[Thagomizer]

Hi all;

Just wondering if anyone out there has cobbled together something like this:

nonaC 40mm f/2.8 | It's been a while, so I thought it's time? | Flickr

to allow for automatic diaphragm/aperture control on reversed Pentax lenses? I know the need for mechanical as well as electrical connection between camera body and lens makes this a much more complicated project than the relatively straightforward device for Canon to which I've linked. But since when has "complicated project" ever stopped dedicated macro photographers?

[pathdoc]

It's only a matter of time before someone successfully hacks the pinout to a PLM lens. Then the world is xatneP's oyster.

Until then, the FA series and their predecessors are your go-to.

[stevebrot]

to allow for automatic diaphragm/aperture control on reversed Pentax lenses?

Ha! Ha! The E-mount Canon has the advantage of having NO physical coupling. Doing something similar with other than a PLM lens would require a powered actuator adapter on the mount end, a source of power for same, and some sort of signaling mechanism to stop down the lens BEFORE exposure.

It is preferable, IMHO, to simply use manual aperture lenses and stop-down metering.


Steve

[stevebrot]

It's only a matter of time before someone successfully hacks the pinout to a PLM lens.

The KAF4 pinouts are known. They are the same as KAF2 (data/control through the data pin and power through the power contacts). What is not known is the PLM protocol, i.e. the signalling that tells the lens how much and when to stop down.


Steve

[maltfalc]

Ha! Ha! The E-mount Canon has the advantage of having NO physical coupling. Doing something similar with other than a PLM lens would require a powered actuator adapter on the mount end, a source of power for same, and some sort of signaling mechanism to stop down the lens BEFORE exposure.

It is preferable, IMHO, to simply use manual aperture lenses and stop-down metering.


Steve

none of that would be required (unless you wanted af for some reason). all you would need is to link the lever to the actuator mechanically with a flexible cable, just like an old shutter release cable.

[pathdoc]

What is not known is the PLM protocol

That's what I was driving at. Sorry if I didn't express it clearly.

IIRC the PLM lens on a non-PLM body is in an always-open state, while the natural state of the DA lenses on a pre-A body is always-closed. So the PLM protocol has to comprise two orders, the order to actually stop down and the parameter for how far. The first can be done by simply passing current through the data pin; the second depends on whether absolute or relative closure is being commanded. Are you sending an analogue signal proportional to the degree of closure, or are you sending a repeated pulse train encoding the desired aperture/number of stops in base 2?

Remember too that there is another datum you need to send, and that is the focusing order. Do we know what the SDM and DC focus command protocols are, which channels do they pass through, and does the PLM focusing protocol copy them or act in some other fashion?

[photoptimist]

The KAF4 pinouts are known. They are the same as KAF2 (data/control through the data pin and power through the power contacts). What is not known is the PLM protocol, i.e. the signalling that tells the lens how much and when to stop down.


Steve

Full aperture automation should be easy for KAF2. The reversing ring would have contacts for all the camera's electrical connections, a second ring would go on the exposed end of the reversed lens with a corresponding set of contacts, and a flexible curly cord would connect the two rings. The protocol for aperture communication and control does not matter because the ring set is just a dumb extension cord. (As a plus, the second ring that goes on the exposed end of the lens could have a bit of a cap to protect the lens internals and a 49 mm filter ring.)

Auto-focus would be much harder, though, because the reversing ring reverses the required direction of motor turning. Focusing "closer" means moving the motor in the direction usually used to focus farther.

[stevebrot]

Full aperture automation should be easy for KAF2. The reversing ring would have contacts for all the camera's electrical connections, a second ring would go on the exposed end of the reversed lens with a corresponding set of contacts, and a flexible curly cord would connect the two rings. The protocol for aperture communication and control does not matter because the ring set is just a dumb extension cord. (As a plus, the second ring that goes on the exposed end of the lens could have a bit of a cap to protect the lens internals and a 49 mm filter ring.)

Auto-focus would be much harder, though, because the reversing ring reverses the required direction of motor turning. Focusing "closer" means moving the motor in the direction usually used to focus farther.

Yep, this would work, though only for PLM lenses. As for AF...focus for a reversed setup is generally done by moving the full assembly in much the same way as with a bellows. Just simply turn AF off.


Steve

[stevebrot]

none of that would be required. all you would need is to link the lever to the actuator mechanically with a flexible cable, just like an old shutter release cable.

That might work except for physical resistance of the cable. Edit: Though some sort of helper spring might work.


Steve

[Not a Number]

That might work except for physical resistance of the cable. Edit: Though some sort of helper spring might work.


Steve

Cables stretch. You'd have to recalibrate the adapter from time to time.

[Thagomizer]

It's the mechanical part that I figure would be most problematic. The electrical contacts could be patched into a deglassed teleconverter with a reversing ring attached on the front end. A second deglassed TC would bo on the now forward facing rear end of the reversed lens. getting one to one correspondence of the lever movement in the body to the lens lever to ensure the correct degree of aperture closure sounds like it could be tricky. Also probably beyond my mechanical ability....

[maltfalc]

That might work except for physical resistance of the cable. Edit: Though some sort of helper spring might work.


Steve

Cables stretch. You'd have to recalibrate the adapter from time to time.

both the actuator and lever are spring loaded and the actuator has plenty of torque. polished steel or chrome plating or a bit of teflon would keep friction to a minimum. as long as one end of the cable has the same sort of length adjusting nut as a cable release to eliminate slack, there shouldn't be any issues.

[victormeldrew]

A potentiometer or fine-stepped rotary encoder, connected to a small microcontroller and a miniature servo would be the easiest way to connect the two. I could write the code for the controller if anyone wants to give it a go.