Setting Render Area
You can export the whole screen as an image or you can export a render area which you can define yourself.
To define a render area:
Click the Set Render Crop button then click and drag in the 3d view port.
A red rectangle appears on the screen.
Resize the render area rectangle by dragging the corners, or move it by clicking in the center and dragging. If you click outside the render area rectangle, the rectangle is erased and you can draw a new one by clicking and dragging with the mouse. You can also resize the render area rectangle directly using the Render Area setting of the Output Size field.
Click the show Render Crop button to turn the red rectangle on and off.
Click the Preview button or press F9 to preview the result.