BR Optics: Exceptional high level of Chromatic Aberration correction
Tag: EF Lens, BR Optics, BR Lens

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.


Structure of Blue Spectrum Refractive Optics Lens (BR lens)


EF 35mm f/1.4L II USM: The first EF Lens to incorporate BR lens.


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.



Cause of chromatic aberration


Example of chromatic aberration (Color fringing)


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.


Cross section of the BR lens and the raw material used to make the BR optical element (organic optical material)


Structure of BR Lens

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.
BR Optics Reduce Chromatic Aberration of Blue Light


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