Extenders on the R5. The relative resolutions at 500mm/f7.1:700mm/f10:1000mm/f14 are: 1:1.14:1.4. And for the 500mm/f7.1:800mm/f11 = 1.4. There's only a 14% increase in resolution in using the 1.4x.

Extenders on the R7. The relative resolutions at 500mm/f7.1:700mm/f10:1000mm/f14 are: 1:1.33:1.4. And for the 500mm/f7.1:800mm/f11 = 1.4. Here, the 1.4x gives a 33% increase. I don't know why there is a difference from with the R5.

R7 vs R5. At 500/7.1, the R7 gives in practice only 14% more; with the 1.4xTC 33% more, and with the 2xTC the aperture is so narrow etc there is no real extra reach with the R7.

To get the best advantage on using TCs, you need a wide aperture lens, like an f/2.8 or f/4, to lower the increases in diffraction above the diffraction limited apertures. To get the best out of the R7, again wide lenses are essential. But optyczne.pl, which measures the sensor resolutions, gives the R7 only a 14-15% advantage of the R7 at f/4.

In practice, it means to me I don't use the 1.4x on the R5 but I will use the 2x.