A common question I get asked is about binoculars with ruby coatings on the lenses and if they are a good or bad thing:
The first thing to mention about “ruby” coatings on binoculars is that the coating has nothing to do with the mineral ruby.
As far as I know, ruby coated lenses were first seen on Steiner binoculars who were just starting to make coatings for specific environments, such as hunting binoculars whose coatings are specifically designed to block the colours of haze and foliage, whilst at the same time enhancing the visibility of browns, reds and other wildlife colors. The idea is that this will help you spot hidden wildlife in the bush and they have developed this idea and it now works very well on the Steiner Predator Binocular range – but it must be emphasised that these binoculars have a specific purpose and are not intended as general use binoculars.
One coating developed by Steiner in the past had a ruby color. It worked acceptably, but to make it work, it partially filters some of the wavelengths of light. The Steiner version did this only a little and like their current predator series were designed for a specific use. The biggest problem was that the red coating looked cool!
Because it looked cool, many low-end binocular producers started churning out ruby coated binoculars but there was a problem: Ruby Coatings don’t really give you a very good image and some manufacturers use it to filter red to compensate for their poor-quality optics that do not properly converge the color spectrum.
By eliminating red from the spectrum, the optics first appear to do a better job of minimizing color abberations, but tf you look through a Ruby coated binocular and compare it with a “normal” coated binocular you will see that the ruby one looks a little washed out and often have an unnatural greenish cast.
The greens and blues stand out too much, the reds are faded. In the worst cases, the view can look like at a photo that has had some sun bleaching. This is because the red portion of the spectrum is being deflected by the Ruby coatings, which low-end binocular producers made with brighter and brighter red coatings that dropped more and more red light.
My general feeling on binoculars with a ruby coated lenses is to stay well clear of them as they are simply a gimmick with no redeeming qualities.