EOS R5 + Canon RF 100-500mm L IS Lens = Canon EOS 5Ds R + Canon EF 100-400mm L IS II Lens
The EOS R5's extended viewfinder rises modestly higher than the 5Ds R's viewfinder, but this increase is advantageous in use.
Let's change the RF vs. EF equation to weights:
In ounces: (26.0 + 48.2) - (32.8 + 56.1) = 74.2 - 88.9 = -14.7 oz
In grams: (738 + 1,365) - (930 + 1,590) = 2,103 - 2,520 = -417g
Overall, the RF lens and R camera combination gives us an additional 100mm of focal length range in a similar-sized package that weighs nearly a pound (over .4 kg) less.
Order the Canon RF 100-500mm F4.5-7.1 L IS USM Lens
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Canon RF 100-500mm F4.5-7.1 L IS USM Lens
Canon EF 100-400mm f/4.5-5.6L IS II USM Lens
Canon EOS R5
Canon EOS 5Ds R