[Ilya Kutuzov]

For all i know:
1)
unless you crop the source image to cut out 16:9 from 4:3 or vice versa, changing aspect ratio should not affect the resolution.
2)
what exactly is the video quality? it comes from two things: the camera matrix resolution and the way your camera stores the footage. after you have your footage captured to a PC, in any aspect ratio, see (1).
3)
Proj Props affect the way you see the preview. However, when you want to deinterlace properly, or adjust between different ratios from different sources, or get a flawless audio preview, PP is where you set the parameters. you do it before editing – you edit properly adjusted material. you can change any param in PPwhenever you want. play with it, take a 4:3 footage and make the PP 16:9, see what happens. use Crop image to get a 16:9 picture out of a 4:3, and see what happens to the resolution – by and large, any software zoom will result in a worse picture unless the source is HD and the project is in DV resolution.
Good example for that – the interlace thing.
Remember, when you want to preview and consequently render an interlaced source as progressive, select both “progresive” AND a deinterlace method (blend fields suits 90% of your needs, sometimes a specific kind of movement is better done by interpolate).
Also, I recommend to match the interlace params in PP and Rendering params to get pure deinterlace if you want a progressive result. If you render your footage to be viewed on a TV, you should leave the interlace both in PP and when rendering.
4)
Connected to (3). right click at a source clip in Meida bins or on the timeline and read the media params. compare them to your Proj Props. you’ll see what you can keep.
But the thing is why you’d want to keep or change anything. Example, slow motion. I had to produce a 20% speed slomo for PAL (25 fps) 16:9 progressive output from NTSC 4:3 source (29 fps). To get a sensible slomo – as good as the source may allow – I had to switch on the Disable Resampling radio button in source clip properties. This thing does not appear in Proj Props and whatever i did in PP never changed the slomo quality. So mind those little things, it’s easy to figure out what does what.