Auto Focus With 2x Teleconverter

[rick75]

Ok so I wanted to do a little bird type photography, the options for me were a slow 200- 400mm lens or a quick 70-200mm lens with a teleconverter, I go with the option of the 70-200mm Sigma f2.8 lens as I figure it will be more useful as an all round lens, the lens is great on its own it auto focuses on moving targets very well and takes very nice photos IMHO, I buy a kenko 2x teleconverter and now the lens won't auto focus properly it simply won't hold focus it will go into focus then drop out, it just can't hold a target.

So where have i gone wrong are 2x teleconverters just too strong even for a f2.8 lens or are kenko crap, I am using a d3000 could this be an issue as well?

[mkooper]

I had exactly the same setup as yourself. Sigma 70-200 f2.8, Kenko x2 all on a Canon 40D. Same issue, as teleconverter seemed to disable the autofocus. Bummer.

I have since bought a Sigma x2 teleconverter. On the same lens I can now autofocus BUT only if the focus is already near optimum. So it will not focus from 10m to infinity or vice versa. But it does help to focus the last little distance to get optimum focus for the shot (eg: from 10m to 12m).

[kiwi]

http://blog.nikonians.org/archives/N...er%20Table.pdf

[rick75]

Kiwi that blog page basicly says, we have no idea what will and won't work if you read the bottom of the last page, I thought the f2.8 going out to f5.6 would be ok, after posting yesterday I have read around a little and it may be the issue is with the Nikon D3000 and it poor performance in low light.

I am really confused, the lens focuses in really quickly and well, but it then wont lock, it still trys to search. Does anybody have advice on what is going to be my best way to go.

[nomis]

I have a 2x kenko pro300 I think, that I only use for macro.

Just tried it on some of my other longer lenses in AF to see what it was like and any advice.
Forget about Live View...World of pain on its own before adding a converter.

First off is the sigma a fixed or variable 2.8.?
It will make a difference - sorry not tech wise on them

To check your lens on a d3000 for sure and not to believe the brochure or specs, put it in full manual focus and shutter priority. Zoom through the ranges and focus spread and watch to see if the f. changes in the display. If it changes it's variable and this will cause grief.

On my variable zooms it just hunts (sometimes stopping, sometimes not) and this would make sense with what it needs for AF to work.

On a fixed 1.8 or 2.8 prime / zoom it seems ok but what is soft at 1.8 or 2.8 is now stunningly soft which also makes sense with how a converter works.

But on a variable 90 2.8 prime the f. constantly changes with focus and this also causes a problem. It's just how they work and it gets too dark when the lens is searching for focus because the f. is always changing. The AF just freaks out.

Sorry if nothing helps or works for you.
Manual focus may be your only friend.
I use mine in full manual mode and focus and the image quality is superb for macro.
Sorry cant help more

[rick75]

The lens is a 2.8f through the range, I also tried it on my Tamron 60mm f2 and it does the same thing, I think it may just be the D3000, but I was hoping someone with a working knowledge might be able to shed some light.
Thanks for the help thus far
Rick

[Riverlander]

Have you got the Kenko Pro 300? If so I would have thought it would be OK. Should give you f/5.6 and the camera should handle that.
Make sure you put the teleconverter on the lens and then mount the combination onto the camera. Do not put the teleconverter on the camera and then mount the lens.

[rick75]

Its a Kenko 2x teleplus pro 300 DG, I read about the compatability first and thought it would be Ok, but they do say that although it should be Ok they can't guarantee it.

[kevinj]

The Kenko works with AF on fast lenses,but they get confused,my 50mm is too sensitive for the TC so have to use MF,the benefits of double range,my 100-400mm becomes 200-800mm,is worth the loss of AF,the loss of IQ is small,try different settings it`s a new learning curve,stick with it,it`s worth it.

[kiwi]

If be interested to see good shots with the kenko 2x, only examples I've ever seen are poo

[rick75]

Kiwi I had a af-s 2x teleconverter but it would not fit the sigma lens, I read an article today on how to modify the TC so it takes the lens, I am now wishing I did'nt sell it, does anybody any experience with other teleconverters or maybe if I went back to a 1.7x would this type of problem be less likely.

Thanks again for the info thus far.

[rick75]

Ok so and update, I tried this teleconverter on a D90 body as well and still no joy with the AF, what is the likelyhood of a faulty teleconverter. Anybody have any ideas on how to test?

Cheers Rick