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Teleconverter Problems

WilliamRoar 13 188 United Kingdom
30 May 2010 3:00PM
I was given a 2 Teleconverter, all it says on it is "C-AF 2 TELEPLUS MC7. It is for Canon, used by canons, but I am having a few problems with it.

1. Auto focus - All of my lenses so far are zoom lenses, a 28-90mm, a 10-20mm and a 75-300mm IS. With my teleconverter is attached, the focusing hunts , but only when I am past halfway focal length.

When it is hunting, I can see a focused picture, before it focuses away & gives up. This makes me resort to Manual focus.

2. Stabilizer Jerks - On my 75-300mm, I have a stabilizer. When I am at full focal length with my teleconverter, having to manual focus, the image jumps round every few seconds, and makes a strange sound. This only seems to happen with the teleconverter.

I have also got a slight problem with quality, but Photoshop can fix it.


Canon EOS 20D & 300D
Canon EF 28-90mm F4-5.6 III
Sigma 10-20mm F4-5.6 DC HSM EX
Canon EF 75-300mm F4-5.6 IS


Any explanation or solution greatly appreciated.

brian1208 18 11.8k 12 United Kingdom
30 May 2010 3:18PM
the first problem is that the 2x converter is taking the wide open aperture to greater than f 5.6 which means that you will not get auto focus with either of your bodies (the limit is f5.6 with most canon bodies (except the 1 series when it is f8) so, its manual focus only with this combo.

The next problem is that the 2X TC will degrade image quality even when you have got focus (its down to the optics of the combination) - the 1.4 TC is a better bet but even then it won't work very well with your lenses.

The stabiliser jerk at full extension may be the fact that you are trying to stabilise a"600mm" lens, I don't think the IS system of the 75-300 is really up to that.
cameracat 17 8.6k 61 Norfolk Island
30 May 2010 4:35PM
Unfortunately one answer would be to sell all your lens, Then replace them with f/2.8 versions......Sad

That would be horribly expensive, Or as Brian has suggested look out for a 1.4 TC.....!!!

If you really need more focal range than 300mm, You may be wiser looking at something like a Sigma 120-400mm Or similar.

WilliamRoar 13 188 United Kingdom
30 May 2010 6:18PM

So if I had a prime lens it would autofocus?

And what about one with a fixed aperture?
StrayCat 17 19.1k 3 Canada
30 May 2010 8:36PM

Quote:So if I had a prime lens it would autofocus?

Not necessarilly Will, it depends on the maximum f-stop of the lens you are useing. As Brian mentioned above f5.6 is the limit to get auto-focus. In order to get good image quality, a good quality TC is required, on a fast lens, and if it's a zoom, preferably a constant f-stop such as the Vanon 70-200mm f2.8, which is used with a TC by many togs. I don't think a 2X TC is going to give the best quality unless it's on a high quality lens.
WilliamRoar 13 188 United Kingdom
30 May 2010 8:56PM
So if the aperture was between F1.4 & F4 it would autofocus, but not if it was over?
Graywolf 14 1.0k United Kingdom
30 May 2010 9:23PM
I think more accurately your minimum aperture would need to be 2.8 or larger to maintain autofocus on anything but a 1 series Canon using a 2x converter, and f4 using a 1.4x converter.

1 series Canons will autofocus at f8 but I think only with the central sensor.
kenp 15 96 United Kingdom
30 May 2010 10:27PM
In my experience a 2x converter will drop you 2 stops! 2.8 will become 5.6! will then be back in the Canon trouble lands of not auto focussing!!...I'm a pentax man using a DA*300MM with a Pentax 1.7 converter,it works a treat!
cheddar-caveman 18 1.2k England
31 May 2010 7:57AM

Quote:In my experience a 2x converter will drop you 2 stops! 2.8 will become 5.6! will then be back in the Canon trouble lands of not auto focussing!!...I'm a pentax man using a DA*300MM with a Pentax 1.7 converter,it works a treat!

You are right that the 2X "extender" as Canon calls it, will drop 2 f stops on any lens it's attached to, but wrong that they won't auto focus at f5.6. Canon lenses will focus at f5.6 which is why I bought their 300mm f2.8 so that I can use it with my 2X extender, and it's brilliant! I also have the Canon 300mm f4 and only get auto focus when used with the 1.4X extender (droping it one f stop to f5.6), also a great combination.

Using extenders/converters with zoom lenses is always a problem as the f stop on a zoom lens invariably changes as you zoom in and out. Therefore at one zoom setting, the wide angle end of the range, the auto focus may work but as you then zoon in a bit, the effective f stop goes past that 5.6 mark and it won't auto focus. Therefore if you want a 2X converter to work over the entire range of a zoom lense, then the smallest f number of that lens must not exceed f2.8 (a very big zoom lense)!
tomcat 16 6.4k 15 United Kingdom
31 May 2010 8:27AM
I personally, would be very cautious about attaching any converter to a zoom lens, unless you have fully embraced any pitfalls.

cambirder 17 7.2k England
31 May 2010 10:27AM
It is pointless using a converter on any of your shorter lenses as it will only give you a focal length that you can already achieve with one of the other lenses. It will give you more reach with the 70-300 lens using MF, but to be honest the drop in image quality and the difficulty in getting anything in focus is not worth bothering with, and I would not even attempt using a 1.4 on any 70-300 zoom lens.
RichBrew 11 151 England
31 May 2010 11:09AM

Quote:It is pointless using a converter on any of your shorter lenses...

Not only pointless, but inadvisable. Converters are designed to be used on telephoto lenses, not wide angles such as your 10-20mm lens. I agree with Cambirder (above) about using a 1.4 on a zoom. You will get the best optical quality mounting a converter on a prime super-telephoto.
Get saving!

StrayCat 17 19.1k 3 Canada
31 May 2010 8:07PM
Example: Scroll down through the photos and read the exposure info here. Wink
cheddar-caveman 18 1.2k England
6 Jun 2010 7:49AM

One of many pix I took of a Peregrine family using Canon 300mm f2.8 with 2X extender fitted (sorry about the watermark. Original pics available from email mentioned on "Home" page of my site).

As has been said, no point, and not advisable to fit extenders/multipliers to zoom lenses. Save up and get a good prime if you need to "get up close" to your subjects.
dlegros 19 217 England
6 Jun 2010 9:21AM
I'm curious as to the problems with TCs on zoom lenses people keep alluding to on this thread.

The following shots were all taken with a Zoom Nikkor 70-300 f4-5.6 D on a cheap Jessops 2x TC, all attached to a Nikon D80. Zoomed to 300mm so 600mm f11 equivalent - AF worked fine.

Prairie Marmot




These are the last shots from this glass though, as it was traded in towards my new Sigma 70-200 f2.8 (also tracked down a used 2x APO TC Smile )

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