Edge-blacking is the term that refers to the black paint that is applied to the outer edge of an element, inside of an optical system. Nearly all elements within a lens will have their edges, or any surface not being used to transmit light rays through the lens, coated with a specialist type of black paint. The reason for this is to dramatically reduce the amount of stray light rays bouncing around internally. If the edge-blacking process was not done, it would have an adverse effect on the image produced by the lens. This would show as bad flare/glare on the final image and produce a more ‘washed out’ image from the optics.

Generally, the surfaces of the element that are not used for transmitting light through the lens (i.e. the edge of the element) are not a polished finish. However, they are a ground surface which will appear physically duller and rougher than the polished front and rear surfaces. This gives an ideal key for the paint to adhere to, while also making it possible to get a decent matt finish on the blackening paint. Ideally, when choosing a suitable paint, it should have a similar refractive index as the glass type that it is being applied to. This will yield the best results.

With age and/or certain conditions, edge-blacking can deteriorate and begin to separate from the element. This can show itself in a variety of ways. Sometimes the paint will blister off the element, which will appear like white specs or spots around the perimeter of the element when looking into the lens. In other scenarios, it will flake away from the element, creating shapes of white/shiny areas around the edge of the element. Another factor is damage from servicing. If the lens is mishandled during any service work, the paint can be chipped of scratched from the element, leaving a visible mark once the lens is re-assembled.

The image here displays a patch of edge-blacking missing on the front element of a 50mm Lomo Anamorphic round front lens.

While it is mainly obvious that the edge-blacking is deteriorating, sometimes it is difficult to diagnose. Especially if the element is internal within a multi-element system. Due to the way that the light can reflect within a lens, it can appear as if the defect is on the polished surface of the element. However, it is in fact a defect on the edge-blacking, being reflected, giving the appearance of a different location.

Once you notice the edge-blacking is coming away from the element it is best to seek professional help to bring it back to original condition. If the defect is minor, it can sometimes be touched in with a careful touch and the right paint. However, in most cases it is best to remove all the existing edge-blacking and re-paint the entire edge(s). To do this, chemicals must be used to remove the existing paint. This can be a time-consuming and skilled process as most the paints used are two- or three-part paints. Therefore, once hardened, they are very difficult to remove. It is always advised to start with mild solvents before moving onto more harsh chemicals when removing existing edge-blacking. It may also be necessary to ‘scrub’ the paint with a soft cotton bud or optical wipe to help remove the edge-blacking.

When all the existing edge-blacking is removed, the surface is then prepared for the new coat of paint. As the exact glass type used is rarely known, a generic edge-blacking paint will be used to replace the old paint. This is more than suitable for 90% of applications, however on the rare occasion, the edge-blacking may appear slightly grey due to the difference in refractive index between the element and paint.

When the technician is ready to paint the element, it is secured onto a turntable and the paint is mixed. There is a short window to paint the element before the paint begins to harden. It is also critical to get the paint the correct consistency for the size of element and thickness required. As the element needs to fit into a bore diameter, the tolerance is generally tight (+/- 0.02mm). Therefore, the thickness of the paint needs to be as thin as possible, while still providing adequate cover to matt the edges. Care needs to be taken to not get any paint on the polished surfaces of the element. If some does spread onto the polished surfaces, it needs to be cleaned off immediately. After the paint has been applied, the paint is left to cure, either at room temperature or in a heat chamber (depending on the type of paint required).

When the paint is cured, the element is ready for inspection before being installed back into the original metalwork. If there are any defects at this point, the whole process needed to be repeated until the element passes the quality control checks.

If you’d like a quote or more information our edge-blacking process, please get in touch with us at sales@truelens.co.uk or calling our office on +44 (0)1455 848411.