For user-facing documentation, call this the focus distance or focal distance setting, not “magnitude.” Photographers generally care about the real-world subject distance the lens is focused on, typically shown in meters or feet, rather than arbitrary values like 0–255.

If possible, map the control range to actual distances and present minimum focus distance through infinity. That will be much clearer than exposing raw numeric values.

For the control wording, Near and Far are understandable, but increase focus distance / decrease focus distance is more precise if the direction matches the behavior. Since you stated that increasing the numeric value moves focus closer, be careful: that means the numeric value is moving opposite to focus distance. In that case, label the user action by the result, such as focus nearer and focus farther, or display the actual distance directly.

If you must keep 0–255 internally, treat those as device control values and document them separately from the user-facing focus distance.