I created a disc image from a DVD using Disk Utility, and the Apple help guide suggested I select—in the drop-down menu of the image creation window—"compressed" before clicking "okay". As the disc image was being created, I found another tutorial, unrelated to Apple's site, that said to select CD/DVD Master.
My resulting image was saved as a .dmg file, and the tutorial's was a .cdr file, though a quick search on Apple Communities told me there's no need to worry about the difference between those two.
So my question is: was choosing "compressed" a bad idea? The DVD itself is just a home movie made using, probably, iMovie and iDVD, and I needed to make one copy.