Blender 3d: Rendering only a small portion of your scene

sometimes i have very large elaborate scenes to render but I need to tweak a particular texture on one small portion of a much larger render..    here's a quick way to only render a small portion of your entire scene at its full size (saves a ton of rendering time)

press Control + B when you are in the camera view and draw a square around the object you want to render.

to release the selected area just do a Control + B again outside of the render area of the main scene.