Thank you, guys, for kind comments.
Cumulonimbus was far enough from my place, so I didn't hear any roar of the thunder.
All images were shot with my trusty D750 and AF-S 50/1.8G.
I had no intention of shooting a 20-minute video from which the lightning images were extracted. Fortunately, my camera seems to have been set appropriately for this situation: 1080p/24, and the Movie Quality was set to "HIGH". It was right after the sunset, but the ISO was automatically adjusted from around 1600 to 5000 by the camera, following the darkening situation.
The aperture was set wide open, for whatever reason.
The still images of the lightening was extracted in iMovie on my humble early 2015 MacBook Air. The extracted JPEG images were not retouched in any way.
If I knew I was going to shoot video, I definitely would have used a tripod. Holding the camera as still as possible for 20 minutes was a pain, but it turned out to be worthwhile.
The dynamic range of the sensor in D750 seemed to have helped to some extent, even in video mode.