The Olympus ® Pen FT (PenFT) was introduced as "the world's smallest SLR system camera." (At that time, it was. It was later eclipsed by the Pentax 110 SLR.) The PenFT is an ingeniously designed 35mm, half-frame, interchangeable lens, single lens reflect (SLR) camera. It has features that can be found in most full frame 35mm SLR cameras as well as features that the others do not have. The PenFT is an improved version of the Pen F and incorporates a through the lens (TTL) CdS (cadmium sulfide) meter. The PenFT film range is ASA 25 to 400. The standard lens is 38mm f1.8. The filter diameter is 43mm.

The longest dimension of a full frame 35mm camera is horizontal, it produces an image on film that is 24x36 (approximately 1 1/2 inches). The longest dimension on a half-frame 35mm camera is vertical. It produces an image on film that is 18x24 (approximately 3/4 of an inch). Therefore, a half-frame camera produces twice as many exposures as a full frame 35mm camera.

35mm SLR systems have prism devices which make it possible to view an image correctly. The full frame 35mm mirror and prism system is what gives the camera its characteristic hump (protruding pentaprism). The unique porroprism design of the PenFT eliminated this hump. The PenFT TTL metering system, uses a focusing screen, angle prism, and a mirror.

The shutter of coventional 35mm SLRs have a varying slit which travels horizontally across the film at a fixed speed. Consequently, images that are moving in the opposite direction appear slightly distorted. Also, the nature of the 35mm SLRs shutter usually does not allow them to sync at electronic flash speeds beyond 1/60 of a second. The PenFTs rotary focal plane shutter is waffle embossed titanium. (Titanium is a light weight, very hard material.) The shutter is placed at such an angle that it opens fully, at some point. Because of this, the PenFT can synchronize flash at any speed.

Olympus engineers determined that there were errors in the conventional f-stop system. They designed a TTL numbering system calibrated from 0 to 7. The TTL numbers appear on the left in the camera's viewfinder. A different lens would have different numbers. The numbers were unevenly spaced on the lens barrel and theorectically corresponded to 1/2 stop of exposure.

The complete PenFT SLR system included interchangeable Zuiko lenses, from 20mm wide angle to 800mm mirror. For close-up photography there were attachment lenses, extension tubes, two types of bellows, camera slider, a reverse ring adapter, and a microscope adapter.

The PenFT SLR system also included a lens mount adapter. This permitted one to use interchangeable lenses for other 35mm SLR systems.