In August 2015, Canon announced that it has successfully developed a new Blue Spectrum Refractive optical element (BR Optics) for use in camera lenses that corrects for chromatic aberrations at an exceptionally high level to make possible superior imaging performance. The first Canon EF Lens to incorporate BR optics technology is the EF 35mm f/1.4L II USM.
Cause of Chromatic Aberration
White light is composed of a mixture of light of various wavelengths (red, green, blue, etc.). Since the refractive index varies according to wavelength, it is not possible to get all wavelengths of light to focus on a single point on the image plane by normal glass material. This appears as color fringing in images, and is called chromatic aberration.
Development Goals: Ideal chromatic aberration correction
To correct chromatic aberration, elements of differing dispersion have been combined to provide reduced chromatic aberration. Material with anomalous dispersion properties (dispersion characteristics that differ from common glass materials) ,such as fluorite, UD and Super UD lenses developed by Canon, can be used to correct chromatic aberration (secondary spectrum) that remains after the above chromatic correction. While these materials are effective in removing secondary spectrum, it had been impossible to achieve ideal chromatic aberration correction for all interchangeable lenses that encompass a wide variety of lens specifications. For example, with large-diameter lenses, chromatic aberration remains despite this correction, and color blurring is still recognizable in enlarged images.
The reason for this was that in selection of glass materials, it is necessary to take into consideration correction of aberration other than chromatic aberration, and there were limits as to how compact and lightweight a lens could be made while providing correction of chromatic aberration.
Therefore, Canon aimed to achieve the development of a new optical material, which possesses anomalous dispersion characteristic equal or greater than fluorite, enables more space-efficient and effective chromatic aberration correction, and achieves a correction amount of chromatic aberration which had not been possible with conventional lens configurations consisting of glass elements only.
Differences in Refractive indexes of optical elements in Canon EF Lens
Normal Glass |
BR Optics Characterized by ability to greatly refract blue light (short wavelength spectrum) |
DO Element The order of colors is opposite to that of normal glass |
Fluorite Low refractive index and dispersion |
Blue Spectrum Refractive Optics (BR Optics)
By reviewing lens materials from its molecular design, Canon succeeded in developing a unique lens material using organic optical material as its raw material – the Blue Spectrum Refractive Optics (BR Optics). BR optics is characterized by its ability to greatly refract blue light (short wavelength spectrum), a wavelength that, until now, had proven particularly difficult to converge to a specific focal point.
To utilize BR optics as camera lens element, a BR Lens is formed by sandwiching the BR optics between a convex and concave glass lens element. As a result, a greater standard of correction has become possible and superior imaging performance has been achieved even for lenses that were impossible to correct for chromatic aberration using conventional technology.
Normal Glass | BR Lens |
Merely combining convex and concave lenses will not correct blue wavelengths refraction and the shifted focal point will appear as blue color fringing. |
Since a BR lens can greatly refract blue light (short wavelength spectrum), placing it between a concave and convex lens allows all visible light wavelengths to be focused onto a single point. |
Application of BR Lens
By incorporating a BR Lens, the EF 35mm f/1.4L II USM achieves a correction amount of chromatic aberration in wide-angle large-diameter lenses not previously possible for significantly reduced color blurring, even at wide open aperture.
Comparison 1: Highly Reflective object
EOS 5D Mark III • EF 35mm f/1.4L II USM • 35mm • ISO100 • f/1.4 • 1/100s | |
EF 35mm f/1.4L USM | EF 35mm f/1.4L II USM Axial chromatic aberration is significantly reduced |
Comparison 2: Landscape
EOS 5DS • EF 35mm f/1.4L II USM • 35mm • ISO100 • f/1.4 • 1/8000s | |
EF 35mm f/1.4L USM | EF 35mm f/1.4L II USM Magenta color fringing is significantly reduced |