When a borescope is being used to inspect the inside wall of a bore the following considerations are important.
If a view directly at the wall is desired, and the scope is a close fit in the bore, the focus and illumination must be optimized for this condition. A direction of view between 110° and 70° can be used depending on the condition of the wall. The reason for choosing a direction will depend on what is trying to be seen, and the reflectivity of the wall.
Highly polished walls are very difficult to view. The polished surface reflects the light away so that almost no light gets reflected back into the scope. When looking at welds that have a flatter finish, the scope will be able to inspect the welded area, but the polished area will be difficult to inspect.
A straight ahead, or 0° direction of view can also be used to view the walls of a close fitting bore. Sometimes a very wide field of view is used to optimize the view of the wall and minimize the view down the center of the bore. The 0° direction can be used to view the entire ID at once without needing to scan the wall as in the side viewing scopes described above.