Why use -vframes and -ss to get thumbnail image from video?

I am using this argument to FFmpeg to get a thumbnail image from a video file:

ffmpeg -i input.mp4 -vframes 1 -s 320x240 -ss 10 output.jpg

I understand that -ss 10 tells FFmpeg to capture the thumbnail image at the 10th second, but what is the purpose of -vframes 1?

1 Answer

That will tell ffmpeg to output one frame. Otherwise it will attempt to use the image file muxer to output multiple images. From the help:

$ ffmpeg -h
...
-vframes number set the number of video frames to record