What is the usable ISO in Nikon d750?

What is the usable ISO in Nikon d750?

Enough.

Until dpreview.com has tested it, we won't know for sure. But I would expect it to match or better the Nikon D610, which is usable up to ISO 6400.
http://www.dpreview.com/...on-d610/10

All of them.

Here's a link showing images taken at all ISO settings. Just realize that the images were taken in full sun, hardly a time when someone would ever shoot at ISO 1600 or higher. Ken's site is notorious for being less than accurate, so take his words with a huge grain of salt. That being said, the photos don't lie http://www.kenrockwell.com/...n/d750.htm