Some of the pieces we've rescued were written off by the manufacturers, who were amazed when the bad tube rose, like a phoenix, from the ashes. Please let me know the results if you try this, along with any suggestions you may have. This will not only make it easier on the tube but will provide a greater depth of field. We're not discussing optical theory here, but remember that the smaller the F stop the greater the depth of field. This is true whether for video, film, night vision, and so forth.
Depth of field is described as the ranges from the lens at which a scene is in focus.
With little depth of field it will be necessary to refocus every time you move from a closer scene to a further one, and vice versa. With great depth of field you may not have to focus at all. The basic idea is to try to keep the iris as small as possible. This makes a good argument for using fast film or a low light video camera even when there is plenty of light. You can demonstrate this easily with any auto iris video system.