Canon 300mm F2.8L IS & Teleconverters

Bernie174

Bernie174

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Bernie
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Ladies & gents

Can anyone shed any light on this question please.

I have a Canon 300mm F2.8 and canon 1.4 & 2x Extenders. I use them on a 7d.
OK, great so far, and yes, I know the theory, once the max aperture goes above 5.6, autofocus bites the dust. BUT, I tried ganging the 2 extenders today, and the autofocus works still.....Oh er!
The shutter speed given when using AV was a bit slow for fast jets, but apart from that it seemed to work...
So anyone else used this combo and got any good results?
 
I could be wrong but I think I recall reading that the camera only "reads" or "detects" one of the extenders when you stack them, meaning the max aperture would still read as f/4 or f/5.6, depending on which way round they were, when in fact it's actually f/8. This would obviously affect the exposure by a stop or two aswell.

But, as I said, I could be talking total rubbish :D
 
Vertigo1 said:
I could be wrong but I think I recall reading that the camera only "reads" or "detects" one of the extenders when you stack them, meaning the max aperture would still read as f/4 or f/5.6, depending on which way round they were, when in fact it's actually f/8. This would obviously affect the exposure by a stop or two aswell.

But, as I said, I could be talking total rubbish :D
Click to expand...

I ahve stacked 2 on my 400 f2.8 and your correct.. still reads f5.6 (they only stack one way) in fact all the readings are out..but cameras still works wiht them
 
Vertigo1
Yes, it does only show 5.6 as the apature, and in Exif data shows the lens as 600mm, not 800 and whatever.

KIPAX

Link and quote as below.

http://www.the-digital-picture.com/Reviews/Canon-Extender-EF-1.4x-II-Tele-Converter-Review.aspx


Though most compatible lenses have widest apertures of at least f/4, some of the longer ones have an f/5.6 native maximum aperture. Since most non-1-Series Canon camera bodies need a minimum wide open aperture setting of f/5.6 for autofocus to work, only Canon 1-Series camera bodies will autofocus these f/8 1.4x combinations. The lens and extender combo can always be used in manual focus mode. A narrow maximum wide aperture creates a dark viewfinder that makes manual focusing even harder.
Click to expand...
 
The quote needs updating, as it now needs to read only Canon 1-Series camera bodies prior to the 1D X because even the newest top of the line body will not autofocus with lenses with minimum wide open aperture setting more than f/5.6
 
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The extender reports its presence to the lens, not the body. The front end of the T/C has three extra contacts (a common and one each for 1.4x or 2x). The lens pass the new focal length and max aperture to the body.

When stacking T/C's, the T/C clsoest to the body isn't seen as second T/C (nearest the lens) doesn't have the extra three contacts on the rear to read the presence of the first T/C (the one next to the body)

Hard to describe but look at the T/C and you'll understand better.

Bob
 
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