikons such as F3 and on, the ridge on the aperture ring will move the aperture coupling ring on the body to a different resting position once the lens is mounted. This tells the body the max aperture of the lens.

This being said, I miss both my F2 and F3 though my newer AI lenses (without prongs) didn't meter on my old F2 🙁

Gary, you are right, I wasn't clear.
I even plan to have a plain prism on the F, so why would I care about any indexing?

The thing is: I have other, newer Nikon bodies, and before I buy a lens I wanted to know if I can adapt it to AI or not.

 

[hans voralberg]

Let's make it simple, you can AI almost any lens as long as you know where to file!

 

[Gary E]

You cannot file any old lens to make it an AI lens. The whole aperture ring on the lens must be replaced. They used to sell kits (from Nikon) to change it to an AI lens. You also cannot put an old non-AI lens onto newer bodies as it can damage the body mount. The F4 had a pin with you can push to move the AI out of the way to allow those old lenses to be mounted onto the body. I believe the F5 can do it too, but you had to send it to Nikon to get it done.

If you have a eye level non-metered prism, you are a lucky guy. They go for big $$$ nowadays. I've tried to look for one for my F2 but they cost more than the body and metered prism. 🙁

 

[sevo]

You cannot file any old lens to make it an AI lens. The whole aperture ring on the lens must be replaced.

Wrong. Anything in AI is merely a matter of machining down the ring in the right places - all of it could at least theoretically be machined into an old ring, with the right tools, you might even engrave the aperture numbers...

Basic filing down the old aperture ring can be done without special tools (other than a JIS-0 screwdriver and metal file).

 

[Nikon Bob]

If you get lucky there is a neater way to convert to ai rather than filing as there are still suppliers of original Nikon ai conversion kits out there. Here is a site that can tell you which kit you need for what lens http://www.photosynthesis.co.nz/nikon/serialno.html .

Bob

 

[ronnies]

As others have said, the tab on the aperture ring only indicates the currently set aperture relative to the maximum aperture and the system doesn't need to know absolute aperture values.

There is however a lever inside the lens mount that indicates the lens maximum aperture to the body but as far as I'm aware, the only Nikon bodies to read this pin are the F4 and FA. It's at 6 o'clock with the lens mounted on the body.

Ronnie