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

Started Jun 20, 2013 | Discussions thread
Steve Balcombe Forum Pro • Posts: 12,777
Re: Canon 200-400 f/4 reviewed by "Chasseur d'Images"

Jerry Fusselman wrote:

Steve Balcombe wrote:

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.

That would make sense if it wasn't for the fact that no difference was observed at other f-stops - only at f/32.

I'm surprised you doubt the potential improvement of an internal 1.4x.

Perhaps you haven't seen how good the MkIII Extenders are.

It's a widely-held assumption that the 200-400's built-in extender must be better because it's specifically made for the lens. But the 1.4x MkIII is also optimised for a very small range of similar lenses, and in practice we know it to be phenomenally good on those lenses. Where is the scope for improvement?

Think of the huge number of extra options the lens designers had about where to locate the extra elements.

And they did take advantage of this - the block diagram shows that they are not the rearmost elements. But I'll wager that was for practical reasons - the design allows the elements to be smaller so they can be switched in and out without the bulge becoming enormous.

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.

That is not what the OP says. For the external converter "They ... found no significant difference for most apertures, f/32 being the exception".

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