The title of this thread is the same as that featured in this YouTube video by Ted Forbes.

Sometimes, lens acquisition or discussion in general is "based on"... - hearsay, even.

One thing I DID NOT know was that MTF charts for lenses are mostly not based on the
actual lens but on the theoretical design. So that that
out of the sky for me

And there's some interesting info on photography in general and a bit of Photoshop.

And MTF? - Well, a variant of Optical Transfer Function (OTF) and quite technical, and even
after you think you've got a hang of it, you soon forget again. Search "mtf charts" for a start.

Now, with less ado than could happen, here's the video...