Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Sigma 35mm F1.2 DG DN Art is the longer of the two lenses at 136mm. The Leica APO-Summicron-SL 28mm F2 ASPH with a length of 102mm is 34mm shorter. Besides being longer, the Sigma 35mm F1.2 DG DN Art also has a larger diameter of 88mm compared to Leica APO-Summicron-SL 28mm F2 ASPH's 73mm diameter.
Weight of a lens is as important as its external dimensions, especially if you are planning to hand hold your camera and lens combination for longer periods. Leica APO-Summicron-SL 28mm F2 ASPH weighs 700g, which means it is 390g (55%) lighter than the Sigma 35mm F1.2 DG DN Art which has a weight of 1090g.
Lens Mounts
Both Leica APO-Summicron-SL 28mm F2 ASPH has the Leica L lens mount whereas the Sigma 35mm F1.2 DG DN Art has the Leica SL lens mount. Some of the latest released cameras that are compatible with these mounts are Sigma fp L, Leica SL2-S and Panasonic S5 for the Leica L Mount and Sigma fp L, Leica SL2-S and Panasonic S5 for the Leica SL Mount.
Sigma 35mm F1.2 DG DN Art is also available in Sony E mount.
Focal Range
Leica APO-Summicron-SL 28mm F2 ASPH is a prime lens with fixed focal lenght of 28mm. Sigma 35mm F1.2 DG DN Art is a prime lens with fixed focal lenght of 35mm.