What I think. (Arguably, mostly rubbish, but...)
It's good you asked here.
What you have quoted in your first response to the Q - sorry and apologies beforehand - is wrong.
It has NOthing, zero, zot, and the NULL set to do with ANY screen magnification. It is simply...
Do NOT resample your image.
INSTEAD, open up your full size image;
then take a selection from it;
then make a a new file. This new file will be the exact size of the selection you made - say 400 x 300 pixels...;
now paste that selection (sitting in Clipboard typically) into the new document;
Now that's the 100% crop.
After getting it, you can flatten the image and save as jpeg/tiff/...etc.
Recap: It's NOT what screen magnification you use; it's JUST copying out a section FROM a full size file WITHOUT re-sampling.
OK, now that that's over, what is re-sampling?
Re-sampling means CHANGING the (BASIC/NATIVE) image size. Say your camera produces 6000 x 4000 pixel files (Wow! A 24 MPx camera!!)
Now, if you use some program - like Photoshop - to change the image to, say, 1500 x 1000 pixels, then that is re-sampling. This does not mean
you are changing the display size - you can do that by scrolling the mouse wheel. It is instead making the image as if it came out of a 1.5 Mpx
camera instead of a 24 MPx camera.
Martycon. Thanks for raising the Q again. It is one that's often confusing/confused/bewildering... etc.
(Cripes! I hope I haven't muddled it!!
)