The LEICA SUMMILUX-M 21mm f/1.4 ASPH extends Leica's series of high speed M lenses into the wide-angle range. It offers excellent performance over the entire image field even at full stop and in the close-up range thanks to a "floating element". Stopping down to 2.8 leads to an absolutely outstanding image quality. In fact, in comparison to the Elmarit lens of the same focal length, its rendition is slightly more uniform across the image field.
The vignetting typical of every optical system is naturally more apparent with a super wide angle lens, particularly a high speed one like this, than on standard lenses or those with a long focal length. At full stop with the 35mm format, its maximum - i.e. in the corners of the image - is around 3.2 aperture stops or, with the Leica M8 models and their slightly smaller format, around 1.9 aperture stops. Stopping down to 2.8 visibly reduces this light fall-off towards the image edges to 1.7 or 0.8 aperture stops. Stopping down further does not bring any noticeable additional reduction, as all that remains is essentially the natural vignetting.
The maximum distortion of the lens is 2.3%,which in practical terms is negligible for the majority of photographic applications. A total of ten lens elements are used to achieve this exceptional performance. To correct color aberrations, five of these are made of glass types with anomalous color dispersion(partial dispersion), while a further two have aspherical surfaces. To maintain performance in the close-up range, one group towards the rear of the optical system is a "floating element" that moves independently of the rest of the mechanism.