Change the project settings to 16 or 32 bit. Drag some footage into the create new comp icon at the bottom of the Project Panel.
Your composition will exactly match the clip. If you check composition settings you'll see:
Or you could just use the HDTV 25 preset. All the standard settings are supported.
You'll probably want to create a custom output module for your blackmagic codec and set that to be the default. Remember to include audio processing if it's important to your workflow.
Thanks for that! What confuses me is the fact that I don't see the same pixel ratio as in the Black Magic file.
Isn't (1) the pixel ratio?! Can you please explain this to me?!
(1) means 1x1 which is square. Square pixels equals a pixel aspect ratio of one. There shouldn't be anything confusing about that. It does not matter what footage you have, if you drag it into the new comp icon you'll get a comp that has the same settings as the footage.
Here's the only caveat. If your footage is non-square pixels Native, you can then go into your composition settings and change it to a square pixel standard sized call using the appropriate frame size. The footage will automatically scale to fit the composition.
For example if you have 1080 x 1440 footage with a non-square pixel aspect ratio, you can change the composition size to 1080 x 1920 and square pixels using the HDTV preset. You just have to make sure that the frame rate doesn't change. Your footage that is non-square pixels will automatically fit the square pixel comp. That is usually the preferred way to work. You want to get whatever you're doing into square pixels at the earliest stages of production.
I hope this helps you figure out how to set up your compositions correctly. Don't make it too difficult. It isn't as long as you stick with the presets.