Hard transition in background [Arnold]

dlz

I don't know how to call  it and maybe it's a newbie question but i didnt found an answer for that
When i preview a render in arnold the background transition is perfect and smooth but when i save it, it does that "bands" that are noticeable.
i play with settings an saved in diferent formats but nothing worked
I hope that picture ilustrate the problem:

Axi5

Yeah this is called banding and unfortunately there's no easy solution. Even 32bit images can appear banded on an 8bit screen. If you're already saving in a 16bpc format and still seeing it, your only solution is to dither it, you can do so in Photoshop.

Edit: That said. On my mobile your banding issue looks fairly minor.

dlz

Axi5 Thank you a lot! i tweak a bit the settings of format and nowit solves the problem a bit. 
Also i notice something i didn't expect, the less samples(and more noise) the less banding (i supose because of that noise).
Anyways it seems strange to me that in the preview of the render it looks perfect and when i save the image i get these artifacts