Sorry, not sure if this should be or travel, but I know some of y'all backpack. I love hiking the national parks and doing landscape photography. Looking for a pack that would accomodate mostly one day trips with the occasional 2-3 day trip carrying normal backpacking gear (not heavy cold weather gear) plus some camera equipment (body, 2 lenses, tripod) that I have a separate bag insert for. This bag will also need to fit as a carryon for plane travel. I was thinking a 36 liter, but that seems a bit small for the occasional longer trips so now I'm looking at a 50l. Too big for the typical day stuff I'll do but too small for the longer ones.

Should I buy the 50 and underpack it and cinch it down to keep everything secure or am I setting myself up for disappointment? For reference the packs I've looked at are Osprey Kestrel 48, Osprey Stratos 50 and 36. I like these packs' features and they fit me well. Seems like the Stratos 50 could be cinched down small enough for carryon and have plenty of room for everything I'd ever need.

I recently bought a Sierra Designs Flex Capacitor after looking at Osprey and Gregory packs. Andrew Skurka (google him, he's a pretty interesting guy) helped design it. It is expandable from 40L-60L, only weighs about 2.5 pounds, and has enough frame to support heavy loads (if you really want to carry them). I absolutely love it. Costs about $200. Sounds like something that might work well for what you want to do.

The 50 L is the right size range.
You have named a couple good brands. My recomendation would be to go to REI and try them on with weight in them and see how your camera equipment fits.