Absolute no ... the AF work perfectly because the AF informations pass through the Macro extension
No differences between with or no EF1,4x and the macro extension
Ok, you know what you know. Note that I
did not mention autofocus, but maybe you like the taste of herring (the red variety, that is). My point was about the distance at which you can focus on a subject with an extension tube in place.
But even though you are apparently very sure of yourself, you might want to do a little research on the effects of an extension tube on infinity/distant focus. Keep in mind that you are discussing this with someone who has a 600/4 II, 1.4 and 2x extenders (EF MkIII and RF), and both the EF 12 II and EF 25 II extension tubes, not to mention a decent knowledge of optics.
You refer to them as ‘macro extension tubes’ for a reason: they increase magnification by allowing a lens to focus closer than it can without the tube. They don’t increase magnification very much on long lenses, the effect is inversely proportional to focal length. But the consequence of reducing the minimum focus distance is that the maximum focus distance is also reduced.
Personally, I use the extension tubes with my 600/4 when I am trying to focus on subjects closer than the native minimum focus distance of the lens (the bare lens can focus at 4.5 m / 15 feet, that drops to 4 m / 13 feet with the EF 12 tube and 3.6 m / 12 feet with the EF 25 tube. When doing so, infinity focus is lost, and subjects further than ~30 m EF 12 and 14 m with the EF 25 cannot be focused on. I have also used the combination of EF and RF extenders with the 600/4 II in a way that does not preclude focusing on distant subjects, namely a
modified 3rd party RF mount adapter, as mentioned by
@tron.
You don't have tt take my word for it, you can consult the Canon documentation (RTFM) for your lens, which lists the minimum and maximum focus distances for the 400/2.8 when used with the EF 12 or EF 25 extension tubes. Below is the relevant section (assuming you have the MkIII version; the max distances with the older versions are a bit longer, but not much).
Using an extender changes the magnification, but not the minimum or maximum focus distances. What that means is that with the EF 12 tube, regardless of whether or not there are zero, one or two extenders in the stack you cannot focus on subjects further away than 13 m / 43 feet. Given that, I question your assertion that the combination with the extension tube is 'perfect for the animal domain in low light'. It may be, as long as your animal subject is closer than 13.6 m / 45 feet. Note that at that distance and the combination of 1.4x and 2x extenders for an 1120mm lens, you are framing an area of 44 x 29 cm, so if your 'animal domain' comprises anything larger than a house cat, you'll be taking perfect pictures of only part of the animals.
But like I said, you know what you know. Good luck with that.
