HELP - Canon EF focusing protocol (electronic)

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

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.<p>Why not just use a manual focus lens?

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

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)??

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

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.

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.

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.

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:

 

1. VBAT

 

2. P-GND

 

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

 

4. VDD

 

5. DCL

 

6. DLC

 

7. LCLK

 

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!

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.

Scott, did Bob say that you, personally, were wrong? No.

 

Did he post pretty much complete agreement with you about the focusing requiring power, rather than simple mechanical effort? Yes.

 

So what's your beef? No need to get defensive, we're all just trying to help here.

Hi Brett,

 

I agree with you a little, Bob did basically repeat what I had said

without adding any really useful additions. But he also stated

"there is no mechanical version of the 200 1.8L, they all require

electrical power" this was a direct contradiction of a statment I

had made, I was the only person to mention the mechanical

version in an answer and in this instance Bob is 100% wrong,

as Gaspar mentions later he is actually useing the FD version

aswell as the EF version, the FD is exactly (except the mount) the

same lens but with mechanical manual focus ONLY.

 

That is the main reason I felt a followup was in order, I don't feel I

overreacted and I very much respect the far deeper knowledge

that many of the Photonet heroes have on these subjects than

me, however this one instance just proved they are not infalable.

 

Best wishes all, Scott.

Hi

 

Take a look at this:-

 

http://www.birger.com/html/ef232_home.htm

 

Basically, it mounts to an EF lens, and gives you full control over it via RS-232

 

"The EF232 adapter allows the use of Canon EF-mount lenses without requiring the use of a Canon EOS camera body.

With the Adapter, focus and iris functions of the lens are precisely controlled over the serial port. The beauty of the Canon EOS lens system is that all of the actuators for the lens are incorporated. But the difficulty is that many of the Canon lenses can not even be manually focused without being attached to a Canon EOS Camera body. That is of course, until now. The EF232 Adapter takes the place of the Camera body, giving functionality such as highly repeatable focus control, and iris control in ½ stop increments."

 

Regards,

 

Steve