Function of circuit board in (old) tamron 1.4 tc?

[markus_franke]

Hi,

 

recently I bought a Tamron 1.4 tc for EOS (one of the older models)

that does not focus correctly on a 10D or 50E/ElanIIe, even with a 1.8

lens attached (so this is not an issue of lens speed being above 5.6).

The lens will start focussing when the shutter button is half

depressed, but then stops somewhere (after maybe a quarter second of

focussing) without aquiring focus. Subsequent presses of the shutter

button do not motivate the lens to move. The correct aperture (though

without tc factor) is shown at all times in the display.

 

Having opened the tc, I noticed that the circuit board looks a bit

corroded here and there. My question is: can I simply bypass it? What

is its function, does it slow down af or is there anything else?

 

Thanks in advance,

maf

[robin_sibson1]

Canon Extenders (and the Life Size Converter) report their presence to the lens through the extra contacts. The logic is very simple. The pin nearest to the standard pins is common, and is connected to either one, or both, of the other extra pins. I cannot offhand remember which connections identify which device. It is the processor in the lens that does all the clever stuff to change how aperture and focal length are reported, slow down the AF, and so on. The standard pins in the Extender/Converter are just connected through from front to back (although I think one connection has a mechanical switch in it closed by mounting the lens). The Extender/Converter contains no electronics. Look at the parts catalogue to confirm all this. It sounds as if your off-brand TC is trying to achieve the same effect by intercepting and modifying the signals on the standard pins. Scarcely surprising if this does not work with more recent kit than it was reverse-engineered for.

[steve_dunn2]

<p>Just a guess, but I'd suspect that the Tamron TC lacks the three extra pins. In that case, its electrical wiring ought to be very straightforward: direct connection between the pins on the camera-side mount and the lens-side mount.</p>

 

<p>I don't have a link to the Canon TC wiring diagram handy but if you search back through the EOS forum for links to the parts diagrams for the 100-400 and 70-200 IS (two separate articles, each within the last week), the diagrams are in the same directory on the same site. That would tell you which pin combinations are used on the extra three pins to distinguish between the 1.4x, 2x, and life size converter (as well as confirming my belief that the rest of the pins are simply wired straight through). But those only matter if you're using a supported lens, and since the only supported f/1.8 lens is an expensive monster (the 200/1.8) I'm guessing you don't have a supported lens :-)</p>

[markus_franke]

Yes, the Tamron tc does not have the three extra pins. All pins it has are directly wired through, except for one that takes the deviation over the circuit board.

 

I think I will just try to bypass the board and see what happens. Since, as you say, the canon extenders are directly wired, this could work - although af may be too fast without the extra pins.

 

Thank for the answers,

 

maf

[markus_franke]

Yup, just did it, killed the circuit board and connected this one pin directly. Works okay indoors (in the middle of the night...), will have to test it in normal light conditions tomorrow.

 

BTW: This was not an issue of old 3rd party equipment-new camera but a defective tc.