video - How to capture first X frames every X seconds into a PNG with FFmpeg?

I am new to using the command line. It is pretty cool and exciting to learn, but I am stuck.

I have a video where, for example, I want to capture the first 8 frames every 5 seconds into an image.

I've written this command:

ffmpeg -i video.mp4 -vf fps=8/20 out%04d.png

I see it captures 8 frames every 20 seconds, and not the first 8 frames. Is there a way I could specify this in the command?

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