jerste

definitely very bad copy, I used that lens and it should be nicely sharp.

almost 100% more pixels 1.4x1.4=1.96 so twice more pixels

Should be much better but you must exclude back/front focus. I use different values for bare 500mm and 500mm with 1,4 tc. Another explanation could be mentioned atmospehere conditions.

Could you post example of blurred shot? With A7r iv I find small spot unreliable, better medium

Yes it is normal, nothing wrong with the lens https://www.flickr.com/photos/162376953@N04/

200-600 not bad too 2,5% crop

R4 does not have problems with AFC. It is the tracking mode that causes some inconsistences.

I prefer d500 + 500 pf for flying birds. A7r iv keeps tracking on busy background a bit better but critical sharpeness is better with D500 (I use group AF). Also with erratic birds you have some ...

I have both D500 is generally faster A7r IV more cropping power and better when used with rear lcd (ground level shotting( 200-600 very good zoom,  500 pf lighter There are many small advantages ...

Only tc 1.4, crop does not have any effect on AF and IQ (depending on a camera, crop may have very small effect on improving AF)

I would go with a6600 if you tend to crop you wildlife images. Technically it will not give you 300mm, the focal lenght stays the same but will give you more pixels (A7III in apsc mode is 10mpx vs ...

you will not get any reach in crop mode, only tc gives you reach. Crop mode is like cropping a picture in postprocessing - you can crop it how much you like, even to 1% but this is not a reach, it ...

Why would you want to use crop mode instead of FF?

Lots of softness seems to be diffraction, maybe with 20mpx R6 the lens would be sharper but anyway disappointing lens for me.

100-400 f4-f6.3 zoom always stays 100-400 f4-f6.3 regardless if you use it on FF, apsc or m43, no need to do any recalculations

not sure about your case but images stabilisation elements move in some lenses even when not powered.

Cheap 800mm f11 could be interesting assuming it has very good sharpeness but some questions remain: is it a dust pump? does optical quality suffer because it it extending lens as opposed to ...

I read somewhere it was f11-f16, but now I see it is only f11 so in fact it is cheaper to produce

At f11 they should have skipped apperture, lenses would have been even cheaper.

Not with D4. The problem exists with lighter, high mpx cameras like d500, d7500 and d850. Generally VR problems are more visible on high mpx, light cameras without AA filter. Of course you can ...