Thanks everyone. My problem is the weight of the larger lens when I go on a plane. Long story short. I have a Sony A73 FF that I love. Also a Tamron 150-500, that I also love, but the Tamron 150-500 (light as it is compared to the Sony's was still a lump to carry on a plane). So I bought the sony 70-350 lens and a Sony A6400 camera. I then put the same settings on both cameras. Then using the 70-350 (yes it's not FF) on both cameras. I took several photos of the same object with the A73 in FF mode (yes the edge was gone), but most of the rest was there. I took several photos in APS-C mode, and several photos with the A6400. As I say all same settings, and the lens set at 350mm. I then picked the best photo from all three ways of shooting. Then with a crop only on the object, I found the best picture was the A73 in FF, then the A73 in APS-C mode and lastly the A6400. So reading a lot on line if I get a bigger MP camera I can take photos of birds and sunsets (for instance) with the Sony 70-350mm as a telephoto lens, and with a crop get almost the same or similar quality as I get with the Tamron 150-500. But it will be a lot lighter. In the UK I will use the Tamron 150-500. But I am worried about to many MPs taking up HD space, hence I am hoping the APS-C setting on the Sony A7R IV will have lower size photo's, whilest using it in telephoto. Complicated I know, not perfect, but it will be so much lighter to take abroad and still give me a degree of telephoto.