BIF with R5 and RF 100-500mm compared to A9II and FE 200-600 G

Steve W wrote:

I'm currently using a Sony A9 and FE 200-600mm f/5.6-6.3 OSS G for BIF work. I am thinking of switching over to a R5 and RF 100-500mm f/4.5-7.1L IS. Originally I didn't consider it because of the slower available aperture of the 100-500 and also the shorter reach of only 500mm vs. 600mm.

Today I woke up and realized that with the A9's 24Mpixel (6000 x 4000) pixel sensor when compared to the R5's 45 Mpixel (8192 x 5464) sensor I can crop my images by ~1.36x and still get a 24 Mpixel image in post. This makes the 500mm give me the equivalent of 683mm equivalent if I only took out 6000 x 400 images.. So that removed one major barrier and now I know I can get a decent resolution out to 683mm.

The next barrier now though is the slower aperture at the log end of the lens. My calculations say that the Canon's f/7.1 is only 2/3 of a stop slower than the f/5.6 of the Sony. If that's the case can the R5 deal with the higher ISO needed to make up the difference?

The R5 + RF 100-500 together cost about the same as the the Sony A1 which I am also considering so its not a done deal other than neither the A1 or RF 100-500 are easy to find. There are other plus and minus between the two systems I am also taking in account and trading off. Currently shooting both but realize I am wasting a lot of money but I admit to being a gear head and enjoying the advantages of owning each system.

Please share your thought. Thank you for your time.

I've have used both systems you're considering (have not tried the A1) and IMO the advantages of the R5 with the 100-500 outweigh any disadvantage compared to the A9.  Both systems are excellent and you would get results with either one.  However, the conclusion I came to is that after having used both, the R5 with the 100-500 is a far more versatile setup.

It is lighter, packs smaller, has a significantly better MFD, and honestly, I did not see much difference in the IQ between the two at their maximum apertures.  The A9 is an amazing camera but in comparison to the R5, it's a significantly more purpose focus setup especially with the 200-600.  If all you're using it for is BIF then I'd say its a harder decision, but if you want a camera that can do extremely well for BIF and then so much more, I'd point you at the R5 and the 100-500 without hesitation.

It can do BIF, is easier to carry for hiking/trekking, can do semi-macro, portraiture, etc.

All other accounts I've seen of people who have used both seem to agree for the most part.  Now, if you do BIF, the A9+200-600 is considered better for things that like to fly low to the waterline as the R5 can get confused and bounce back and forth between the waterline, but outside of that, the R5 can handle almost every situation you would likely throw at it and pass with flying colors.