Nikon Corp. has in recent years halted development of single-lens reflex cameras as the Japanese camera maker is shifting its focus to mirrorless cameras which are lighter than SLR types, company sources said Tuesday. Nikon, which began selling SLR cameras in 1959, once captured a large share of the camera market worldwide. The decision to suspend the development of new SLR cameras came as small mirrorless cameras rose to dominate the digital camera market. The company will continue sales of existing products and customer support services, the sources said, adding that Nikon may resume develop…