Aspects of a Camera Lens: Diameter
One of the most noticeable properties of a camera lens is its diameter. Some lenses are thick and wide, while others appear narrow. Diameter is the length of a line, through the center of a circle, which intersects the circle twice. For a lens, it is measured across the front glass of the lens, the part which can be covered by the lens cap.
Now, the measurement of lens diameter presents a unique problems. What if the the glass itself does not sit at the tip of the lens? Or what if the lens body is much wider than the glass? What should be measured then?
The solution is this: lens diameter is functionally referred to as filter size.
This is the size of the threaded ring on the front of a lens. Now, the lens itself may have a larger body, or the lens elements may be of a different size, but the lens has this ring at its very tip. Photographers need the size of this ring for the proper fitting of lens accessories, such as the lens cap or various colored or polarizing filters. For this reason, filter size is the important measurement for all lens functions that are affected by lens diameter.