What are the electrical contacts in a Nikon F-mount?

Question

Current Nikon camera bodies and lenses have electrical contacts. What exactly are they used for and what sorts of electrical signalling is used? Why are there so many?

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Some background to my question.

Early Nikon F-mount lenses communicated to the camera with physical couplings:

• at 7 o'clock: the original aperture-indicating meter-coupling "prong" (pre-AI).
• at 10 o'clock and at 6 o'clock black aperture-indicating ridges (AI).
• at 3 o'clock a slot to engage the pin that locks lens to body.
• just below that a machined dimple indicating linear stop-down (AI-S).

In 1990 Nikon filed US Patent 4896181 which described "a camera system … provided with terminals for transmitting information data signals [between body and lens]" this described a five contact system

The Patent is long and tedious to read but I gather that these five contacts implement a simple synchronous serial interface:

• "a" - Vdd (positive battery voltage)
• "b" - P1/P2
• "c" - SCLK (serial clock)
• "d" - SI
• "e" - Vss (common ground)

When Nikon introduced autofocus (AF), at that time they introduced lenses and bodies with these five electrical contacts (by the way, at 11 o'clock is the coupling for the focus-motor in the body that drives the lens focussing mechanism)

However, later they increased the number of contacts to seven in the body.

And (later?) to eight contacts as shown in this lens, although some contemporary Nikon DSLRs (with this kit lens) still only had seven contacts! (by the way note at 7 o'clock is the focus-motor coupling in this entry-level DSLR - stare and weep ye D3200/D5100 owners with AF-D lenses)

More recent lenses have up to ten contacts!

I wonder why you need more contacts (surely the old 5-contact serial interface can just be used for newer data)? What do the "extra" contacts do?

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For comparison the Nikon 1 CX mount has twelve contacts arranged

• with spring pins in body not in lens
• at the bottom of the opening (not at top as in F-mount)
• parallel to the front surface of the camera body (not radially perpendicular)

Answer 1

This blog post is by someone who wondered the same thing and got a response from someone with (apparently) detailed knowledge. In short, the number of different contact counts is due to the number of different versions of features that Nikon has included in their lenses.

The gap in the contacts in the second lens image is because the 5 contact interface was retrofitted to a design for a 7 contact interface. Those 5 pins are still all used for serial communication with the lens.

The extra three contacts on 8-contact lenses are for: an additional high power line for the lens motor, and two lens motion detection lines for direction and speed.

The two extra contacts on 10-contact lenses are for communication with teleconverters.

The response isn't sure about 7-contact lenses and speculates that they might be for lower end lenses without motion detection, but in the thread linked to by @Unapiedra a person going by the same name as the response provider in the blog post states that 7-contact lenses simply don't need the extra power that 8-contact lenses do.

The response in the post also lists what each contact in each of the 5, 7, and 10-contact interfaces is used for.

Answer 2

I wonder why you need more contacts (surely the old 5-contact serial interface can just be used for newer data)? What do the "extra" contacts do?

Two reasons:

• Maintaining forward and backward compability. Old lenses on new bodies and new lenses on old bodies should still work. This means if you have additional functionality, the interface (even if it is a generic serial connection) might not be able to handle it.
• Not all of these connections are for data. Some are actually for powering the motor and for teleconverters.

Some speculation on what the pins do: http://forums.dpreview.com/forums/read.asp?forum=1030&message=26958272

On the changes to the Nikon 1 CX mount, I would think that Nikon completely redesigned the interface, incorporating any feedback of 50 years of F mounts.
12 pins: The more the better, allow motor voltage, some data interface, control of aperture, maybe even zoom.
Put the spring pins in the body maybe to reduce cost of lens manufacturing.

Answer 3

For completeness: There was also an early F-mount autofocus system around the F3AF and a few dedicated lenses which did have contacts in the same place but had an incompatible pinout (at least two pins were raw motor connections). Using the lenses for that system on any other contact-bearing F mount camera than the F3AF, F4 or F501 is deprecated, nonfunctional and claimed to be potentially deleterious to the electronics.