I have noticed a lot of products for setting your back focus to the ?micron, but as I recall back focus adjustment was set for the lenses. Has that changed? I remember a year ago that people were setting the back focus specifically for different lenses, yet now things like the Red null and others make it sound like one size fits all. What have people found? Since I don't own one of these I always check back focus with the widest lens in a set on any new project.