Inheriting the optics of the EF 400mm f/2.8L IS III USM, the same high image quality is maintained in the RF400mm F2.8 L IS USM with outstanding sharpness and contrast, center to corner, even at maximum aperture. A combination of Fluorite and Super UD (Ultra-low Dispersion) glass is at the heart of this lens’s outstanding optical performance. At the widest aperture of f/2.8, chromatic aberration is significantly reduced, to achieve tack-sharp images with minimal color fringing. This means detailed results in any shooting condition, particularly in low light levels.