Select the Custom option to analyse grain in the back clip.

To analyse the grain in the back clip:

1. From the Grain Signature box, select Custom.

   
   

2. Look closely at the image and try to find three patches that best represent areas of shadow, midtone, and highlight. You will create control points for these on the curves for each of the red, green, and blue channels.
3. To analyse the grain for the red channel, go into the View menu, then enable Excl and the R button.
4. Viewing the back clip, press Ctrl and drag a selection box over an area in the back clip that is relatively uniform and predominantly red.

   Control points appear for the red channel modifying the shape of the red curve based on the analysed sample.

5. Repeat steps 3 and 4 to analyse the grain in the green and blue channels.
6. For even more precise grain matching, repeat steps 3 and 4 for different luma ranges of the red, green, and blue channels.

   The results produced by grain analysis may not be exactly what you want. If this is the case, you can modify the RGB Gain curves and adjust the size of the grain using the RGB X Radius and RGB Y Radius sliders.