Labs #4: Lens Diameters

By Eric Johnston, Product Specialist

Often we receive questions about fitting lenses to the OConnor O-Box mattebox. There are two ways to do this; by either rods supporting the unit or by clamping it directly onto the camera lens. With a clamp-on configuration there is the advantage of having a lighter weight set-up supported by the lens with a specific clamp ring. The disadvantage is the exclusion of a rubber bellows that has the potential to act as a third rotatable stage when using a 138mm screw-in round filter. With a bellows, it is important to use properly fit screw-in step down rings around the lens to prevent light leaks. To help identify the right step down rings, we have created a chart of the most common PL-mounted outer lens diameters.

Pro Tip - if you are unsure of a lens’ outer diameter, look behind the lens cap. On most you will see the Ø symbol plus a number. This symbol means “connection at” and the number, measured in millimeters, represents the outer diameter for both lens caps and connection rings.

The OConnor C1243-1117 Bellows Ring has an outer diameter of 150mm matching the rear opening of the mattebox, a 138mm screw in retainer for a round filter, and a threaded 114mm rear opening towards the lens.  Common step down rings can be screwed into these 114mm threads to achieve the proper fit around the lens. For lenses greater than 114mm in outer diameter, an inexpensive neoprene doughnut can be used to prevent light leaks.