Originally posted by volosong What focal length is "best" to use when photographing a total solar eclipse? What exposure mode? Metering mode?
"It depends".
For a closeup of the sun, you want 300mm or longer. A solar filter is required; the sun's heat plus telephoto magnification can fry your camera. At totality, that ~2 minute period when the moon will fully block the sun's disk, remove the solar filter and reset exposure. Be careful here; when totality ends even a small amount of direct sun can harm your eye and camera. If you aren't near the eclipse centerline you'll never be in totality.
For wide landscape images you can't use a solar filter at all because it makes the foreground black. Don't look through the optical finder to protect your eye, and only use liveview briefly to frame the shot.
Baader solar film is any easy way to make a custom filter that fits your lens.
Making solar filters for total solar eclipses shows one way to build the lens cell from foam board. Note that the article is a bit old and mentions solar film products that might not be available any more. Telescope dealers also have pre-built filters that might fit your camera lens.
Given that you have a K-1 and K-3, I would use the K-3 with your 100-300 or older 400 lens. Get a solar filter for one of those lenses. Put the K-1 on a tripod, wide angle lens, set the intervalometer to take a set of landscapes as the eclipse progresses.
Or, for the contrarian approach,
experience the eclipse and forget about photographing it. Your first eclipse might not be the best time to practice astrophotography.
Here's a great set of eclipse photos combining telephoto and wide angle. Not my photos.
IN THE SHADOW OF THE MOON | Stan Honda