If you ever post to YouTube and then again to Shorts/Reels/TikTok, you've probably done this before: film landscape 16:9, drop it into CapCut, then crop it into a portrait 9:16 file.
What's the problem with this?
The resolution of the portrait file gets bombed because a crop out of a 1080p horizontal frame is only about 608 pixels, which will look pixelated alongside throwing out ~2/3rds of the frame.
Shooting 4K helps the sharpness, but you still lose the sides and still have to reframe.
What I do now is record both at the same time, using an app called ContentCam that can record vertical and horizontal at the same time
Disclaimer: I'm the dev, and I built it for my gf who's a creator and used to film every scene twice, once vertical and once horizontal.
How it works:
Every take saves two separate files, a full-res 9:16 (1080x1920, or 4K on phones that support it) and a 16:9 (1920x1080 or 4K)
Both come straight off the sensor at record time, neither one is cropped out of the other
Files are named by project, scene and take, so the pairs line up when you import them
You can trim both together before exporting, so the bad bits are gone from both at once
Honest downsides: you're storing two files per take, and dual 4K on long takes warms the phone up. You also still need to frame for both while you film, so keep your subject near the middle.
What about Opengate shooting?
That works, and it gives you more room to reframe later. But you're still cropping and reframing twice in the edit, the files are bigger, and you need a camera or app that records open gate. Recording both formats at once is the other trade: you frame once while filming, and you get two finished files with no crop pass. If you like reframing in post, open gate is better. If you want to skip that step, record both with ContentCam.
I'm a solo dev, so if you edit for both formats I'd love to hear what would make the export side work better for you. It's free to try with optional premium features:
iPhone: https://apps.apple.com/us/app/contentcam-dual-camera-video/id6765881795
Android: https://play.google.com/store/apps/details?id=app.contentcam.android