Changing the Preview Resolution
                                                        The Preview Resolution command allows you to quickly get a preview at three-quarter, half, one-third, or one-quarter of the scene resolution. You can even customize the preview resolution size.
                                                        
How to use the Preview Resolution command
                                                            
                                                                
                                                                    - From the top menu, select View > Preview Resolution and one of the following: 
 
                                                                
                                                                
                                                                    
                                                                    
                                                                    
                                                                    
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                                                                        ‣
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                                                                        Same as Scene Resolution
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                                                                        ‣
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                                                                        3/4 of Scene Resolution
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                                                                        ‣
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                                                                        1/2 of Scene Resolution
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                                                                        ‣
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                                                                        1/3 of Scene Resolution
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                                                                        ‣
                                                                         | 
                                                                        1/4 of Scene Resolution
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                                                                If you selected the Custom option, the Custom Preview Resolution dialog box opens.
                                                                
                                                                    
                                                                
                                                                
                                                                    - In the dialog box, select a resolution from the presets or type a new X-Y value.
 
                                                                
                                                                
                                                                    - Click OK.
 
                                                                
                                                                
                                                                    - Preview your effects animation.