When R is attached to the edge of a workpiece, it may not be seen clearly.

You can see clearly by devising magnification, lighting, background.

I tried to observe the edge of dry cell
1. Difference in depth of focus (compare at 80 times)
Open the aperture	Close the aperture
If you open the aperture, the depth of focus will become shallow and the edge may become blurred.
2.Difference in background color (compare at 80 times)
When the background color is black	When the background color is white
The edge may be blurred due to reflection of the background, reflection of the edge part due to lighting, shadows.
3. Difference in magnification
The depth of focus also varies depending on the magnification. (open the aperture and compare)
at 30 times	at 80 times
Even with the same object, the edge may become unclear if the magnification increases.