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To crop render in Render View, simply click and drag in the image to draw a crop region.  IPR will immediately update with the new region.  To draw a new region, click and drag again.  The crop region can be removed by just clicking once in the image.

Rendering Selected Objects

To prevent objects from being exported, you may select the objects and place them into a display layer. Then switch this layer to Template. The objects that are members will not be exported. Otherwise, objects are exported regardless of visibility so that during IPR rendering you can quickly make objects visible/invisible without having to re-export or restart the render.

You may also render selected objects by choosing the option from the Maya Render View, Windows > Rendering Editors > Render View > Render > Render Selected Objects Only, or by toggling a mel shelf button using this mel command: switchSelectObj;