So when I use an aspect ratio other than 1:1 my image is stretched. How do I prevent this?
Don’t use anything other than a 1:1 aspect ratio unless your intended display format requires it.
This setting determines the shape of each pixel in the rendering. Computer monitors use square pixels, which means a 1:1 aspect ratio. If you change that, the pixels will be stretched or squashed to fit the hardware’s display ratio.
Some other formats require non-square pixels – I think the general term for this is “anamorphic” – in which case you can set that when needed. But it will still look odd on any display using square pixels, unless the playing software has the means to “un-squash” the pixels ratio prior to display, which depends on the format the movie is in.
BTW, pixel aspect ratio is NOT the same as image aspect ratio – that is determined by the measure of a rendered frame’s horizontal and vertical dimensions.