Canon Teleconverter at >f5.6

Chr1stof · 4 Apr 2015

I'm sure this or similar has been asked before but having searched I cant find anything.

My camera is a Canon 550d crop body, I also have a 400mm f 5.6.
I have read that autofocus is lost above f5.6 (not bothered) and that IQ can suffer when using an extender. Obviously it will affect shutter speed etc, but other than that is there any other technical reasons why an extender wouldn't work or any other issues to consider?

HoppyUK · 5 Apr 2015

Chr1stof said:
I'm sure this or similar has been asked before but having searched I cant find anything.

My camera is a Canon 550d crop body, I also have a 400mm f 5.6.
I have read that autofocus is lost above f5.6 (not bothered) and that IQ can suffer when using an extender. Obviously it will affect shutter speed etc, but other than that is there any other technical reasons why an extender wouldn't work or any other issues to consider?

No. AF will work in live view.

Chr1stof · 5 Apr 2015

HoppyUK said:
No. AF will work in live view.

Even a 2x?

admirable · 5 Apr 2015

Canon call them extenders so do a search for extender, there's heaps of stuff on here about them

Chr1stof · 5 Apr 2015

admirable said:
Canon call them extenders so do a search for extender, there's heaps of stuff on here about them

Did a search with "extender" too and strugglerd to find much to answer what I was asking to be honest. Saw the stuff about people taping over contacts etc but just wondered if there was any further issues to consider.

HoppyUK · 5 Apr 2015

Chr1stof said:
Even a 2x?

Yes. Live view AF uses the contrast-detect method and is independent of lens aperture (unlike normal phase-detect AF).

Edit: taping the pins fools the camera into believing the lens aperture has not gone above f/5.6, when it would normally simply switch out with most Canons, so will attempt to focus. But the aperture is still physically reduced in size (as seen by the AF system) so performance is seriously compromised.