title: Difference between various AVAssetExportPreset
body: <p>I am compressing video so i have come across following Export Presets</p>
  
  <pre><code>AVAssetExportPresetLowQuality
  AVAssetExportPresetMediumQuality
  AVAssetExportPresetHighestQuality
  
  AVAssetExportPreset640x480
  AVAssetExportPreset960x540
  AVAssetExportPreset1280x720
  AVAssetExportPreset1920x1080
  AVAssetExportPreset3840x2160
  </code></pre>
  
  <p>I have read about all of them in apple documents and found that second list compressed in given fixed dimensions but found nothing about first list , i am confused that why it has 2 lists one with low, medium and high quality and other with fixed sizes and which one is better to use?</p>