To put it simply — does focal length affect diffraction?

I've done a search to find this answer, but the only thing I keep coming across is, yes, diffraction will be different at 24mm compared to 200mm, but I'm specifically curious about diffraction when using the same lens.

If I'm shooting with a standard 24-70mm or 16-35mm, is there a big difference between both ends? If I'm trying to determine the sweet spot for a zoom, do I need to test both extremes and perhaps the middle, or will testing just the middle be close enough?