New FCP user

Hi all - have been using FCP now for about 7 months (translation stumbling through). Learning lots as I go. I am trying to get my head around aspect ratios, and fps and everything else. My head is spinning.
When I start a new project I enter the aspect ratio (based on guess). The last project I did (a drone flight) I let FCP set up the project and the view is totally different. It's in a aspect ratio that I have never used before (wish I had of used this way earlier for my other videos). My question is whats the best way to start a new project. I know now to let FCP do it if I am using video from one source, but how would I do it if I am using multiple cameras (my cameras are a P3S drone and iphone 8+). I have tried looking for answers but I guess I am not using google friendly search criteria.

Welcome, Brad, to fcp.co

your question is a bit beyond FCPX… more fundamental. Aspect ratios are depending on the delivery method… in tube-telly-days, the AR was 4:3; nowadays AR is 16:9, at Instagram we see 1:1, and 99% of phone-philmers prefer a 9:16 vertical AR…. In cinematography we had and have all sorts of others, read at en.wikipedia.org/wiki/Aspect_ratio_(image)

Now, in FC… It offers two options:
Set the AR automatically based upon the first clips AR placed in the timeline; or select it manually when creating a new project.

ANY clip placed into such a timeline will get 'conformed', allowing 3 methods fit/fill/none, the Manual explains in-depth:
help.apple.com/finalcutpro/mac/10.4.4/?lang=en#/ver3363b44e

… really no rocket-science: for 'normal' video, use 16:9

Karsten Schlüter wrote: your question is a bit beyond FCPX… more fundamental.

Thank you - apologies if I posted in the wrong section. I am still new to all this and trying to understand it all (its a lot to take in). When I use the auto section it adds the video but I have black bars left and right side. Selecting fill it then completes the frame. Is that the correct thing to do, or leave it as is? Apologies once again if im in the wrong section. Happy to be directed else where.

Select (highlight) clip in project. Set viewer size to 50%. Then click the Transform button lower left side of viewer. Finally in Conform menu, select None. Let us know what happens.

NTKING wrote: ...(a drone flight) I let FCP set up the project and the view is totally different. It's in a aspect ratio that I have never used before (wish I had of used this way earlier for my other videos). My question is whats the best way to start a new project. I know now to let FCP do it if I am using video from one source, but how would I do it if I am using multiple cameras (my cameras are a P3S drone and iphone 8+)...

Ideally you want all cameras to be the same AR or close to the same as possible. The P3S can shoot 16:9 and I think the iPhone 8 does 16:9. If the AR differs between cameras this will require zooming, cropping or accepting black borders.

FCPX will automatically set the project (aka timeline) characteristics based on the 1st clip added. You can inspect that by clicking on the project and viewing the Inspector (CMD+4).

I think the P3S can also do a more rectangular 1.9:1 AR, which might be what happened. In that case your options would be use a 16:9 project and show the top & bottom black bars for the P3S content, or use a 1.9:1 AR and crop slightly the top and bottom of the 16:9 iPhone material.

A similar situation exists for frame rate. Make sure all cameras are using the same frame rate. FCPX will convert frame rates automatically but this is not without cost. E.g, mixed 24 and 30 fps may require 3:2 pulldown which could change the visual "motion cadence" of some clips.