Traduction de Luc Moreau
 

In 1975, Pentax abandons the M42 screw mount in favor of the type K bayonet mount, which we had dreamed for a moment could become an industry standard.  Even if the diverging interests of various brands have aborted this great idea, the K bayonet remains a reference because it still allows the latest Pentax bodies, including the digital ones, to exploit the qualities of all the lenses of the brand or other compatible brands.
These cameras were the first in the K-series: KM, KX, K2 and K1000.
In 1976 the K series is followed by the M series which is more compact. The "Asahi Pentax" logo is replaced by just "PENTAX".
Two lines of cameras constitute the M series: on one hand, the semi-auto MX model comes in two successive versions (Metal memo-film then later a plastic one also with somewhat simplified electronics), and on the other hand the fully automatic ME and its derivatives : ME Super, MV, MG...
Upon its release, promotional materials show the MX as the smallest and lightest in the world of motorized bodies.  In reality it would be more accurate to say “motorizable”.  It was delivered in two finishings: chrome or black. The latter, a little more expensive, was supposed at the time, to be more like a "pro" look...

Features of the Pentax MX

SLR with semi-automatic exposure settings.
The shutter is focal plane, rubberised silk curtain with horizontal travel. Speed: Bulb, 1 second to 1/1000 sec. The flash sync speed is 1/60 sec.  Quick return mirror with pneumatic bumper. Cocking lever.  It has a delayed action timer: the lever to the right of the lens, which also allows manual stop-down of the diaphragm for depth of field preview.
The shutter speed setting is done by the knurled knob located on the camera top, next to the shutter release button.  This same knob also serves to adjust the film sensitivity by disengaging the ring gear knob with a button.
The MX is ranked in the category of semi-professional equipment. Pentax enthusiasts often choose as the second body in addition to an LX.
Upon its release, it introduces a series of innovations that make a new camera immediately sought after by amateurs: Robust, compact and lightweight, although it has a gallium arsenide phosphide cell which was fast and low-light sensitive, performing a weighted average metering. The indication of exposure in the viewfinder, with 5 LEDs (2 red, 2 orange and 1 green) indicates intuitively over or under exposures. The cell is powered on by the halfway action of the shutter release, it remains active until the film advance lever is put back into its fully lowered position (1st position). The viewfinder is bright and clear and provides all the necessary information: aperture, shutter speed, exposure. Through a small window near the bottom of the viewfinder, the user can read the aperture directly on the lenses' diaphragm ring (with Pentax original lenses only).
The film loading, fast and safe, is provided by a new "magic needle" system.
The standard lenses are: SMC Pentax 1:1.2 f = 50mm, SMC Pentax M 1:1.4 f = 50mm, SMC Pentax M 1:1.7 f = 50 mm, or "pancake" SMC Pentax M 1:2.8 f = 40mm as in the picture.
The MX accepts all the K bayonet lenses, including the most recent ones (Series A, F and FA), except for lenses without an aperture ring or specifically developed for digital cameras.
A wide range of accessories extends the capabilities of the device. In addition to the conventional focal multipliers, extension tubes and bellows, it can accommodate two accessories (MX Winder or MX Motor), a back for 250 frames or a data back. The focusing screens are interchangeable (8  different ones were available).