So Bill it was exactly as i said in my earlier post in that you are using the wrong lens for your requirements. You talk about the distance to the subject and the crop ratios but the same can ve said of any lens. Sure the 600 has a long MFD but then i wouldnt normally shoot macro or portraits with long lenses. The 600 when used correctly for its intended purpose is stunning however the bigger the crop the more detail and data you lose- same for any image from any lens/camera combo. You have picked on the 600 lenses weakness which is the MFD and honed in on it - use the correct lens for the situation you find yoursel in be it for small birds a metre away or birds of prey 50m away.
Maybe my posts are confusing
I think that we are saying the same thing
in part - the 600mm f4 is a great lens not a weak lens but it has to be used at the correct distance to get the best results ..... and I have tested it at various distances from 6 to 15 metres .. along with the 300's at MFD upwards
what I am also saying is that if you use the 300mm f4 or f2.8 with a 5" inch bird, ( for example), at its MFD which is less than that of the 600mm - you will still have to crop to 20% to get the same detail ..... the ratios are the same
That's why I said that when I am shooting birds with the 600mm, I take two cameras with me D810/600mm f4 and the D750/300mm f2.8/f4 with the 3 x TC's and then depending on how near I am to the subject I can use the correct set up ...... or depending where I am
I also said don't buy the 600mm f4 if you expect it to take great shots when, for example, you are miles away from the subject, and that the 300mm lens with the 3 TC's is more flexible than the 600mm f4 on it's own ........ because you have a choice of MFD's with the TC's and can use which is the best depending on how near you are.
I am not using the wrong lens for my requirements as I chose which set up to use depending on my requirements
read my post again, I said
" if I am out shooting with the 600mm f4 I always have another camera with the 300mm f4 PF VR and the range of TC's , with me" and "What i am saying is that the 600mm f4 is quite a specific, specialised lens"
what I am also suggesting is that the 300mm f2.8 or 300mm f4 with the TC's is a more flexible arrangement that just the 600mm f4
alone
I would not buy a 600mm f4 unless I had a 300mm with the TC's that are appropriate to the version - the 1,4 and 1,7 work well with the 300mm f4 PF VR and all three work well with the 300mm f2.8VR, the 1,4 works well with the AFS 300mm f4
what I have also eluded to is that it you want to reduce the MFD on the 600mm then maybe a set of extenders (tubes) should be tried so that you can use the 600mm f4 when the birds are nearer than the "native" MFD of the 600mm, i.e. 5,6 metres and in doing so this may make the 600mm f4 a more flexible lens ............. this was suggested to me and at first I thought it was not a good idea - but maybe I am wrong and I have not tried it yet