Complex sentence: Compared to single-lens reflex cameras (SLR), mirrorless cameras are usually simpler, smaller and lighter because they do not have an optical viewfinder, which is composed of a mirror housing, a movable mirror, or a viewing prism with reticle.
Split into simpler sentences: Compared to single-lens reflex cameras (SLR), mirrorless cameras can be made simpler, smaller and lighter because they do not have an optical viewfinder. Such a viewfinder is composed of a mirror housing, a movable mirror, or a viewing prism with reticle.