EF 400 F2.8 L IS USM + Extender 2x

I am using my 400mm F2.8 L IS USM as my main wildlife lens, very often with an extender 1.4X II and getting excellent results. I tried a 1.4X III and do not notice a difference. I photograph mammals (notably red deer, roe deer, wild boar), birds only very occasionally.

My experience with the extender 2X II on shorter distances is quite OK, at larger distances the image quality in my experience is affected (e.g. softness, contrast). I would be quite happy to obtain a extender 2X III if this would address the IQ issues I have noted.

I would appreciate any applicable comments, has anyone made a direct comparison of extender 2X II vs III on the 400mm F2.8?
A different thought/question: Is there a software solution to the softness/contrast issue? In the past I tried Piccure+ (trial version), which had some promise. Unfortunately this appears no longer available. Any suggestions?

Thanks in advance

Guus

The distance issue probably has more to do with atmospheric conditions than any issues with the converter. I'd you are shooting from a distance where you are at 560mm and framing large mammals you are seeing the effects of haze, heat distortion, and dust between you and the subject. It has a slight but noticeable effect on sharpness and contrast.

I had a link to The Digital Picture about the 1.4 and I recall it said version III had less CA. I found this to be accurate as I had both versions. Will anyone notice it? Unless you really pixel peep you may not see much difference. The question about performance is discussed in these links.

https://www.pointsinfo​cus.com …non-extender-ef-1-4x-iii/

https://dancarrphotogr​aphy.com …mkii-and-mkiii-extenders/

https://www.the-digital-picture.com …der-EF-2x-III-Review.aspx

https://www.fredmirand​a.com/forum/topic/1460​336/