100-400 ii lens won't autofocus with 1.4 iii extender on a 5D mark iii

[janericster]

I recently purchased the new 100-400 ii zoom lens and read that it would autofocus on a 5D MK 3 with the 1.4 iii extender. Mine isn't working. Not sure what the problem is but it just searches back and forth and never locks in. If I remove the extender, it works great. Does anyone have any insight? 

I updated the firmware on the 5D with the latest version and it doesn't help. Also tried all the shooting and focusing modes that I can think of. I heard the single point focus mode was the only one that might work, but not for me.

I'm completely boggled by this (and obviously disappointed).

Thanks.

Eric

[ScottyP]

I checked the review at The Digital Picture and it says flatly that the combo will work. 5d3 + 100-400 II + 1.4 mk.3 TC. Not sure what to suggest. Of those 3 pieces of equipment the only one I own is the TC.

I assume you tried it in an easy test with blinding sun on a contrasty target to be sure it is truly not operating at all rather than that it is just operating badly due to the TC?

[ebiggs1]

I am trying to come up with an answer for you.  I no longer have my 5D Mk III but I do still have the 1.4x and a EF100-400mm zoom.  The combo as I recall did work, however, and it works on my 1D Mk IV.

Put it all together, go outside on a sunny daylighted day.  Find a vertical object, like a fence post, and try again.  The most likely is, you didn't have enough light or contrast in what you were trying before.  Also set it to just use the center focus point.

Remember, it will be slow because you are on the back end of what the 5D can do.  It will miss some shots with the 1.4x on.

You might try a clear all settings just to make sure you didn't set something wrong before you try again.

[jrhoffman75]

Are you trying to focus on something too close to the camera?

[janericster]

Thanks guys.

No, all the subjects were at least 30 feet away.

I think the issue was that the lighting wasn't quite optimal when I did the test shots (even though it was afternoon with full sun, but apparently just not enough contrast that it needs). The lens never locked in focus on anything: tree trunks, fence, brick wall people, etc.) It would focus in and out and then stop completely out of focus.

So I tried it again this morning with full sun and now it is a 50/50 success rate depending on the subject matter. Using aperature priority with single point focus. 

I am probably going to keep the 1.4iii on my 70-200 2.8 (which works great) and leave the 100-400ii alone without any extender. It obviously isn't a fast enough lens to make it worthwhile for me with the extender (wanted to use it for soccer games) and can't take the chance of over 1/2 the shots being out of focus. Disappointing but completely understand the technical reasoning.

Without the extender on, I'm really happy with the lens and its performance. So much cheaper than buying a 300 or 400 prime and gives the flexibility of the zoom for shots when the players are close.