How to stop my videos from being bad quality?

I've recently just purchased a Nikon D5200, mainly for the purpose of filming, as I am studying film at A level. I recently submitted a short film, but was disappointed with how bad the quality was of the video. When I had originally put the videos onto my computer and played them back (using widows 8 video) they had seemed okay quality, yet as soon as I try and put them on any kind of editing software, they immediately lose quality. How do I stop this from happening?

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Lens for a manual camera?

Lens for a manual camera?
Added (1). I have a minota srt-102 and would like to try renting a new lens. Would I need a mirrorless lens or would a DSLR lens work? I've never changed the lens before so low price recommendation for a beginer to play with would be great. Thanks!

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