Hi there! I am looking for real experts, possibly some folks working in the optics industry. I am new to this forum, but not really new to photographic optics. However, one question really is a tough nut to crack for me. Here is my problem: Does anybody have reliable information on how Canon's BR lenses (Blue Spectrum Refractive Optical Elements) REALLY work? They are kind of achromatic lenses trying to get all wavelengths / colors of light to the same focal length in order to reduce chromatic aberrations in the photograph. On their website (https://hk.canon/en/consumer/BR-Lens/article) Canon claims that "Merely combining convex and concave lenses will not correct blue wavelengths refraction and the shifted focal point will appear as blue color fringing." The image above that explanation shows that a combination of a convex lens with a concave lens will bring red and green light to the same focal point but not blue. Canon announced these BR lenses in 2015 as a major invention where they found a new way to better control blue light / short wavelengths in order to bring that blue light to the same focal length as red and green (see the second image where all color rays meet at the same point). Here is the trouble I have with these explanations: The combination of a (1) convex lens with lower refractive index and (2) a concave lens with a higher refractive index is called an achromatic doublet, and it was invented roughly 200 years ago. In such a configuration the lens element (1) is called a crown glass and (2) is called a flint glass. The combination of both lens elements brings the two extreme wavelengths (red and blue) to the same focus whereas green isn't corrected so well. But that concept can be found all around the internet and technical literature, and it is almost always red and blue that is being adjusted to the same focal length. See the image on https://en.wikipedia.org/wiki/Achromatic_lens and note how only red and blue is corrected as these are the outermost wavelengths within the visible spectrum. So, does anybody understand what the specialty with Canon's BR lenses really is? Why do they show on https://hk.canon/en/consumer/BR-Lens/article that red and green have the same focal length and blue is way off? That looks like an incorrect representation of the existing color correction options. What does the BR lens really do - e.g. does it provide an identical focal length for all wavelengths (red, green, and blue)? If so, please note that bringing all three colors together, again, isn't a new thing. This is typically done by apochromatic lenses which consist of three lens elements and provide an identical focal length for three colors. I hope that somebody finds this question interesting and might be able to help clear up my confusion. Thanks! MK
