Canon 200-400 f/4 reviewed by "Chasseur d'Images"

Started Jun 20, 2013 | Discussions thread
Jerry Fusselman
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Re: Canon 200-400 f/4 reviewed by "Chasseur d'Images"
In reply to Steve Balcombe, Jun 20, 2013

Steve Balcombe wrote:

Oldvic wrote:

Thanks for quote. Just want to be clear that in above sentence - external 1.4x TC is better or worse than internal 1.4x TC? It doesn’t make sense if external TC actually is better.

That's what I can conclude from the graphs: at f/32, the external converter gives better results than the internal one; at other apertures, there's no difference.

At f/32 diffraction will totally dominate. Any difference which is not seen at larger apertures is just a blip - a statistical fluke or a bad measurement.

Just as a general comment, it's hard to imagine that the internal TC would be any better than the 1.4x MkIII which is stunningly good on the MkII big whites. The point of the internal TC isn't better IQ, it's instant (and weatherproof) availability.

The results at f/32 make sense to me. As I explained two weeks ago in the "Strange thing about the built-in 1.4x teleconverter" thread, the internal 1.4x considerably reduces focal length when focusing close. It probably reduced focal length (compared to shooting at infinity) at whatever distance "C d'I" was using in their tests. The lens's chip must know about this, so to keep the aperture constant at relatively closer focusing distances, it has to close down the aperture more. If Canon did not do this, they should have (though perhaps not when shooting wide open). Smaller physical aperture size, more diffraction. No surprise there.

It would be easy to verify my hypothesis by stopping the lens down somewhat, engaging the internal 1.4x converter, and watching the size of the working aperture (from in front of the lens) shrink a bit while focusing closer. My hypothesis is that the degree of shrinkage when focusing closer is greater with the internal 1.4x than with an external 1.4x. If I am right, then the results at f/32 from C d'I are merely a consequence of their mistaking marked aperture numbers for actual aperture numbers. That is, with the external 1.4x, there was less diffraction at f/32 because the size of the aperture was larger. Another way to check is exposure levels in the images both ways.

I'm surprised you doubt the potential improvement of an internal 1.4x. Think of the huge number of extra options the lens designers had about where to locate the extra elements. Besides, we have this result quoted by the OP when using the internal 1.4x: " sharpness: same as above." I've never seen a result that wonderful with an external teleconverter. Indeed, as quoted by the OP, the results with the external 1.4x were not as sharp.

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Jerry Fusselman

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