Size and Weight Comparison
Size of a lens is an important deciding factor when comparing two lenses.Panasonic S 50mm F1.8 is the longer of the two lenses at 82mm. The Sigma 45mm F2.8 DG DN C with a length of 46mm is 36mm shorter. Besides being longer, the Panasonic S 50mm F1.8 also has a larger diameter of 74mm compared to Sigma 45mm F2.8 DG DN C's 64mm 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. Sigma 45mm F2.8 DG DN C weighs 215g, 28% (85g) lighter than the Panasonic S 50mm F1.8's weight of 300g.
Both Panasonic S 50mm F1.8 has the Leica L lens mount whereas the Sigma 45mm F2.8 DG DN C 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 45mm F2.8 DG DN C is also available in Sony E mount.
Panasonic S 50mm F1.8 is a prime lens with fixed focal lenght of 50mm. Sigma 45mm F2.8 DG DN C is a prime lens with fixed focal lenght of 45mm.