[bertwert]

I have a Meade telescope (800mm, f/8.8) that has big knobs for focusing:

(not my photo)

I was wondering if I could attach a DC motor to one of the knobs and wire it to the power pins on my camera to make this an autofocus lens...
Is this technically possible and would it work? Or am I just being silly?

Is there anyway this could damage my camera attempting this, as this would be a fun experiment.

[shardulm]

I was wondering if I could attach a DC motor to one of the knobs and wire it to the power pins on my camera to make this an autofocus lens...

Yes you can however it depends upon what DC motor you are using and if the power supplied by your camera is sufficient enough to drive it. I am also not sure if power is accessible to tap from outside of the camera (unless you are talking about tapping into the SDM motor contacts that sit inside the mount). If you are trying to use SDM contacts at all you need to understand how these contacts behave when AF is activated.

Is this technically possible and would it work?

Theoretically possibl and would work theoretically speaking. But won't be usable mostly.

Or am I just being silly?

Great idea. But its an overkill. Those big knobs are there for a reason and may actually give you the pleasure of manual focusing (which you should do in this case any way).

Is there anyway this could damage my camera attempting this, as this would be a fun experiment.

Subject to what motor you are using it may or may not damage the camera. The SDM motors are very low power motors and draw very less current and the camera contacts and power lines are designed accordingly. Most likely it will damage the camera if you end up using a motor that is power hungry. Almost instantly it will burn the internal wiring if that happens to be the case. That won't be a fun experiment unless you have a spare body that you can sacrifice to see the fun (burn) part.

[bertwert]

Yes you can however it depends upon what DC motor you are using and if the power supplied by your camera is sufficient enough to drive it. I am also not sure if power is accessible to tap from outside of the camera (unless you are talking about tapping into the SDM motor contacts that sit inside the mount). If you are trying to use SDM contacts at all you need to understand how these contacts behave when AF is activated.

I was thinking of those contacts, so it depends on the motor, OK.

Theoretically possibl and would work theoretically speaking. But won't be usable mostly.

I didn't think it would be useable, more for fun.

Great idea. But its an overkill. Those big knobs are there for a reason and may actually give you the pleasure of manual focusing (which you should do in this case any way).

I don't mind MF using the telescope, probably faster than any AF. It's more a proof-of-concept idea.

Subject to what motor you are using it may or may not damage the camera. The SDM motors are very low power motors and draw very less current and the camera contacts and power lines are designed accordingly. Most likely it will damage the camera if you end up using a motor that is power hungry. Almost instantly it will burn the internal wiring if that happens to be the case. That won't be a fun experiment unless you have a spare body that you can sacrifice to see the fun (burn) part.

OK I probably won't try this then (unless I find a cheap body).

[DougieD]

I'm sure you could make a telescope auto focus, if you really want to. You could use an external battery for the telescope focuser, and use the camera supply as a reference to start and stop the focusing. I'm not familiar with how the auto focusing system really works; I'm sure there are if's, and's, and but's. For example, the focuser moves back and forth. Do they change the polarity at the camera, or does that happen inside the lens? There must be a pinout for the mount somewhere...

I use a Vixen ED80sf telescope with my K-50. It's 600mm f/7.5, and I often use it with an Orion 0.8X reducer for 480mm f/6. It's fairly easy to manually focus. And if you remove the paint or anodizing off of the T-ring in the area where the contacts are (so they short to the T-ring), you can use catch-in-focus.

[bertwert]

I'm sure you could make a telescope auto focus, if you really want to. You could use an external battery for the telescope focuser, and use the camera supply as a reference to start and stop the focusing.

Interesting idea...

[UncleVanya]

Catch in Focus may be easier and simpler and also more rewarding.

[bertwert]

Catch in Focus may be easier and simpler and also more rewarding.

I do that already

This idea was more a proof-of-concept / fun thing to do.

[DougieD]

I'm not sure whether the camera sends power to the focus motor as needed, or the power is always there and the focusing instructions are sent to the lens through other pins. Maybe even serially, using pins that send and receive other information. The first scenario is the preferable one for what you are thinking of doing. However, as is so often the case in electronics, the way you wish they did it is not necessarily the way they did it, lol!

If you don't have some easily accessible way of controlling your motor, both the direction it rotates and starting/stopping it, you might have to dive inside the camera. Or if the focusing instructions are sent as digital data, you would have to intercept and decipher them.

But maybe there is a couple of dedicated control pins. You would have to look into this.

[bertwert]

I'm not sure whether the camera sends power to the focus motor as needed, or the power is always there and the focusing instructions are sent to the lens through other pins. Maybe even serially, using pins that send and receive other information. The first scenario is the preferable one for what you are thinking of doing. However, as is so often the case in electronics, the way you wish they did it is not necessarily the way they did it, lol!

If you don't have some easily accessible way of controlling your motor, both the direction it rotates and starting/stopping it, you might have to dive inside the camera. Or if the focusing instructions are sent as digital data, you would have to intercept and decipher them.

But maybe there is a couple of dedicated control pins. You would have to look into this.

I wonder where I could get this info?

[UncleVanya]

Another option - albeit weird - would be to build a physical interface to the screw drive and read the direction of the drive as it turned and translate that to motion of the focus mechanism. I don't actually know how the mechanism knows when it reaches the end of the available focus range (infinity or MFD) there has to be some feedback.

[VisualDarkness]

Focusing most likely works like this:
1. Camera asks the lens for info.
2. Lens answers.
3. Camera gives an order
4. Lens reacts
5. Lens gives feedback
6. Camera asks for info or gives a new order
Steps repeated in different loops

I highly doubt it is a simple as just reading directions at random.

[shardulm]

Focusing most likely works like this:
1. Camera asks the lens for info.
2. Lens answers.
3. Camera gives an order
4. Lens reacts
5. Lens gives feedback
6. Camera asks for info or gives a new order
Steps repeated in different loops

I highly doubt it is a simple as just reading directions at random.

It may be simpler than you think it is. And you may start pondering over how a manual lens focusing works on Pentax bodies. If and when you use a manual lens the camera still gives you a focus conformation despite no communication via any contacts. Since manual lenses can be used with ease like this my hunch is that the camera does offer that simple kind of focusing ability through the lens center point restricted for manual lenses.
It may however not be the case for newer digital lenses or the ones that have firmwares in them to do the focus math and communication to the camera.

[bertwert]

Another option - albeit weird - would be to build a physical interface to the screw drive and read the direction of the drive as it turned and translate that to motion of the focus mechanism. I don't actually know how the mechanism knows when it reaches the end of the available focus range (infinity or MFD) there has to be some feedback.

I thought of using the screwdrive as well.
I wonder if I can get a broken lens...

---------- Post added 20-10-15 at 09:49 ----------

Focusing most likely works like this:
1. Camera asks the lens for info.
2. Lens answers.
3. Camera gives an order
4. Lens reacts
5. Lens gives feedback
6. Camera asks for info or gives a new order
Steps repeated in different loops

I highly doubt it is a simple as just reading directions at random.

I will study the lens mounts and any info I can find and see if I can work this out.

---------- Post added 20-10-15 at 09:52 ----------

It may be simpler than you think it is. And you may start pondering over how a manual lens focusing works on Pentax bodies. If and when you use a manual lens the camera still gives you a focus conformation despite no communication via any contacts. Since manual lenses can be used with ease like this my hunch is that the camera does offer that simple kind of focusing ability through the lens center point restricted for manual lenses.
It may however not be the case for newer digital lenses or the ones that have firmwares in them to do the focus math and communication to the camera.

I don't think the lenses do any focus math.
I think they just tell the camera this:
Lens Model
Max and Min f-stop
Type of focus (srewdrive/motor)
Focus range (infinity - min focus)
Then the connection to the AF.

The camera can decide when it's focused and/or how far to turn the focus.