500F4IS & 1.4 TC & Kenko Extension Tube (20 mm)

I recently tried to use the Canon 1.4TC in combination with the 20 mm Kenko extension tube on my 500F4IS/40D rig. Either with 40D/TC/ET/500F4IS or 40D/ET/TC/500F4IS (can't remember which way it was), I observed massive oscillations attempting to autofocus. Upon switching the order of the TC and ET, the AF was fine.

I photographed with a tripod, in good light and a contrasty subject with clean background. So, that shouldn't be a factor.

Anyone has similar experience and/or knows why that is?

Thanks. JR

The extension tube needs to be between the lens and the 1.4TC to get the most magnification, I have also noticed that when trying to focus at the new minimal distance I sometimes get oscillation and will switch to manual focus using live view if possible.

I regularly use the 40D/ET/TC/500F4IS combination and haven't experienced any problems focussing. I don't use 40D/TC/ET/500F4IS because it reduces the maximum distance too much for my purposes, so I can't comment on how well this works.

Joerg,

The two configurations have two different effects.

1) lens - extension tube (ET) - TC - camera
The allows closer focusing without changing the magnification of the TC.

2) lens - TC - ET- camera
Increasing the TC to camera distance increases the magnification of the TC, and also changes focus. Depending on the lens, this may cause the AF system to mis-focus resulting in hunting. Changing the magnification may also change aberrations, in particular giving more spherical aberration.

If you are trying to focus on a close subject, configuration #1 is the one you want to use. Note that adding the ET also means the lens is working beyond it's designed specs and aberrations may increase too.

Roger

Clear info Roger !!! Excellent !!

Would recommend going for maximum magnification (depending on what you are doing) and focusing manually. Also remember you can use more than one tube ... will loose some light !! Brand wise I like the Canon which seem sturdier and made out of metal.

Quote:

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Originally Posted by Alfred Forns

Clear info Roger !!! Excellent !!

Would recommend going for maximum magnification (depending on what you are doing) and focusing manually. Also remember you can use more than one tube ... will loose some light !! Brand wise I like the Canon which seem sturdier and made out of metal.

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Alfred,
I think one needs to experiment with each lens/TC combo because of the potential for increasing aberrations, it isn't clear which configuration will give the best result. How's that for being unclear!;)

I did play around with this one time with my 180 mm macro 1.4 and 2x TCs and extension tubes (all Canon), trying to get magnifications above 1:1. If I remember right, the extension tubes between the camera and TCs didn't help, and contrast seemed lower, probably because of spherical aberration. The best solution I've come up with to get more macro resolution is go to the 65 mm 1:5 macro lens and go to a 7D with its smaller pixels. A 7D with 5x macro gives 0.8 microns per pixel. Adding a 25mm extension tube increased scale to about 0.75 microns/pixel. Adding a TC lowered true resolution (while in theory a 2x TC should have doubled it, ignoring diffraction limits).

Roger

There's a not insignificant magnification with the lens-TC-ET-body configuration on supertele's:

http://www.naturescapes.net/phpBB3/v...6215&p=1199530

Thank you all for the feedback. I hope I'll be able to test in the coming days which combo gives me the AF problems.

JR