2x convertor question

chris954

chris954

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pretty obvious really.
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will a 7D still auto focus using a 2x convertor with an f2.8 lens
 
You lose two stops with a 2x converter so F5.6 will be your maximum. From memory only the centre focus point worked on my 7D at F5.6. At any other aperture you will need to manual focus.
 
Grumps1974 said:
You lose two stops with a 2x converter so F5.6 will be your maximum.
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Why is this the case? Won't it will focus as long as it has enough light to do so?

(I'm not arguing against your suggestion, just wondering why).


Steve.
 
Not one to argue , but as long as the lenses max aperture is f5.6 it should work fine, even if you have stopped down to f8. The aperture doesn't close until you lock the exposure.
 
I think we are at cross purposes here ... or I am anyway. ;)

The lens itself must be no slower than f4 to focus with a 1.4 TC or f2.8 with a 2x (therefore max f5.6) ... once it has the converter on it will focus as normal (on certain focus points)

As an example my 7D will focus with a 70-200mm f4 and 1.4x (effective aperture f5.6) but not with a 2x (f8)
It will focus with my 70-200mm f2.8 MKII using both 1.4x (f4) and 2x (f5.6)
 
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Some non Canon ones (eg kenko) will allow AF beyond f5.6. For example I used AF on my 7D with a Tamron 70-300VC @ 300mm with a 2x pro 300dgx TC attached, the EXIF from the photo states 600mm / f16.
 
Most cameras will retain focus providing your lens/TC combo doesn't go past f5.6

f2.8 lenses - you can use a 2x TC = f5.6

f4 lenses - you can use a 1.4X TC = f5.6

Some cameras retain focus at f8 so you can use f4 lenses with a 2x TC

Taping the last 3 pins down on the TC will prevent the aperture being reported and 'fool' the camera. It's a bit hit and miss and different lenses seem to behave differently.

As somebody said - some Kenko TCs seem to work. I still have a 2X one somewhere. It retained AF at f8 on my 1D3 which I would expect anyway howere AF was also retained at f8 on a 40D.
 
I just tried a 2x out on my newly acquired sigma 120-300 f2.8 and it seems to work fine.
 
Grumps1974 said:
You lose two stops with a 2x converter so F5.6 will be your maximum. From memory only the centre focus point worked on my 7D at F5.6. At any other aperture you will need to manual focus.
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I am afraid you memory has not served you well ! A f2.8 lens + 2x tc (e.g. f5.6) will AF on any focus point (lens reliant) or zone with any Canon DSLR - you have full access to all the AF options , it is only when the max aperture exceeds f5.6 that you have restrictions with certain Cameras. You can of course stop the lens down to any aperture and get full AF as the Camera only closes down to your chosen aperture after obtaining AF.
 
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ChrisHeathcote said:
Its a very good combo, I recently swapped my 150 -500 for the 120-300 with TC, so far I've been very impressed.
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What TC are you using? The Canon or Sigma one?

I have been looking at the 150-600 Sigma sport but am wondering if the 120-300 with 2 x would be a better option although more expensive
 
Roy C said:
I am afraid you memory has not served you well ! A f2.8 lens + 2x tc (e.g. f5.6) will AF on any focus point (lens reliant) or zone with any Canon DSLR - you have full access to all the AF options , it is only when the max aperture exceeds f5.6 that you have restrictions with certain Cameras.
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Nope. Some combinations of camera/lens/extender will result in a subset of focus points being available.

If I mount my 70-200 II on my 5D3 with my 2x, only a central subset of focus points are available, the outer ones aren't usable.

EDIT: Actually I'm wrong there (will 'fess up rather than editing and trying to hide :p) - all 61 focus points remain available but only the central 21 cross-type points are available - the outer 20 cross-types function as standard non-cross type points at f/5.6, so if you have your camera set to only use cross-type (as I do), these become unavailable and only the central group can be used.

Basically there is a sliding scale of focus point accuracy and cross-type availability as you move from f/2.8 to f/4 and f/5.6 by adding extenders and then not at all at f/8, unless you're using a 1-series body, 5D3 or 7D2.
 
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mipevo6 said:
What TC are you using? The Canon or Sigma one?

I have been looking at the 150-600 Sigma sport but am wondering if the 120-300 with 2 x would be a better option although more expensive
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I got the new Sigma Converters. So far working very well. If you look on my Flickr in the Thundersport GB album, most were taken using the 2x.
 
Steve Smith said:
Why is this the case? Won't it will focus as long as it has enough light to do so?

(I'm not arguing against your suggestion, just wondering why).


Steve.
Click to expand...

With normal phase-detect AF, the critical factor is the physical diameter of the aperture (it compares one side against the other). It is far less sensitive to brightness and will focus in very dim light if a) the aperture is wide enough, and b) there's some decent contrast for the system to lock on to. Contract-detect AF (eg live view) works at any aperture, though it also needs a contrasty subject and also decent brightness, too, so it doesn't get confused with image noise.

Canon has set the upper limit for phase-detect AF at f/5.6 (a few high-end Canon DSLRs will run to f/8). Canon doesn't make any lenses that are higher than f/5.6, but if you add a telecon that takes them over, the AF system just switches out. You can fox the camera by not reporting the correct aperture (with some TCs, or by taping the pins) but that doesn't change the fact that the physical aperture has changed (as seen by the lens). Some camera/lens combos respond to this, albeit slowly and with inconsistent results, and some will just rack back and forth hopelessly. It's never as good as it is below f/5.6. There's another step at f/2.8, where AF is faster/better/more accurate at f/2.8 and lower.

Nikon doesn't set a hard f/5.6 limit, but if you go over f/5.6 (eg to f/6.3 with some lenses) AF is not as good/fast/reliable.
 
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