Will changing lens focal length by TC corrupt image stabilizer?

[dentheman]

Hello all; I had previously posted concerning use of a Kenko 1.4 pro 300 TC and EF70-300 f/4-5.6 IS USM lens causing the image to jump around in my viewfinder and the lens to click.  I had assumed there was a problem with the electrical communication through the TC; but now I'm not so sure.

What I am trying to find out is how altering a lenses' focal length by adding a TC would affect the operation of in-lens image stabilization, for both between the lens TC or a screw-on front of the lens TC.  I have been spending at least a couple hours a day researching this and haven't found an answer.  A call to THKphoto (kenko distributer) did not help.

Are there any mathmaticians or physicists or photographers with guesses, theories, or facts that can help me out?   Thanks, Dennis.

[KeithB]

I have no problems with my 70-300 mm and Tamron 1.4 X.

How about an engineer? 8^)

Bob might have other ideas, but the 1.4X should reduce the amount of maximum travel required, but increase the "gain" of the system, i.e., a much smaller change in shift is required to get the same amount of stabilization.  To a first order, this should not affect things, but if the feedback loop is underdamped, it may need to oscillations and overshoot.  Whether this is your issue or not, I don't know.  One way to tell would be to put on the extender and set the zoom to around 210 mm.  This should be effectively the same as the lens set to 300 mm without the extender.  Then remove the extender and set the lens to 300 mm and see if there are any differences in behaviour.

[Tallyther]

Given the fact you say you have done quite a bit of research, I would add to it including contacting Canon about acceptable operating impedence limits in the communication circuit.  It is possible by addition of additional circuitry (extender) that they are borderline or excessive and are causing the problem.  An simple analogous indicator of this is when people get error messages and can get rid of them by cleaning the contacts.  Naturally checking the same lens with another extender would be the best check to localize the problem.  The truth is, both the lens and the extender could be borderline and the combination resulting in the problem. But if you try a Canon extender that may have tighter specs and the problem goes away, well at least you have some idea of what you can do even if it's not desirable.

[Bob Atkins]

Image stabilization is self contained within the lens. It should not care if a rear mounted TC is attached or not.

Since it's trying to keep the image stable as it leaves the lens, it should be effective with a rear mounted TC (and I've used a rear mounted Tamron 1.4x on my 70-300IS and found that stabilization is still good).

I'm not 100% sure about the front mounted TC though. The lens calculates how much to shift the optics to get a stable image based on the focal length setting. If you add a front mounted TC I'm not sure if the stabilization algorithm would still be as effective since the focal length of the lens has now changed. With a rear mounted TC the TC takes effect AFTER the stabilized lens and so works on the stabilized image.

IS can sometimes result in the viewfinder image jumping when it's turned on, plus it may click and produce a whirring sound. These are normal.

[dentheman]

Thanks to all for the inputs.  I will probably just have to order another TC to determine if mine was faulty (I sent it back).  Bob, I do hear the whirring of my lens IS, but never clicking or other noise, except when I first activate it I hear it 'center' (I assume).  But  never jumping of the image, unless the TC was installed.  I think the clicking is the sound of the IS assembly hitting the stops HARD.  I am sorry to disagree with with you on this point, but I don't see how this could be normal.  Maybe we are talking about two different things, my clicking was quite loud and happened whenever the image jumped severely (which happened about once or twice a second).  My immediate impulse was to turn off IS so as not to damage it.  And that is my real concern with using a TC with this IS lens.  I will order the same TC for a second try and add my new findings to this thread.       Thanks again...Dennis.

ADDED LATER:  I attached the Canon 1.75x tele converter (from point n' shoot) to the front of the 70-300 lens; the IS works great!  (But the photos are lousy.)