# Meter Coupling Lever



## astroNikon (Oct 23, 2014)

So what does it do ?
A little flat thing just above the lens mount that rotates about 1/4 around the mount.

I think my old AF 300/4 lens moves it but the G's don't.


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## Derrel (Oct 23, 2014)

It is spring-loaded, and the spring holds the tab against the Ai-coupling ridge on the rear of newer Nikon F-mount lenses. On lenses that have an aperture ring, the position of the tab on the body indicates the f/stop that is set on the lens's aperture control ring, based on its location within the arc of travel.

This system is a mechanical interface that is a holdover from the earlier "buckhorns and pin" system premiered in the mid-1960's on the Nikon F's Photomic series of finders. With the "buckhorn gap" straight up, at 12 noon, the lens was set to f/5.6. When a lens was mounted, it was supposed to be put on at f/5.6, for the easiest drop-in of the pin into the buckhorns, and then the lens was to be MANUALLY, by hand, "racked" to maximum aperture, so the meter would "know" the max aperture of the lens in use. Basically, Nikon uses a simple direction-and-distance reading system that says, "The follower is way off to the side: ergo, the lens is at f 1.4" or "The lens aperture ring is at f/5.6, because the follower is straight up!"

In 1977, Nikon decided to move to Automatic aperture Indexing, or the "Ai" system. The Ai coupling tab is on the bodies; the Ai coupling ridge is on the back of the lens barrels along with a smaller set of f/stop numbers called the secondary aperture scale, which was read out by a nifty prism and lens system, and relayed to the viewfinder.

One reason that some of the newer Nikon camera bodies STILL have the Ai-indexing tab on the bodies is so that users can still set apertures on the lens itself, and still maintain light metering, and this AI coupling tab allows older lenses with no CPU to interface with the light metering system is most newer Nikon bodies(but not all of them). It's a backward compatibility insurance system for Nikon and their users who have legacy lenses.

The "baby Nikons" D40,D40x,D60,D3xxx,and D5xxx series all LACK this AI coupling system, and they also lack the minimum aperture sensing pin, that dealie-bob located at the 7 o'clock position. The LACK of the AI coupling system AND the lack of the minimum aperture sensing pin allow the reallllly old 1959-1977 (approximate end date,since lens model Ai updating did NOT occur in one year for all lens models) Nikkors to be mounted on the baby Nikons without shearing off the min.,ap.sensing pin, or damaging the fixed Ai tab that most other models have.


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## astroNikon (Oct 23, 2014)

secondary aperture scale --> You mean the lenses with the metal triangle with cutouts on them ?


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## Derrel (Oct 23, 2014)

astroNikon said:


> secondary aperture scale --> You mean the lenses with the metal triangle with cutouts on them ?



No, the secondary aperture scale is a separate SMALL set of f/stop numbers, located at the verrrry back of Ai and Ai-S lenses. The actual setting ring and index numbers are written larger,and are more forward on the barrel.

The metal triangle with the cutouts is the "prong" or *the "buckhorns" or the "meter coupling assembly"*. EARLY lenses had triangular-shaped metal; later ones had some skeletonizing done on them, which allowed more light thru, to allow the secondary numbers for f/8 and f/4 to be seen more-clearly in poor light. It also helps f/5.6 be seen more easily as well.


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## Derrel (Oct 23, 2014)

Here is an old, 50mm f/2 Nikkor, in Ai mount.


By the white rear lens cap, you can see the Ai coupling ridge on the lens, which ends at f/11 and 1/3 stops on an f/2 lens. 

The lens aperture-setting ring, the one with the COLORED f/stop numbers,always has f/5.6 positioned directly straight up at NOON on any Nikkor lens which is set to f/5.6. The "notch" in the buckhorns is also straight up when the lens aperture is set to f/5.6.

The "buckhorns" are the silver-colored stainless steel mechanism, held on with the two screws. The buckhorns are not required except to connect to OLDER, Photomic metering prisms for the F or F2, and the older Nikkormat cameras. The secondary aperture scale is that set of small, all-white engraved numbers at the very back edge of the lens barrel.

Notice the color-coded apertures and depth of field "bars"? That was a Nikon exclusive, and it made their lenses easier to use than other brands when scale-focusing.


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## wfooshee (Oct 28, 2014)

As stated, that tab "reads" the position of the aperture ring on AI-type lenses. CPU lenses don't use the aperture ring, they are set by the camera's controls. CPU lenses that have an aperture ring (D-series, for example) work on "modern" cameras by leaving the ring at minimum aperture, or with older cameras by turning the ring as needed. A "modern" camera with the AI sensing ring will only use it with non-CPU lenses. It's a compatibility feature on the "higher-up" cameras, allowing them to meter correctly and auto-expose in aperture-priority if desired. The "lower" cameras will mount these lenses but the camera's meter will not work, having no way to read the aperture setting of the lens.

Those secondary aperture numbers were visible in the viewfinder via a periscope on the front of the pentaprism so the photog could see the lens setting while his eye was at the camera.


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## 480sparky (Oct 28, 2014)

Derrel said:


> By the white rear lens cap, you can see the Ai coupling ridge on the lens, which ends at f/11 and 1/3 stops on an f/2 lens......




Minor technical correction.... it ends at 8 2/3 stops, or f/10.  The scale reads right to left.


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## Derrel (Oct 28, 2014)

480sparky said:
			
		

> Minor technical correction.... it ends at 8 2/3 stops, or f/10.  The scale reads right to left.



As stated f/11, and then 1/3 stop. Minor reading comprehension problem, on your end. F/11 AND, meaning "added exposure"...as in a third-stop more than f/11. Look at it again and see if you can comprehend what "and" means, mmkay?

I wrote, "f/11 and 1/3 stop", NOT f/11 MINUS 1/3 stop. That means f/11, PLUS 1/3 stop.

If the lens were an f/1.4 lens, the Ai ridge would be at f/8.


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## 480sparky (Oct 28, 2014)

Sorry. F/11 and 1/3 is f/13.

I guess the error is the way it's written on your end.

I've had my say here.


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## Derrel (Oct 29, 2014)

480sparky said:


> Sorry. F/11 and 1/3 is f/13.
> 
> I guess the error is the way it's written on your end.
> 
> I've had my say here.



Seems like you cannot understand that when we describe an exposure, opening the diaphragm up means, "Moving to a larger hole, one that lets in more light."

F/11 *and* 1/3 stop means you click to f/11 and then ADD light. F/11 MINUS means the aperture is closed down, to a hole that is smaller, and lets in less light. This is the way the terms "and" and "minus" have been used for well over a century.

I had a hundred dollars, but my buddy payed me back the twenty dollars I had loaned him for lunch last Friday, so now I have _one hundred *AND* twenty dollars_.* More dollars*.

A second example of how the language has been used for a century: f/5.6 and two-thirds of a stop means you are at f/5.6 and then you tick the aperture OPEN more, 2/3 stop wider, in the direction of f/4--*adding* more light.

"And" means added light. "Minus" is the word used to describe closing the aperture. This is about the fifth time you've pulled this same stunt on a post I've actually taken the time to answer. Maybe next time, do some actual work yourself. Is that too much to ask?


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