Sharpness decreases with image straightening?

I read on another thread that, if you adjust a picture to (for instance) straighten a horizon, some of the sharpness goes. The same also applies, apparently, if you use the perspective tool. Would anyone like to explain this, or comment on it?

It is because in either case resampling has to be done. When you straighten an image two corners get cut off. In order to maintain image size it has to be resampled. Perspective changes either compress or stretch the image in certain areas. A lot of resampling is done. And like every interpolation where the computer has to invent new data based on a weighted average of the neighborhood, local contrast is reduced and details get blurred.

How important, or noticeable, do you think this is in real terms? For instance, if you were doing an A3 print from, say, a 40D, would there be much difference?

At A3 I think the difference would be visible in a side-by-side comparison. But since the accepted practice is to always do a final sharpen at the very end of the workflow, the question becomes moot.

Very true. Thanks for your input.

I straighten wedding photos without any concerns for this. I've never noticed it, the prints look good, which is what matters.

tzalman wrote in post #5584030
In order to maintain image size it has to be resampled. Perspective changes either compress or stretch the image in certain areas. A lot of resampling is done.

None of my image editors make any attempt to match the original size of the image (though I think that the option exists in one of them). They simply rotate it and leave white areas around the angled image, for which two of them automatically provide a suggested crop and the other leaves you to do it yourself. I have never seen any resizing or perspective change. I would rather end up with a smaller image, as I imagine that it involves less resampling.

Same here.... no resizing or resampling takes place.