To create Rembrandt lighting the subject must turn slightly away from the light. The light must be above the top of their head so that the shadow from their nose falls down towards the cheek. Not every person’s face is ideal for creating Rembrandt lighting. If they have high or prominent cheek bones it will probably work. If they have a small nose or flat bridge of the nose, it may be difficult to achieve.
In loop lighting the shadow of the nose and that of the cheek do NOT touch. Keep the shadow small and slightly downward pointing, but be aware of having your light source too high which will create odd shadows and cause loss of the catchlights. Loop light is probably the most common or popular lighting pattern as it is easy to create and flatters most people.