I was experimenting with my settings and doing lots and lots of renders as I often do and I noticed some interesting results. I done three renders of the same female and adjusted the Pixel Filter Radius on each one...
The first one is set to 0.010, an extremely low setting. You'll noticed it results in very sharp hair, where you have fine lines, in this case, too sharp (in my opinion) as it leads to a lot of aliasing (jagged edges)...
This next one I use an even 1.0 setting... which still has fine lines, but less aliasing, it's not too bad and one I have used a lot...
And finally, this next one is the default 1.50 setting which has the least amount of aliasing in fine lines...
And all these observations are as expected, but there is something about these I didn't expect! Look at the texture of her lips! You would expect the lips texture to be less and less sharp as the pixel radius went up, but they're not. The lips texture appears more and more detailed with the default pixel radius and really smooth and less detailed on the top image which had the smallest pixel filter radius.
I may just leave the Pixel Filter Radius at the default from now on as I dislike aliased lines and the aparant higher texture detail is nice! I wonder if shrinking down the Pixel Filter Radius, if that doesn't cause it to leave out details of the textures? <shrug>
I rendered these at a fixed 400 iterations, only denoising on the last iteration. I also tested this at 800 iterations and the texture detail didn't really improve, the Pixel Filter Radius made most of the difference. Not the iterations.
Interesting anyhow, at least to me.