The Spotmatic SP was introduced by Asahi Optical Company in 1964, and was one of the first 35mm SLR cameras with through-the-lens light metering. The camera was entirely mechanical apart from the light meter, which was powered by a 1.35 volt PX400 mercury cell. Mercury batteries are now banned but Zinc-Air batteries can be used instead, or it is possible to carry out a minor modification to the meter circuit to allow the use of 1.55 volt 387S silver-oxide batteries. A small switch on the (photographer's) left side of the lens housing was pushed up to stop down the lens and activate the meter; the exposure controls would then be adjusted to center a needle on the right edge of the viewfinder. The body took lenses with an M42 screw thread, providing a large range of available lenses. The system became the workhorse of many professionals of the period. Three variants of the Spotmatic SP are common: 1) Black and chrome body with Asahi Pentax badge on prism housing, Spotmatic badge on front below shutter release and SP badge on top near the rewide knob. 2) All black body with Asahi Pentax badge on prism housing, Spotmatic badge on front below shutter release and SPbadge on top near the rewind knob. 3) Blank and chrome body with Honeywell Pentax badge on prism housing and Spotmatic badge on front below shutter release (but no correspond SPbadge).