Hello there and welcome to Just Answer. My name is ***** ***** I will be assisting you today. I am an engineer with over 30 years of electrical and electronic training, repair and installation experience. I will try and answer your question accurately and precisely so that we can get you on your way.
Nikon guy here.
You have purchased a crop frame DX lens. Your D600 is a full frame FX camera. Basically, the sensor in an FX camera is about 1.5x larger than the sensor in a DX camera. A DX lens is designed to project the image onto that smaller DX sensor so on the larger sensor, you will get the dark ring.No worry. What you can do is put teh camera into DX mode when you use that lens. You will not notice the difference in photo quality - I promise you.So, go into the Shooting Menu, find IMAGE AREA and set it to Auto DX Crop.Now you will see a square in the viewfinder that represents the smaller sensor.OR you can return the lens and get the FX version.Please let me know if you need further help.Steve