Look carefully at your spectacles and you will see, on the inside face of one of the sides, a set of numbers; such as 47-18-140. These numbers show you the fundamental dimensions of spectacle frames.
47 is the width in millimeters of the width of each lens. The width of the lens is usually measured at its widest position horizontally.
18 is the width of the bridge. The width of the bridge is the dimension of the closest points between the two lenses. I initially thought that this would be a dimension the dispensing optician could actually measure on the frame and on the customer’s face. But no. Perhaps this is too difficult to achieve. This is a pity because I am beginning to find out that this is important for finding a frame that fits properly. The width between the nose pads is crucial in getting a fit that allows the spectacles to give you the best vision.
140 is the length of the sides. Not shown here.
Range of sizes.
All major suppliers of spectacles have a range of sizes in stock for most styles of frames. But the dimensions are only an approximate guide to how well each frame will fit. It is still the skill of the optician to find the best fit in a style that complements the customer’s face. The designer must still stick to the conventions of dimensioning so that the optician is confident in selecting the best options.