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HELP - Canon EF focusing protocol (electronic)

Gaspar Bakos , Aug 20, 2003; 06:31 p.m.

Dear all,

We are desperately seeking the electronic protocol of a Canon EF autofocus lens. Here is the problem we are facing:

An EF 200mm f/1.8 Canon lens is used on an astronomical instrument WITHOUT a Canon EOS body attached. We would like to MANUALLY focus the lens by turning the "manual focus ring" and setting the lens focus mode switch to "manual". However, the focus does not change at all with the above approach; for example, the image of stars remains exactly the same: rather blurred.

We are afraid that this "focus mode" switch works only (thus, can be switched to manual) if the lens is attached to an EOS body, and some kind of voltages are applied to the little golden 'plates' (pins) on the bayonett of the lens. Here comes the question: does anyone know the protocol of these pins? Does anyone has a multimeter and could do a 5 minute reverse engineering for us on a Canon EOS body to see what voltages are set to the pins?

If we knew this, we could apply the same voltages, and focus the lens.

Any help is appreciated,

Gaspar Bakos


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Maureen M , Aug 20, 2003; 06:45 p.m.

I'm not sure a simple multimeter would be useful in measuring/analyzing the signals between the camera and lens. Perhaps an oscilloscope would, but even then I would never just start poking around the electronic contacts on a good camera. If reverse engineering were that simple, Sigma probably wouldn't have any compatibilty issues with the EOS lineup.

Why not just use a manual focus lens?

Minh Thai , Aug 20, 2003; 08:29 p.m.

I think all Canon EF lenses have mechanically linked manual focus ring. The 200/1.8 has ring USM so that's definitely not a problem. I think the blurry pictures are the result of the incorrect distance from the lens mount to the film plane. I'm not sure how the lens is mounted to your instrument, but I would suggest a bellow type setup where you can move the whole lens fore and aft. My 2c. M

Phil S. , Aug 20, 2003; 09:22 p.m.

My $0.00000001 guess is that the focus isn't controlled by voltage, but by frequency, maybe a square wave??? so you would need an osciloscope (sp?) to check it. The focal plane comment is more likely the problem. Can you put the lens on a canon body, focus to infinity (and lock the focus there), and then try it on your telescope (looking at a distant object)??

Jim Vanson , Aug 20, 2003; 09:56 p.m.

Gaspar...why not go straight to Canon USA's Camera Divison Technical Information Guru Chuck Westfall. Try contacting him through CPS.

Scott Ferris , Aug 20, 2003; 10:05 p.m.

Hi Gaspar,

Doubt if I can be much help, but the 200 f1.8 was made in an FD manual focus version so would it be possible to swap it out with somebody like KEH ?(who i think have one).

The second thought is that the 200 1.8 focus ring is not attached to the focus group mechanicaly, as you turn the focus ring it transmits an electronic signal to the USM motor that in turn turns the focus elements (this is true in all the tele lenses with variable manual focus speed control). This being the case you need to either supply power to the lens somehow to drive the USM motor, or make a mechanical linkage to the USM/focus group.

Hope this might be of some help, Scott.

Mark U , Aug 20, 2003; 10:35 p.m.

I think the problem is much more fundamental. A camera lens is designed to produce a real image at the film plane or equivalently on a focussing screen. It is not a telescope, which is designed to produce a real image in your eye. You would need additional optics to convert from one to the other.

Bob Atkins , Aug 20, 2003; 11:20 p.m.

No, no, no ,no, no.

Why do people give advice when they don't know what they are talking about!

The 200/1.8L is one of the lenses that requires power in order to focus. It has a USM motor, but it's not the same as in the cheap consumer ring USM lenses. The 300/2.8L, 500/4.5L and 600/4L are the same. The 300/4L is different as are the new IS telephoto lenses and all the consumer USM ring motor lenses. There is no mechanical focus version of the 200/1.8L, they all require electrical power.

Unfortunately I can't help with the protocol. The lens has a CPU as does the camera. I don't know if it just needs a DC voltage from the camera or if it needs to see some sort of correct handshake protocol with the camera before it will focus.

Your best bet, if it's possible, would be to get an old canon body (almost any one would do) and remove the back so you have access to the film plane. Of course that may not be useful in your application. Otherwise you'll have to dig around for reverse engineering data. I know Canon have not made public their EF lens protocol, but that doesn't mean someone somewhere hasn't figured out what it is. Sigma, Tamron and Tokina probably have since they make compatible lenses.

I don't think the voltages on the body pins will do you much good, plus they are probably different whan a lens is attached anyway.

Chuck Dowling , Aug 21, 2003; 12:41 a.m.

It's not just voltages, there is a serial communication protocol being used.

Pinout and pin functions:

Left to right, looking at the front of the body:


2. P-GND 3. P-GND (pins 2 & 3 are common on the lens)

4. VDD

5. DCL

6. DLC


8. D_GND

Pin Functions

VBAT - Motor power

P_GND - Motor power ground

VDD - Logic circuitry power

D_GND - Logic circuitry ground

DCL - Data from body to lens

DLC - Data from lens to body

LCLK - Clock

Data protocol: Motorola SPI; 8 bit serial; Such as the protocol used with the 68HC05 chip.

You will need access to the pins to experiment. Hack up the cheapest body you can find, such as a broken Rebel or whatever. Good luck, experiment on a cheap lens first!

Scott Ferris , Aug 21, 2003; 11:06 p.m.

Chuck answered the question, but Bob you are wrong, as I said the EF version of the 200 f1.8 needs power to focus as the focus ring is not attached to the focus group mechanicaly, but the optically identical FD version is mechanical manual focus only.

Tell me I'm wrong again. The advice I gave was 100% accurate.

Take care, Scott.

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