The Pentax ME F was an amateur level, interchangeable lens, 35 mm film, single-lens reflex (SLR) camera.

Although it autofocused poorly and was a commercial failure, the pioneering ME F was a major milestone in the history of camera technology that pointed the way to all present day AF SLRs.

The M-series remain among the smallest and lightest full-frame 35 mm film SLRs ever made.

Unlike all later Pentax AF SLRs, the ME F did not have the focusing drive motor and keying shaft built into the camera body.

In the module, beam splitter mirrors sent the light to fall on a two row, segmented, linear metal oxide semiconductor (MOS) silicon sensor.

Focus indication also came in the form of a green hexagonal in-focus light-emitting diode (LED) flanked by two red arrowhead shaped, out-of-focus, turning direction LED indicators visible at the bottom of the viewfinder.

After lightly pressing the shutter button, the photographer would turn Pentax brand manual focus lenses in the direction of whichever red arrow LED lit until the green LED lit to indicate sharp focus.

This system lit LEDs alongside a vertical shutter speed scale on the left side of the viewfinder.

The split image rangefinder was a good indication of the width of the AF sensor field of view.

Although the ME F was a highly electronic camera normally dependent on battery power, it had a backup ability to operate without batteries, though in a very limited fashion: completely manual mechanical control with two shutter speeds (1/125th second, marked 125X, and Bulb; both accessed from the mode dial) and without the light meter or AF.

The 1970s and 1980s were an era of intense competition between the major SLR brands: Pentax, Nikon, Canon, Minolta and Olympus.

In addition, because of rapid advances in electronics, the brands continually leap frogged each other with models having new or more automatic features.

The industry was trying to expand out from the saturated high-end professional and advanced amateur market and appeal to the large mass of low-end amateur photographers itching to move up from compact automatic leaf shutter rangefinder (RF) cameras to the more versatile and glamorous SLR but were intimidated by the need to learn all the gritty details of operating a traditional SLR.

Even the normally complex control of flash exposure for auxiliary light in dim situations had been completely automated.

The ME F (and its ilk) could be expected to autofocus only under near-ideal conditions – brightly lit, high-contrast, well-centred and stationary subjects – that could be manually focused with ease.

Instead, it might continuously but hopelessly turn its lens; "hunting" in vain for proper focus but succeeding in wasting limited battery power.

Photographers saw the ME F as little more than a curiosity, not worth its initial 50% price premium over the equivalent manual focus ME Super.

The selling price of the ME F crashed by a third in early 1983 as photographers made it obvious that the time for autofocus had not yet come and dealers scrambled to clear out stock.

It would not be until the introduction of the landmark Minolta Maxxum 7000 (Alpha 7000 in Japan) in 1985, with its superior and better integrated phase comparison AF system and far wider lens and accessory selection, that autofocusing captured the 35 mm SLR buying public's imagination and the AF SLR camera revolution truly began.