Mist Pass viewport render and f12 render different?

Hi guys, i saw a tutorial regarding the mist pass here - https://www.youtube.com/watch?v=Hkm1R02UDfU

In the artwork I uploaded recently I went through messing around far to much to get a mist and depth pass into photoshop but the artists in the youtube video gets one out so fast. When I try follow his way I get the attached image. Does anyone know why my viewport looks great but the f12 render looks well… crap?