Re: Struggling between RF USM and STM lineup as an amateur
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I was in a very similar situation as you, my baby boy now is 7 month old. Here are my experiences with the R5, an old EF 50mm 1.4, the 135mm f2 and the RF 24-105 f4 L.
R5: Love it, however the R6II is plenty enough for your use case. If money is no big object for you, I wouldn't take the compromises of an R8 (handling, battery, IBIS), the size/weight difference is neglectable for your use case.
EF 50mm: in the first months, 50mm was quite perfect. In the meantime, I'm looking already for something wider, because the baby is mobile now and obviously larger. So much larger that for 50mm I need to be further away than what I want. Therefore, from my experience, I wish I had the RF 35mm right from the beginning. The 0.5x macro is neat for taking those close detail shots of hands and feet.
EF 135mm: believe it or not, I've taken some of my favorite shots so far with this lens. Indoors! I sometimes sit at the other side of the room while my wife plays with the baby. Or the baby chills on his mum. Or plays by itself. Or sleeps. With the 135mm I can take pictures unnoticed, with nice emphasis on the subject but not too close. We live in a flat, the living room is decently sized. Bonus for this lens: it's perfect for distant shots on the playgrounds, when the baby grows older. Tried it alot with my nephew already, love the results. Can't wait until my baby is old enough for it.
RF 24-105 f4: the most surprising for me. I never took videos. Completely changed with the baby. The RF 24-105 is the perfect video lens. Completely silent AF, incredible stabilization, instant focus speed. Often, it's the more fleeting moments I capture on video. For this, the zoom range is very valuable. F4 is enough since ISO is even less of an issue on video and the shutter speed is slower than with stills.
Hope this helps a little.
One further thing: Get a "camera lens doll". A little fabric animal that you can put on your lens. Some have little squeekers in it. The baby loves it and always smiles into the camera 😉