https://i.ytimg.com/vi/94UsZw6mTaE/hqdefault.jpg

Oftentimes we need to render just a part of our camera view just for previewing details of our render (such as materials textures etc)
Blender 3D has a very handy feature for this,called “Render Border”
In this video tutorial we present how you can use this little yet useful feature of Blender 3D

source