It's not too often I photograph the very same scene with two different camera / lens combinations, but when we came across this Tiger Heron I had the chance to do exactly that. I first reached for the R5 with the 100-500 attached, since it's the fastest way to photograph a bird. After firing a few shots and seeing that it wasn't going anywhere, I then reached for the 1DX3 with the 500 f/4, to see the difference that a couple extra stops would do. I did minimal post-processing, just enough to match the images as closely as possible.

R5, 100-500 lens, 1/500 sec @ f/7.1, ISO 2500 (shot at 400 mm then cropped to match the other image)

1DX3, 500 f/4 lens, 1/640 sec @ f/4, ISO 800 (this was underexposed a bit, so I boosted it in post)

The improvement in bokeh is dramatically obvious with the f/4 lens, and while the 100-500 f/7.1 lens is no slouch there is clearly no substitute for big glass. Another interesting result is that the 1DX3 did a much better job at choosing the white balance than the R5. I changed the R5 white balance in Lightroom to match the 1DX3 auto setting, because the 1DX3 image came out so much better.

I ran both images through Topaz Denoise to clean them up. But in the raw version, the 1DX3 file has much more detail thanks largely to the lower ISO. I'm sure if I had the 500 f/4 on the R5 body, it would have given an equally impressive result. But I sure love the 1DX3, and it won't be leaving my kit anytime soon.