Anti-Reflection Techniques
Technology - Technology
Flare and ghostings are a major problem in contra-light situations. The problem is caused by reflected light - either from the lens elements or the inside of the lens barrel. There are certain techniques to reduce these reflections:
  • Lens coating: Normal uncoated glass reflects about 5% of the light at each glass-air boundary. A lens with a bigger number of elements (say 10-15) would have a big problem here because A) the effective speed of the lens is reduced and B) the accumulated reflections would reduce contrast performance and produce extreme flare problems. In the early days where no coating techniques were available it was paramount to reduce the amount of elements within a lens. E.g. Zeiss developed the Tessar design (4 elements) lens because the Planar construction (9? elements) had heavy problems due to internal reflections. Today coated glass with extremely thin transparent films on the surface reduces the amount of reflections down to 0.5% or less. This is sufficient for normal shooting situation but you will still see reflections from very bright objects (sun, lamps, candles). There're some differences between the manufacturers here. It is known that e.g. Pentax SMC and Carl-Zeiss T* lenses produce much less reflections due to superior coating than the rest of the gang. The coating has also an additional function: Dependent on the coating film material it can also alter the transmitted color and can therefore be used to correct color shifts caused by the transmission characteristic of the used glass.
  • Electrostatic flocking techniques: Light rays can also be reflected by the inner lens barrel. Extremely fine pile applied to the surface can absorb these reflections.
  • Anti-reflection construction: There're various construction techniques to reduce flare. E.g. flare-cutting diaphragms (FCD) block unwanted light rays from entering the rear part of the lens.

It is surely a problem to compare the different solutions from the various manufacturers because concrete information are very rare and difficult to interpret. Nonetheless even using a simple lens hood reduces the flare problems significantly.