The purpose of a lens is to control and focus the light that enters your camera. For this reason, the more light it admits the sharper it focuses, thus enabling you to take bright, crisp photographs. The secret behind the superior optics of Pentax lenses is Super-Multi-Coating (SMC).
Usually, between 4 and 8 percent of the light rays passing perpendicularly through a glass surface are lost due to reflection. Because of the complex optics of photographic lenses, there is a 10-20 percent loss of light, even when lenses are covered with conventional coating.
Super-Multi-Coating, developed by Asahi Pentax, reduces loss of light to a mere 0.2 percent. Increased light transmission means brighter photos, easier focusing and improved performance in complex lenses such as zooms and ultra-telephotos.
SMC also eliminates three other annoying lens problems. The first is flare which results in distracting bright spots on your photos. With Pentax SMC lenses, you can shoot directly into the sun and still get perfect pictures. Under some circumstances, conventionally coated lenses produce ghost images, like a poorly tuned TV. But ghosting is extremely rare with Pentax lenses.
Finally, some lenses achieve good color rendition only by sacrificing effective lens speed. SMC gives you both: maximum light transmission and optimum color. Not all lenses produce photos of equal brightness and beauty. Naturally, you want the best, and that's why you should choose SMC Pentax lenses.
Source: The many eyes of Pentax: SMC Pentax interchangeable lenses (PUB. 06071).