A perfect sub area length prevents the camera from panning too far.

Use a Minimal Sub Area Length

When creating a sub area, be aware of the length of the level. This is especially important when a level has only one entrance/exit for the sub area.

A sign of the sub area being too wide is when you approach the return warp pipe, and find the camera keeps panning.

A too-wide sub area causes the camera to keep panning with Mario's movement.
A too-wide sub area causes the camera to keep panning with Mario’s movement.

A poor solution is to fill in the gap with blocks.

Filling in extra screen length prevents an open gap when the camera pans.
Filling in extra screen length prevents an open gap when the camera pans.

This may be necessary when a level has multiple separate sections in the sub area, but should never be used when there is only one section in the sub area.

The proper solution is to resize the level to only the maximum length required.

The cat's paw is pointing out where the level length slider needs to be for this sub area.
The cat’s paw is pointing out where the level length slider needs to be for this sub area.

This is important to remember, because when you first drag Mario into a warp pipe, Super Mario Maker will set the sub area to the main area’s length, and position you at the same location into the sub area as you were into the main area. Move to the far left, resize the sub area length, and fill in your sub area. When the proper level length is used, the camera will not pan beyond the edge of the sub area.

A perfect sub area length prevents the camera from panning too far.
A perfect sub area length prevents the camera from panning too far.