You can cross bad image quality off of your list of potential reasons to not get the new Canon EF 200-400mm f/4L IS USM Ext 1.4x Lens. Because this lens totally rocks in that regard. Image quality results from the 200-400mm f/4L IS Lens are now available on the site. These are among the best results I've ever seen from a zoom lens - or from any lens.
I'll let you decide if the 200-400 L bests the previous zoom lens image quality title holder, the Canon EF 70-200mm f/2.8L IS II USM Lens. Here is the 200mm comparison (use the mouseover or toggle buttons feature). Note that you are going to see some distortion differences in these results as the 70-200 is at its longest focal length and the 200-400 is set to its widest focal length. Also note that the 70-200 has a 1-stop wider aperture in this comparison. I'll let you decide if they would be more-fairly compared at equivalent (f/4) apertures (you can change the comparison tool settings if you vote this way).
There are some things you need to know about the Canon EF 200-400mm f/4L IS USM Ext 1.4x Lens image quality test results. The built-in extender with external extender compatibility complicates complete image quality presentation of this lens in our tool. So, here is what we are showing.
The first tested copy of this lens (we have two right now) is presented as two lens samples – sample "1" and "2". Sample "1" is tested at all focal lengths (including those with extenders) with the built-in extender switch set to 1.0x (not being used) with the only exception being the first of the two 560mm focal length tests – the one that indicates "1.4x Extender Int". Sample "2" results were all captured with the built-in extender in place – the switch was set to 1.4x with no exceptions. Sample "2" results showing one of the "III" extenders in use also had the built-in 1.4x in use. You will notice the ultra-high focal lengths in these results.
Here are some interesting comparisons that can be made:
Compare the image quality of the 200-400mm L lens at various focal lengths with and without the built-in extender in place by comparing lens sample "1" to lens sample "2". The with-extender image quality is really good, but there is a modest image quality penalty for using the extender. This is not unexpected from adding 8/4 elements/groups to the light path. Stop down 1 stop to eliminate most of that penalty.
Compare the image quality of the 200-400mm L lens' built in 1.4x extender to the external Canon 1.4x III Extender. The first difference I notice is that the internal extender appears to add pincushion distortion while the external extender adds a touch of barrel distortion. This difference is what would account for the difference in test chart detail sizes. Otherwise, I don't think you will see a difference in image quality between these two extenders used on this lens.
While extended focal lengths up to 1120mm are supported with Canon's blessings, I doubt that many will want to go to this extreme.